[HiCache] Buffer-only mode for HiCache host memory layer (#34798)

benchmark/hicache/bench_buffer_mode.py

@@ -0,0 +1,404 @@
+"""Benchmark driver for HiCache buffer_only mode.
+
+Runs a matrix of (model, host-memory-mode) server configs through the two
+workloads buffer mode targets, and reports hit rates, latency, and the
+buffer-mode pipeline counters side by side:
+
+- multiturn: growing-history conversations (delegates to bench_multiturn.py,
+  offline random tokens, round barrier, fixed seed);
+- longctx: long shared prefix + divergent continuations, measured warm
+  (device tier) and again after /flush_cache (through-storage tier).
+
+Example (dense, cache-vs-buffer):
+    python benchmark/hicache/bench_buffer_mode.py \
+        --model /path/to/model --modes cache,buffer \
+        --workloads multiturn,longctx --buffer-size-gb 2 --cache-ratio 2
+
+SWA / Mamba hybrids run the same way (add --tp for large hybrids); the
+unified radix tree is selected automatically for them.
+"""
+
+import argparse
+import json
+import os
+import shutil
+import subprocess
+import sys
+import tempfile
+import time
+
+import requests
+
+BENCH_DIR = os.path.dirname(os.path.abspath(__file__))
+
+BUFFER_METRICS = [
+    "sglang:hicache_existence_cache_skipped_pages_total",
+    "sglang:hicache_backup_dropped_tokens_total",
+    "sglang:hicache_pending_write_queue_depth",
+    "sglang:hicache_host_used_tokens",
+]
+
+
+def parse_args():
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument("--model", type=str, required=True)
+    parser.add_argument("--tp", type=int, default=1)
+    parser.add_argument("--port", type=int, default=31212)
+    parser.add_argument(
+        "--launch-module",
+        type=str,
+        default="sglang.launch_server",
+        help="server entry module (e.g. sglang_meta.launch_server for "
+        "Meta-internal model families)",
+    )
+    parser.add_argument(
+        "--modes",
+        type=str,
+        default="cache,buffer",
+        help="comma list of deployments to compare: cache (alias cache_wt), "
+        "cache_wb (write_back), buffer",
+    )
+    parser.add_argument(
+        "--workloads", type=str, default="multiturn,longctx", help="comma list"
+    )
+    parser.add_argument("--buffer-size-gb", type=int, default=2)
+    parser.add_argument("--cache-ratio", type=float, default=2.0)
+    parser.add_argument(
+        "--host-ratio",
+        type=float,
+        default=0.0,
+        help="if > 0, size EVERY mode's host pool as ratio x device pool "
+        "(overrides --buffer-size-gb / --cache-ratio) for apples-to-apples runs",
+    )
+    parser.add_argument(
+        "--max-total-tokens",
+        type=int,
+        default=0,
+        help="cap the device KV pool so the working set overflows into the hierarchy",
+    )
+    parser.add_argument("--mem-fraction-static", type=float, default=0.5)
+    parser.add_argument("--page-size", type=int, default=64)
+    parser.add_argument("--attention-backend", type=str, default="")
+    parser.add_argument(
+        "--storage-backend",
+        type=str,
+        default="file",
+        help="L3 backend: file, or meta_cache_store (spawns a local-mode store "
+        "under the run's storage dir; requires --launch-module "
+        "sglang_meta.launch_server)",
+    )
+    parser.add_argument(
+        "--sub-question-input-length",
+        type=int,
+        default=0,
+        help="Input tokens per FOLLOW-UP turn (0 = same as --request-length). "
+        "Set with a large --request-length for long-context multiturn: "
+        "100K first prompt + small follow-ups.",
+    )
+    parser.add_argument(
+        "--prefetch-policy",
+        type=str,
+        default="wait_complete",
+        choices=["best_effort", "wait_complete", "timeout"],
+        help="hicache storage prefetch stop policy (server default is "
+        "timeout; benches historically used wait_complete)",
+    )
+    parser.add_argument(
+        "--storage-extra-config",
+        type=str,
+        default="",
+        help="JSON merged over the backend's default extra-config "
+        '(e.g. \'{"capacity_gb": "120"}\')',
+    )
+    parser.add_argument(
+        "--extra-server-args",
+        type=str,
+        default="",
+        help="space-separated extra args appended to the server command",
+    )
+    # multiturn shape
+    parser.add_argument("--num-clients", type=int, default=12)
+    parser.add_argument("--num-rounds", type=int, default=4)
+    parser.add_argument("--request-length", type=int, default=1024)
+    parser.add_argument("--output-length", type=int, default=64)
+    parser.add_argument("--request-rate", type=int, default=4)
+    # longctx shape
+    parser.add_argument("--prefix-tokens", type=int, default=6144)
+    parser.add_argument("--num-prefixes", type=int, default=4)
+    parser.add_argument("--continuations", type=int, default=4)
+    parser.add_argument("--out", type=str, default="bench_buffer_mode_results.json")
+    return parser.parse_args()
+
+
+def scrape_metrics(base_url):
+    try:
+        text = requests.get(f"{base_url}/metrics", timeout=30).text
+    except Exception:
+        return {}
+    out = {}
+    for name in BUFFER_METRICS:
+        total, found = 0.0, False
+        for line in text.splitlines():
+            if line.startswith(name + "{") or line.startswith(name + " "):
+                total += float(line.rsplit(" ", 1)[1])
+                found = True
+        if found:
+            out[name.split(":")[1]] = total
+    return out
+
+
+class Server:
+    def __init__(self, args, mode, storage_dir):
+        self.args = args
+        self.mode = mode
+        self.storage_dir = storage_dir
+        self.base_url = f"http://127.0.0.1:{args.port}"
+        self.proc = None
+
+    def launch(self):
+        a = self.args
+        extra_config = {"prefetch_threshold": 64}
+        if a.storage_backend == "meta_cache_store":
+            # Local-mode store: in-process server on rank 0, data under the
+            # run's storage dir (same isolation as the file backend).
+            extra_config.update(
+                {
+                    "cluster": "mks",
+                    "local_mode": "true",
+                    "data_root": os.path.join(self.storage_dir, "mcs"),
+                    "capacity_gb": "120",
+                }
+            )
+        if a.storage_extra_config:
+            extra_config.update(json.loads(a.storage_extra_config))
+        cmd = [
+            sys.executable,
+            "-m",
+            a.launch_module,
+            "--model-path",
+            a.model,
+            "--port",
+            str(a.port),
+            "--tp-size",
+            str(a.tp),
+            "--mem-fraction-static",
+            str(a.mem_fraction_static),
+            "--page-size",
+            str(a.page_size),
+            "--enable-hierarchical-cache",
+            "--enable-cache-report",
+            "--enable-metrics",
+            "--hicache-write-policy",
+            "write_through",
+            "--hicache-storage-prefetch-policy",
+            a.prefetch_policy,
+            "--hicache-storage-backend-extra-config",
+            json.dumps(extra_config),
+        ]
+        if self.mode == "buffer":
+            cmd += [
+                "--hicache-host-memory-mode",
+                "buffer_only",
+                "--hicache-storage-backend",
+                a.storage_backend,
+            ]
+            if a.host_ratio > 0:
+                cmd += ["--hicache-ratio", str(a.host_ratio)]
+            else:
+                cmd += ["--hicache-size", str(a.buffer_size_gb)]
+        else:
+            write_policy = "write_back" if self.mode == "cache_wb" else "write_through"
+            cmd += [
+                "--hicache-ratio",
+                str(a.host_ratio if a.host_ratio > 0 else a.cache_ratio),
+                "--hicache-storage-backend",
+                a.storage_backend,
+            ]
+            # Override the default write policy appended below.
+            cmd = [c if c != "write_through" else write_policy for c in cmd]
+        if a.max_total_tokens > 0:
+            cmd += ["--max-total-tokens", str(a.max_total_tokens)]
+        if a.attention_backend:
+            cmd += ["--attention-backend", a.attention_backend]
+        if a.extra_server_args:
+            cmd += a.extra_server_args.split()
+        env = {
+            **os.environ,
+            "SGLANG_HICACHE_FILE_BACKEND_STORAGE_DIR": self.storage_dir,
+        }
+        log_stem = os.path.splitext(os.path.basename(self.args.out))[0]
+        self.log = open(f"/tmp/{log_stem}_{self.mode}_server.log", "w")
+        self.proc = subprocess.Popen(cmd, stdout=self.log, stderr=self.log, env=env)
+        deadline = time.time() + 2400
+        while time.time() < deadline:
+            if self.proc.poll() is not None:
+                raise RuntimeError(f"server died during launch; see {self.log.name}")
+            try:
+                if (
+                    requests.get(f"{self.base_url}/health", timeout=5).status_code
+                    == 200
+                ):
+                    return
+            except Exception:
+                pass
+            time.sleep(2)
+        raise RuntimeError("server did not become healthy in time")
+
+    def flush(self):
+        requests.post(
+            f"{self.base_url}/flush_cache", params={"timeout": 60}, timeout=90
+        ).raise_for_status()
+
+    def kill(self):
+        if self.proc is not None:
+            from sglang.srt.utils import kill_process_tree
+
+            kill_process_tree(self.proc.pid)
+            self.proc = None
+
+
+def run_multiturn(server, args):
+    log_file = tempfile.mktemp(suffix=".jsonl")
+    cmd = [
+        sys.executable,
+        os.path.join(BENCH_DIR, "bench_multiturn.py"),
+        "--model-path",
+        args.model,
+        "--port",
+        str(args.port),
+        "--disable-auto-run",
+        "--disable-random-sample",
+        "--enable-round-barrier",
+        "--num-clients",
+        str(args.num_clients),
+        "--max-parallel",
+        str(args.num_clients),
+        "--num-rounds",
+        str(args.num_rounds),
+        "--request-length",
+        str(args.request_length),
+        "--output-length",
+        str(args.output_length),
+        "--request-rate",
+        str(args.request_rate),
+        "--seed",
+        "1",
+        "--log-file",
+        log_file,
+    ]
+    if args.sub_question_input_length > 0:
+        cmd += [
+            "--sub-question-input-length",
+            str(args.sub_question_input_length),
+        ]
+    subprocess.run(cmd, check=True, timeout=7200)
+    with open(log_file) as f:
+        records = [json.loads(line) for line in f if line.strip()]
+    record = records[-1]
+    result = {
+        "hit_rate": record["summary"]["cache_hit_rate"],
+        "avg_ttft_s": record["summary"]["average_ttft"],
+        "p90_ttft_s": record["summary"].get("p90_ttft", 0),
+    }
+    for round_key, round_data in record.get("round", {}).items():
+        result[f"{round_key}_hit"] = round(round_data["cache_hit_rate"], 4)
+        result[f"{round_key}_ttft_s"] = round(round_data["average_ttft"], 3)
+    return result
+
+
+def _gen_token_prompt(tokenizer, n_tokens, seed):
+    import random as _random
+
+    rng = _random.Random(seed)
+    vocab_size = min(tokenizer.vocab_size - 1000, 32000)
+    ids = [rng.randrange(1000, vocab_size) for _ in range(n_tokens * 2)]
+    text = tokenizer.decode(ids)
+    ids = tokenizer.encode(text)[:n_tokens]
+    return tokenizer.decode(ids)
+
+
+def run_longctx(server, args):
+    """Long shared prefix + divergent continuations, warm then post-flush."""
+    from sglang.benchmark.utils import get_tokenizer
+
+    tokenizer = get_tokenizer(args.model)
+    prefixes = [
+        _gen_token_prompt(tokenizer, args.prefix_tokens, seed=900 + g)
+        for g in range(args.num_prefixes)
+    ]
+    tails = [
+        _gen_token_prompt(tokenizer, 128, seed=9900 + c)
+        for c in range(args.continuations)
+    ]
+
+    def one_pass(tag):
+        latencies, cached, prompt_tokens = [], 0, 0
+        for prefix in prefixes:
+            for tail in tails:
+                start = time.perf_counter()
+                res = requests.post(
+                    f"{server.base_url}/generate",
+                    json={
+                        "text": prefix + tail,
+                        "sampling_params": {
+                            "temperature": 0.0,
+                            "max_new_tokens": 8,
+                            "ignore_eos": True,
+                        },
+                    },
+                    timeout=600,
+                ).json()
+                latencies.append(time.perf_counter() - start)
+                meta = res.get("meta_info", {})
+                cached += int(meta.get("cached_tokens", 0))
+                prompt_tokens += int(meta.get("prompt_tokens", 1))
+        return {
+            f"{tag}_hit_rate": round(cached / max(prompt_tokens, 1), 4),
+            f"{tag}_avg_latency_s": round(sum(latencies) / len(latencies), 3),
+            f"{tag}_max_latency_s": round(max(latencies), 3),
+        }
+
+    result = one_pass("warm")
+    # Give write-backs a moment to settle before dropping device state.
+    time.sleep(5)
+    server.flush()
+    result.update(one_pass("replay"))
+    return result
+
+
+def main():
+    args = parse_args()
+    results = {}
+    for mode in args.modes.split(","):
+        mode = mode.strip()
+        storage_dir = tempfile.mkdtemp(prefix=f"bench_buffer_{mode}_")
+        server = Server(args, mode, storage_dir)
+        print(f"\n=== launching {mode} server ===", flush=True)
+        try:
+            server.launch()
+            mode_result = {}
+            for workload in args.workloads.split(","):
+                workload = workload.strip()
+                print(f"--- {mode}: running {workload} ---", flush=True)
+                if workload == "multiturn":
+                    mode_result["multiturn"] = run_multiturn(server, args)
+                elif workload == "longctx":
+                    mode_result["longctx"] = run_longctx(server, args)
+                server.flush()
+            mode_result["hicache_metrics"] = scrape_metrics(server.base_url)
+            results[mode] = mode_result
+        finally:
+            server.kill()
+            shutil.rmtree(storage_dir, ignore_errors=True)
+
+    print("\n===== results =====")
+    print(json.dumps(results, indent=2))
+    with open(args.out, "w") as f:
+        json.dump(
+            {"model": args.model, "args": vars(args), "results": results}, f, indent=2
+        )
+    print(f"saved to {args.out}")
+
+
+if __name__ == "__main__":
+    main()

python/sglang/srt/managers/cache_controller.py

@@ -17,7 +17,7 @@ import logging
 import threading
 import time
 from queue import Empty, Queue
-from typing import TYPE_CHECKING, List, NamedTuple, Optional
+from typing import TYPE_CHECKING, Callable, List, NamedTuple, Optional
 
 import torch
 
@@ -203,6 +203,13 @@ class StorageOperation:
         self.completed_tokens = 0
         self.hash_value = hash_value if hash_value is not None else []
         self.prefix_keys = prefix_keys
+        # Full queried page-hash chain, set by _storage_hit_query before
+        # hash_value is truncated to the hit boundary; the tail is the
+        # absence signal that invalidates buffer-mode existence beliefs.
+        self.all_hash_values: Optional[List[str]] = None
+        # Prefetch-outcome accounting, set at enqueue by the tree cache.
+        self.stats_requested_tokens = 0
+        self.stats_total_tokens = 0
 
         self.id = StorageOperation.counter
         StorageOperation.counter += 1
@@ -211,6 +218,15 @@ class StorageOperation:
         return self.id < other.id
 
 
+# Buffer-mode staging budgets. Prefetch staging is latency-critical
+# (wait_complete gates TTFT), so loads may fill the pool up to this fraction
+# before new prefetches are declined.
+HICACHE_LOAD_POOL_USAGE_FRACTION = 0.9
+# Write-staging floor: writes are deferrable, so the flush gate grows the
+# write window dynamically into whatever load staging is not using.
+HICACHE_WRITE_STAGING_POOL_FRACTION = 0.2
+
+
 class PrefetchOperation(StorageOperation):
     def __init__(
         self,
@@ -262,8 +278,10 @@ class HiCacheController:
         model_name: Optional[str] = None,
         storage_backend_extra_config: Optional[dict] = None,
         enable_storage_metrics: bool = False,
+        host_memory_mode: str = "cache",
     ):
         self.tp_group = tp_group
+        self.host_memory_mode = host_memory_mode
         self.attn_cp_group = attn_cp_group
         self.attn_tp_group = attn_tp_group
         self.pp_group = pp_group
@@ -283,6 +301,9 @@ class HiCacheController:
         self.storage_backend = None
         self.storage_backend_type = None
         self.enable_storage_metrics = enable_storage_metrics
+        # Buffer mode: wired by the tree cache after attach; the load rate
+        # limiter subtracts write staging from actual pool usage.
+        self.host_write_staged_tokens_fn: Optional[Callable[[], int]] = None
 
         # Draft KV pool support (best-effort piggyback on target L2/L3 ops).
         self.has_draft = False
@@ -501,8 +522,16 @@ class HiCacheController:
             self.enable_storage = True
             # todo: threshold policy for prefetching
             self.prefetch_threshold = max(prefetch_threshold, self.page_size)
-            # Budget speculative prefetch at half the host pool, leaving the rest for the write-back staging path.
+            if self.host_memory_mode == "buffer_only":
-            self.prefetch_capacity_limit = int(0.5 * self.mem_pool_host.size)
+                # The whole pool is transient staging; loads may fill it up
+                # to this fraction, and the tree's write flush gate yields
+                # to live fetch demand (the write fraction is a floor).
+                self.prefetch_capacity_limit = int(
+                    HICACHE_LOAD_POOL_USAGE_FRACTION * self.mem_pool_host.size
+                )
+            else:
+                # Budget speculative prefetch at half the host pool, leaving the rest for the write-back staging path.
+                self.prefetch_capacity_limit = int(0.5 * self.mem_pool_host.size)
             # tracking the number of tokens locked in prefetching, updated by the main scheduler thread
             self.prefetch_tokens_occupied = 0
 
@@ -1050,6 +1079,16 @@ class HiCacheController:
         """
         Rate limit the prefetching operations to avoid overwhelming the storage backend.
         """
+        if self.host_memory_mode == "buffer_only":
+            # Gate on real pool usage: buffer mode allocates hit-sized, so
+            # prefetch_tokens_occupied's requested spans overstate it. Pool
+            # state mutates only at scheduler-thread lockstep points, so this
+            # stays TP-deterministic. Write staging is the write budget's
+            # usage; charging it here would park hits behind its storage drain.
+            used = self.mem_pool_host.size - self.mem_pool_host.available_size()
+            if self.host_write_staged_tokens_fn is not None:
+                used -= self.host_write_staged_tokens_fn()
+            return max(0, used) >= self.prefetch_capacity_limit
         # cancel prefetch if too much memory is occupied
         if self.prefetch_tokens_occupied >= self.prefetch_capacity_limit:
             return True
@@ -1066,6 +1105,7 @@ class HiCacheController:
         page_hashes = self.get_hash_str(
             tokens_to_fetch, last_hash, page_size=self.page_size
         )
+        operation.all_hash_values = page_hashes
 
         for start in range(0, len(page_hashes), STORAGE_BATCH_SIZE):
             batch_hashes = page_hashes[start : start + STORAGE_BATCH_SIZE]

python/sglang/srt/managers/scheduler.py

@@ -2698,8 +2698,28 @@ class Scheduler(
         if self.enable_hicache_storage:
             req.init_next_round_input(self.tree_cache, cow_mamba=False)
             tree_cache = self.tree_cache
-            if tree_cache.is_backuped(req.last_host_node) or tree_cache.is_root(
+            buffer_mode = self.server_args.hicache_host_memory_mode == "buffer_only"
-                req.last_host_node
+            last_host_node = req.last_host_node
+            # Buffer mode host-backups nothing, so match_prefix anchors at
+            # root; re-anchor on the deepest device node. The anchor is only
+            # read for hash/extra-key context here (never locked), so a device
+            # node serves. Cache mode keeps the is_backuped gate below: its
+            # write-through prefix is contiguous from root, so an unbacked
+            # anchor means a guaranteed storage miss.
+            if (
+                buffer_mode
+                and tree_cache.is_root(last_host_node)
+                and not tree_cache.is_root(req.last_node)
+            ):
+                last_host_node = req.last_node
+
+            if (
+                tree_cache.is_backuped(last_host_node)
+                or tree_cache.is_root(last_host_node)
+                or (
+                    buffer_mode
+                    and tree_cache.get_last_hash_value(last_host_node) is not None
+                )
             ):
                 matched_len = len(req.prefix_indices) + req.host_hit_length
                 match_end = req._compute_max_prefix_len(
@@ -2707,16 +2727,17 @@ class Scheduler(
                 )
                 new_input_tokens = req.full_untruncated_fill_ids[matched_len:match_end]
                 prefix_keys = (
-                    tree_cache.get_prefix_hash_values(req.last_host_node)
+                    tree_cache.get_prefix_hash_values(last_host_node)
                     if tree_cache.hicache_storage_pass_prefix_keys
                     else None
                 )
                 tree_cache.prefetch_from_storage(
                     req.rid,
-                    req.last_host_node,
+                    last_host_node,
                     new_input_tokens,
-                    tree_cache.get_last_hash_value(req.last_host_node),
+                    tree_cache.get_last_hash_value(last_host_node),
                     prefix_keys,
+                    matched_prefix_tokens=req.full_untruncated_fill_ids[:matched_len],
                 )
 
     def _add_request_to_queue(self, req: Req, is_retracted: bool = False):
@@ -3329,6 +3350,23 @@ class Scheduler(
                     req.storage_hit_length = loaded_tokens
 
             req.init_next_round_input(self.tree_cache)
+            if (
+                self.enable_hicache_storage
+                and self.server_args.hicache_host_memory_mode == "buffer_only"
+            ):
+                # Buffer mode: surface a staged prefetch as the request's host
+                # hit (consumed through init_load_back) plus its SWA window,
+                # which consumption allocates and the request lock pins —
+                # uncharged, the batch alloc can OOM. Set AFTER
+                # init_next_round_input (which recomputes host_hit). Mamba
+                # (fenced in init_hicache) will need the same charge via
+                # mamba_host_hit_length.
+                held_tokens = self.tree_cache.staged_prefetch_tokens(req.rid)
+                if held_tokens > 0:
+                    req.host_hit_length = held_tokens
+                    req.swa_host_hit_length = (
+                        self.tree_cache.staged_prefetch_swa_tokens(req.rid)
+                    )
             res = adder.add_one_req(
                 req,
                 has_chunked_req=(self.chunked_req is not None),
@@ -4146,6 +4184,11 @@ class Scheduler(
                 if tc.enable_storage:
                     idle &= len(tc.ongoing_prefetch) == 0
                     idle &= len(tc.ongoing_backup) == 0
+                    if self.server_args.hicache_host_memory_mode == "buffer_only":
+                        # Queued writes, staged prefetches, and in-flight
+                        # storage writes still hold host staging
+                        # (buffer-mode unified tree only).
+                        idle &= tc.buffer_pipeline.is_idle()
 
         return idle
 

python/sglang/srt/mem_cache/buffer_mode/__init__.py

@@ -0,0 +1,9 @@
+"""Buffer-only HiCache host memory mode (--hicache-host-memory-mode buffer_only).
+
+Host RAM is a transient staging buffer between the GPU and the L3 storage
+backend, never an L2 cache tier: writes stage device KV through op-owned
+host bounces into storage and free them at the storage ack; reads fetch
+storage hits into op-owned bounces and publish them into the device tree at
+prefill admission. ``UnifiedRadixCache`` composes ``BufferModePipeline`` for
+the two transfer pipelines and dispatches to it at the mode branches.
+"""

python/sglang/srt/mem_cache/buffer_mode/buffer_page_cache.py

@@ -0,0 +1,396 @@
+"""Refcounted content-addressed page cache over the buffer-mode host pool.
+
+NOT WIRED YET: staged spans register as ``(pool, page_hash) -> (slots,
+refcount)`` so prefetches can be served zero-copy from local staging
+(write-around / promote-on-read retention, zero-ref LRU reclaim); keys are
+the content-chained page hashes, so entries survive node deletion, splits,
+and recompute. Counterpart of ``StorageExistenceCache`` (beliefs about
+STORAGE, dedupes writes); this tracks LOCAL HOST RAM and dedupes loads.
+
+TP determinism: replicas must stay identical across attention ranks — the
+cache feeds scheduler-visible structure, so divergence is a collective
+hang, not a soft miss. Preconditions when wiring: (1) mutate only on the
+scheduler thread at lockstep points; (2) rank-reduce any fold anchored by a
+per-rank storage outcome (hit count, revoke) before it picks a mutation;
+(3) controller queues stay FIFO and single-threaded so MIN-count drains
+process the same prefix on every rank.
+"""
+
+from __future__ import annotations
+
+from collections import OrderedDict
+from typing import Callable, Optional, Sequence
+
+import torch
+
+from sglang.srt.mem_cache.hicache_storage import (
+    PoolHitPolicy,
+    PoolName,
+    PoolTransfer,
+)
+
+
+class _PageRef:
+    """One cached page: host slot span, reader refcount, and whether to
+    retain at refs==0 (write-around: write staging frees at its storage ack
+    unless a read promoted it). ``first_slot`` mirrors ``slots[0]`` as a
+    plain int because per-page tensor-scalar reads are too slow in
+    ``release``."""
+
+    __slots__ = ("slots", "first_slot", "refs", "retain")
+
+    def __init__(
+        self, slots: torch.Tensor, first_slot: int, refs: int = 1, retain: bool = True
+    ):
+        self.slots = slots
+        self.first_slot = first_slot
+        self.refs = refs
+        self.retain = retain
+
+
+class BufferPageCache:
+    def __init__(self) -> None:
+        # (pool, page_hash) -> _PageRef; slots stay allocated in the host
+        # pool for as long as the entry exists.
+        self._entries: dict[tuple[str, str], _PageRef] = {}
+        # Per-pool zero-ref LRU (head = coldest): reclaim victims.
+        self._zero_ref: dict[str, OrderedDict[str, None]] = {}
+        # Per-pool slot tokens held by the cache (refed + zero-ref).
+        self._held_tokens: dict[str, int] = {}
+        # Per-pool slot tokens at refs=0 (reclaimable under pressure).
+        self._zero_ref_tokens: dict[str, int] = {}
+
+    def __len__(self) -> int:
+        return len(self._entries)
+
+    def num_zero_ref_pages(self) -> int:
+        return sum(len(lru) for lru in self._zero_ref.values())
+
+    def held_tokens(self, pool: str) -> int:
+        return self._held_tokens.get(pool, 0)
+
+    def zero_ref_tokens(self, pool: str) -> int:
+        """Slot tokens reclaimable right now (zero-ref cached pages).
+        Occupancy/rate-limit gates must treat these as free-able, not used:
+        a pool full of zero-ref cache is one reclaim away from empty."""
+        return self._zero_ref_tokens.get(pool, 0)
+
+    def register(
+        self,
+        pool: str,
+        hashes: Sequence[str],
+        host_indices: torch.Tensor,
+        page_size: int,
+        retain: bool = True,
+    ) -> int:
+        """Cache a staged span, one entry per page, refs=1 (the staging op);
+        returns the number of pages newly cached. ``retain=False`` =
+        write-around (freed at last ref unless a read hit promotes it); a
+        duplicate hash keeps the existing entry and the newcomer's slots
+        stay op-owned for a raw free at release."""
+        assert len(host_indices) == len(hashes) * page_size
+        registered = 0
+        entries = self._entries
+        # One batched read of the page-boundary slot ids (see _PageRef).
+        first_slots = host_indices[::page_size].tolist()
+        for i, page_hash in enumerate(hashes):
+            key = (pool, page_hash)
+            existing = entries.get(key)
+            if existing is not None:
+                existing.retain = existing.retain or retain
+                continue
+            entries[key] = _PageRef(
+                host_indices[i * page_size : (i + 1) * page_size],
+                first_slots[i],
+                retain=retain,
+            )
+            registered += 1
+        if registered:
+            self._held_tokens[pool] = (
+                self._held_tokens.get(pool, 0) + registered * page_size
+            )
+        return registered
+
+    def contains(self, pool: str, page_hash: str) -> bool:
+        """Non-mutating presence probe (no LRU touch)."""
+        return (pool, page_hash) in self._entries
+
+    def peek_run_len(self, pool: str, hashes: Sequence[str]) -> int:
+        """Length of the leading run of cached pages. Non-mutating."""
+        entries = self._entries
+        run = 0
+        for page_hash in hashes:
+            if (pool, page_hash) not in entries:
+                break
+            run += 1
+        return run
+
+    def acquire(self, pool: str, hashes: Sequence[str]) -> Optional[torch.Tensor]:
+        """refs++ on every page and return the gathered slot tensor (pages
+        expanded to token slots, in page order). All-or-nothing: returns
+        None without mutating if any page is missing."""
+        entries = self._entries
+        refs = []
+        for page_hash in hashes:
+            entry = entries.get((pool, page_hash))
+            if entry is None:
+                return None
+            refs.append(entry)
+        zero_ref = self._zero_ref.get(pool)
+        for page_hash, entry in zip(hashes, refs):
+            if entry.refs == 0 and zero_ref is not None:
+                if zero_ref.pop(page_hash, None) is not None:
+                    self._zero_ref_tokens[pool] -= len(entry.slots)
+            entry.refs += 1
+            # Read demand proven: promote write-around pages to retained.
+            entry.retain = True
+        return torch.cat([entry.slots for entry in refs])
+
+    def release(
+        self,
+        pool: str,
+        hashes: Sequence[str],
+        host_indices: torch.Tensor,
+        page_size: int,
+    ) -> Optional[torch.Tensor]:
+        """Drop one ref per page of a span: at refs==0 retained pages move
+        to the zero-ref LRU tail while write-around pages return their
+        slots for an immediate free. Duplicate-staging slots (canonical
+        entry lives elsewhere) are returned for a raw free without touching
+        the canonical refcount."""
+        assert len(host_indices) == len(hashes) * page_size
+        leftover: list[torch.Tensor] = []
+        entries = self._entries
+        # One batched read of the page-boundary slot ids (see _PageRef).
+        first_slots = host_indices[::page_size].tolist()
+        for i, page_hash in enumerate(hashes):
+            entry = entries.get((pool, page_hash))
+            if entry is None or entry.first_slot != first_slots[i]:
+                leftover.append(host_indices[i * page_size : (i + 1) * page_size])
+                continue
+            assert entry.refs > 0, "release without a matching acquire/register"
+            entry.refs -= 1
+            if entry.refs == 0:
+                if entry.retain:
+                    self._zero_ref.setdefault(pool, OrderedDict())[page_hash] = None
+                    self._zero_ref_tokens[pool] = self._zero_ref_tokens.get(
+                        pool, 0
+                    ) + len(entry.slots)
+                else:
+                    del entries[(pool, page_hash)]
+                    self._held_tokens[pool] -= len(entry.slots)
+                    leftover.append(entry.slots)
+        if not leftover:
+            return None
+        return torch.cat(leftover)
+
+    def reclaim(
+        self,
+        pool: str,
+        need_tokens: int,
+        free: Callable[[torch.Tensor], int],
+    ) -> int:
+        """Pop zero-ref LRU heads, free their slots back to the host pool,
+        and drop the entries. Called under allocation pressure only. Returns
+        the number of slot tokens freed (may undershoot when everything
+        left is refed)."""
+        zero_ref = self._zero_ref.get(pool)
+        if not zero_ref or need_tokens <= 0:
+            return 0
+        freed = 0
+        batch: list[torch.Tensor] = []
+        while zero_ref and freed < need_tokens:
+            page_hash, _ = zero_ref.popitem(last=False)
+            entry = self._entries.pop((pool, page_hash))
+            batch.append(entry.slots)
+            freed += len(entry.slots)
+        if batch:
+            free(torch.cat(batch))
+            self._held_tokens[pool] -= freed
+            self._zero_ref_tokens[pool] -= freed
+        return freed
+
+
+class BufferPageCacheOps:
+    """Pool-facing operations over a :class:`BufferPageCache`: span/hold
+    registration and release keyed the way the storage write keys them,
+    pressure reclaim, and the SWA-folded continuation fold. The caller owns
+    the collectives — rank-reduce any fold anchored by a per-rank storage
+    outcome before acting on it (see the module docstring)."""
+
+    def __init__(
+        self,
+        page_cache: BufferPageCache,
+        mem_pool_host,
+        sw_window_pages_fn: Callable[[], int],
+    ):
+        # Rebound by the owner when the structure is recreated (reset).
+        self.page_cache = page_cache
+        self._mem_pool_host = mem_pool_host
+        # SWA window in KV pages when SWA stages through a host pool
+        # (0 = KV-only: no trailing window in the fold).
+        self._sw_window_pages_fn = sw_window_pages_fn
+
+    def aux_window_keys(
+        self, hash_values: list[str], transfer: PoolTransfer
+    ) -> Optional[list[str]]:
+        """Trailing KV page hashes keying an aux transfer's staged window
+        (one key per aux-pool page), recomputed from the rank-synced span
+        hashes so registration and release always agree across ranks."""
+        if transfer.host_indices is None or transfer.host_indices.numel() == 0:
+            return None
+        if transfer.indices_from_pool is not None:
+            return None  # sidecar rides another pool's slots; nothing to key
+        entry = self._mem_pool_host.entry_map.get(transfer.name)
+        if entry is None:
+            return None
+        pool_page_size = entry.host_pool.page_size
+        num_keys = len(transfer.host_indices) // pool_page_size
+        if num_keys == 0 or num_keys > len(hash_values):
+            return None
+        return hash_values[-num_keys:]
+
+    def register_span(
+        self,
+        pool: PoolName,
+        hashes: list[str],
+        host_indices: torch.Tensor,
+        retain: bool = True,
+    ) -> None:
+        """Cache a page-aligned staged span (refs=1 for the staging op)."""
+        if not hashes:
+            return
+        entry = self._mem_pool_host.entry_map.get(pool)
+        if entry is None:
+            return
+        self.page_cache.register(
+            pool, hashes, host_indices, entry.host_pool.page_size, retain=retain
+        )
+
+    def release_span(
+        self,
+        pool: PoolName,
+        hashes: list[str],
+        host_indices: torch.Tensor,
+    ) -> None:
+        """Drop the staging op's ref on a span; zero-ref pages stay cached
+        (servable) until pressure reclaims them. Op-owned duplicate slots
+        (their hash was cached elsewhere) are freed raw, as before."""
+        if host_indices is None or host_indices.numel() == 0:
+            return
+        entry = self._mem_pool_host.entry_map.get(pool)
+        if entry is None:
+            return
+        if not hashes:
+            entry.host_pool.free(host_indices)
+            return
+        leftover = self.page_cache.release(
+            pool, hashes, host_indices, entry.host_pool.page_size
+        )
+        if leftover is not None and leftover.numel() > 0:
+            entry.host_pool.free(leftover)
+
+    def register_hold(
+        self,
+        hash_values: list[str],
+        host_indices: torch.Tensor,
+        aux_xfers: list[PoolTransfer],
+        retain: bool = True,
+    ) -> None:
+        """Register a staged KV span plus its aux windows (SWA/Mamba states
+        keyed by their trailing KV page hashes, same keying the storage
+        write uses). ``retain=False`` = write-around: servable only while
+        the staging op pins the slots, freed at the last release unless a
+        read hit promotes it."""
+        self.register_span(PoolName.KV, hash_values, host_indices, retain=retain)
+        for transfer in aux_xfers:
+            keys = self.aux_window_keys(hash_values, transfer)
+            if keys is not None:
+                self.register_span(
+                    transfer.name, keys, transfer.host_indices, retain=retain
+                )
+
+    def release_hold(
+        self,
+        hash_values: list[str],
+        host_indices: torch.Tensor,
+        aux_xfers: list[PoolTransfer],
+    ) -> None:
+        """Mirror of register_hold for every hold retirement path
+        (storage-ack, fill H2D-ack, staged drop, abort)."""
+        self.release_span(PoolName.KV, hash_values, host_indices)
+        for transfer in aux_xfers:
+            if transfer.indices_from_pool is not None:
+                continue
+            keys = self.aux_window_keys(hash_values, transfer)
+            self.release_span(transfer.name, keys or [], transfer.host_indices)
+
+    def reclaim(self, pool: PoolName, num_tokens: int) -> int:
+        """Free just enough zero-ref cached pages for an allocation of
+        num_tokens to succeed. Scheduler-thread only (lockstep pressure
+        points: staging-hit alloc, prepare_prefetch, cc.write)."""
+        entry = self._mem_pool_host.entry_map.get(pool)
+        if entry is None:
+            return 0
+        shortfall = num_tokens - entry.host_pool.available_size()
+        if shortfall <= 0:
+            return 0
+        return self.page_cache.reclaim(pool, shortfall, entry.host_pool.free)
+
+    def continuation_run(self, chain: list[str], start_pages: int) -> int:
+        """Longest cached run continuing the span at page ``start_pages``
+        (0 = leading run), folded for SWA: the joint span's trailing window
+        must be fully cache-servable, mirroring batch_exists_v2's
+        trailing_pages fold. Non-mutating and rank-deterministic."""
+        page_cache = self.page_cache
+        kv_run = page_cache.peek_run_len(PoolName.KV, chain[start_pages:])
+        if kv_run == 0:
+            return 0
+        sw_pages = self._sw_window_pages_fn()
+        if sw_pages == 0:
+            return kv_run
+        for cont in range(kv_run, 0, -1):
+            joint = start_pages + cont
+            window = min(sw_pages, joint)
+            if cont < window:
+                # Window straddles into the head; only possible for
+                # anchored runs, and shrinking cont cannot fix it.
+                break
+            if all(
+                page_cache.contains(PoolName.SWA, chain[i])
+                for i in range(joint - window, joint)
+            ):
+                return cont
+        return 0
+
+    def acquire_span(
+        self, chain: list[str], start_pages: int, cont_pages: int
+    ) -> Optional[tuple[torch.Tensor, list[PoolTransfer]]]:
+        """Acquire a folded continuation run: its KV pages plus the JOINT
+        span's trailing SWA window (refs++ on every page). Returns
+        (kv_slots, aux_xfers) or None (with no refs held) if a page
+        vanished since the fold — defensive; fold and acquire run in the
+        same lockstep step."""
+        page_cache = self.page_cache
+        cont_hashes = list(chain[start_pages : start_pages + cont_pages])
+        kv_slots = page_cache.acquire(PoolName.KV, cont_hashes)
+        if kv_slots is None:
+            return None
+        aux_xfers: list[PoolTransfer] = []
+        sw_pages = self._sw_window_pages_fn()
+        if sw_pages > 0:
+            joint = start_pages + cont_pages
+            window_hashes = list(chain[joint - min(sw_pages, joint) : joint])
+            swa_slots = page_cache.acquire(PoolName.SWA, window_hashes)
+            if swa_slots is None:
+                self.release_span(PoolName.KV, cont_hashes, kv_slots)
+                return None
+            aux_xfers.append(
+                PoolTransfer(
+                    name=PoolName.SWA,
+                    host_indices=swa_slots,
+                    keys=window_hashes,
+                    hit_policy=PoolHitPolicy.TRAILING_PAGES,
+                )
+            )
+        return kv_slots, aux_xfers

python/sglang/srt/mem_cache/buffer_mode/pipeline.py

@@ -0,0 +1,816 @@
+"""Buffer-only mode transfer pipelines for the unified radix cache.
+
+``BufferModePipeline`` owns all buffer-mode state and the two pipelines that
+move KV through the transient host staging buffer:
+
+- backup (write path): admission-gated FIFO intents, head-of-line D2H
+  staging launches, storage writes at the D2H ack, staging freed at the
+  storage ack;
+- load back (read path): completed storage fetches parked as op-owned host
+  bounces, consumed at prefill admission via a device alloc + layer-gated
+  H2D + plain tree insert, bounce freed at the H2D ack.
+
+The pipeline is an intimate collaborator of ``UnifiedRadixCache``: it is
+constructed by ``init_hicache`` only when ``--hicache-host-memory-mode
+buffer_only`` is active, and it drives tree/controller operations (insert,
+match, evict, lock refs, cache actions) through the owning cache. All
+buffer-mode-only state lives here; the cache dispatches to this object at
+its mode branches.
+
+TP-lockstep contract: every mutation runs on the scheduler thread at
+rank-synchronized points (insert walks, rank-MIN-reduced drains, ack
+drains), so per-rank state never diverges. There is no runtime
+verification; a violation surfaces as an unexplained collective hang.
+"""
+
+from __future__ import annotations
+
+import logging
+from array import array
+from collections import deque
+from typing import TYPE_CHECKING, Optional
+
+import msgspec
+import torch
+
+from sglang.srt.managers.cache_controller import HICACHE_WRITE_STAGING_POOL_FRACTION
+from sglang.srt.mem_cache.base_prefix_cache import (
+    DecLockRefParams,
+    EvictParams,
+    InitLoadBackParams,
+    InsertParams,
+    MatchPrefixParams,
+)
+from sglang.srt.mem_cache.hicache_storage import PoolHitPolicy, PoolName, PoolTransfer
+from sglang.srt.mem_cache.radix_cache import RadixKey
+from sglang.srt.mem_cache.unified_cache.cache_action import RebuildFullToSWAMapping
+from sglang.srt.mem_cache.unified_cache.components import (
+    BASE_COMPONENT_TYPE,
+    ComponentType,
+)
+from sglang.srt.mem_cache.unified_cache.unified_tree_core import (
+    NodeId,
+    UnifiedTreeNode,
+)
+
+if TYPE_CHECKING:
+    from sglang.srt.mem_cache.unified_cache.components import SWAComponent
+    from sglang.srt.mem_cache.unified_radix_cache import UnifiedRadixCache
+
+logger = logging.getLogger(__name__)
+
+
+class _UnifiedBackupIntent(msgspec.Struct):
+    """Buffer-mode backup intent, unpinned while queued.
+
+    Snapshots node identity at enqueue time: a split rewrites the node's
+    key/hash in place while these copies stay intact, so
+    ``node.hash_value != hash_values`` doubles as split detection and a None
+    FULL device value as eviction detection (``_backup_intent_stale``).
+    """
+
+    node: UnifiedTreeNode
+    node_id: int
+    hash_values: list[str]
+    key: RadixKey
+    prefix_keys: Optional[list[str]] = None
+
+
+class _UnifiedBufferBackupEntry(msgspec.Struct):
+    """A buffer-mode backup after its D2H launch: intent + staging slots.
+
+    ``host_indices`` are FULL-pool staging slots; ``aux_xfers`` carry the
+    staged aux-pool slots (e.g. the SWA window). All are freed at the
+    storage-write ack — host memory is never retained as a cache tier.
+    """
+
+    intent: _UnifiedBackupIntent
+    host_indices: torch.Tensor
+    aux_xfers: list[PoolTransfer]
+    lock_params: DecLockRefParams
+
+
+class _StagedPrefetch(msgspec.Struct):
+    """A completed buffer-mode fetch parked until prefill admission: only
+    the op-owned host bounce exists (no device state, nothing in the tree).
+    """
+
+    req_id: str
+    key_tokens: list[int]
+    extra_key: Optional[str]
+    matched_len: int
+    num_tokens: int
+    occupied_tokens: int
+    host_indices: torch.Tensor
+    aux_xfers: list[PoolTransfer]
+    hash_values: list[str]
+    operation_id: int
+
+
+class _OngoingBufferLoadBack(msgspec.Struct):
+    """A buffer-mode load-back awaiting its H2D ack: the span is already
+    tree-resident; only the host bounce remains to free.
+    """
+
+    req_id: str
+    num_tokens: int
+    occupied_tokens: int
+    aux_xfers: list[PoolTransfer]
+    host_indices: torch.Tensor
+    hash_values: list[str]
+
+
+def _track_content_refs(refs: dict[str, int], hash_values: list[str]) -> None:
+    """Add one content ref per page hash (at D2H launch). Refcounted,
+    not a flag: several launched entries can carry the same content
+    (duplicate staging of republished spans)."""
+    for h in hash_values:
+        refs[h] = refs.get(h, 0) + 1
+
+
+def _untrack_content_refs(refs: dict[str, int], hash_values: list[str]) -> None:
+    """Drop one content ref per page hash (at storage-ack)."""
+    for h in hash_values:
+        n = refs.get(h, 0) - 1
+        if n <= 0:
+            refs.pop(h, None)
+        else:
+            refs[h] = n
+
+
+def validate_buffer_only_stack(
+    sidecar_pool_specs: list, swa_component: Optional[SWAComponent]
+) -> None:
+    """Post-assembly buffer-mode fences.
+
+    Sidecar pools (DSv4 compressed regions) and unified_kv SWA (device-only
+    ring, never offloaded) have no per-pool staging path yet.
+    """
+    if sidecar_pool_specs:
+        raise ValueError(
+            "--hicache-host-memory-mode buffer_only does not support "
+            "sidecar storage pools (DeepSeek-V4 compressed regions)."
+        )
+    swa = swa_component
+    if swa is not None and swa._swa_kv_pool_host is None:
+        # Only reachable on SWA models with the unified_kv layout (SWA as
+        # a device-only ring): without a host pool the window can neither
+        # stage for writes nor fetch for load-backs.
+        raise ValueError(
+            "--hicache-host-memory-mode buffer_only on SWA models "
+            "requires an SWA host staging pool; the unified_kv layout "
+            "keeps SWA as a device-only ring."
+        )
+    if swa is not None and swa._swa_kv_pool_host is not None:
+        # Below two windows the pool cannot hold a staging write AND the
+        # loads-priority reserve (_aux_loads_margin floors at one
+        # window), so every window-carrying intent would be dropped as
+        # oversize and SWA storage coverage would silently be zero.
+        window_tokens = swa.full_window_pages * swa._swa_kv_pool_host.page_size
+        if swa._swa_kv_pool_host.size < 2 * window_tokens:
+            raise ValueError(
+                "--hicache-host-memory-mode buffer_only requires an SWA "
+                f"host pool of at least two trailing windows "
+                f"({2 * window_tokens} tokens; got "
+                f"{swa._swa_kv_pool_host.size}): one staging a write "
+                "while one stays reserved for prefetch window allocs."
+            )
+
+
+class BufferModePipeline:
+    """All buffer-mode state plus the backup and load-back pipelines.
+
+    Constructed by ``UnifiedRadixCache.init_hicache`` when host memory mode
+    is ``buffer_only``; ``cache.buffer_pipeline is None`` elsewhere, which
+    the cache's mode branches use as the dispatch test.
+    """
+
+    def __init__(
+        self,
+        cache: UnifiedRadixCache,
+        swa_window_pages: int,
+        write_backlog_cap: int,
+    ):
+        self._cache = cache
+        # SWA window size in KV pages when the SWA component stages through
+        # a host pool (0 = KV-only: no trailing window staged). Static after
+        # pool assembly.
+        self._swa_window_pages = swa_window_pages
+        # Metadata-only pending-write backlog cap; beyond it new intents
+        # are dropped at admission (re-trigger on a later hit).
+        self.write_backlog_cap = write_backlog_cap
+        self.reset()
+
+    def reset(self) -> None:
+        # Load pipeline: hits awaiting a staging grant (park-and-retry),
+        # enqueue-time prefix context, completed prefetches staged until
+        # prefill admission, and load-backs in flight (keyed by synthetic
+        # negative ack id).
+        self.pending_hit_allocs: deque = deque()
+        self._prefetch_prefix_ctx: dict[str, list[int]] = {}
+        self.staged_prefetches: dict[str, _StagedPrefetch] = {}
+        self.ongoing_buffer_load_back: dict[int, _OngoingBufferLoadBack] = {}
+        # Backup pipeline: FIFO intents awaiting a D2H slot, node ids
+        # anywhere in flight (dedupes re-triggers), and a content refcount
+        # of every page hash between D2H launch and storage-ack — admission
+        # skips content covered by beliefs + launched writes.
+        self.pending_write_queue: deque[_UnifiedBackupIntent] = deque()
+        self.inflight_backup_node_ids: set[int] = set()
+        self.inflight_backup_hashes: dict[str, int] = {}
+        # Backups between D2H launch and D2H ack (keyed by node id), then
+        # between storage-write launch and storage ack (keyed by operation
+        # id). Mirrors the cache-mode ongoing_write_through/ongoing_backup
+        # stages, with buffer entries.
+        self.ongoing_write_through: dict[int, _UnifiedBufferBackupEntry] = {}
+        self.ongoing_backup: dict[int, _UnifiedBufferBackupEntry] = {}
+        self.write_staged_tokens_ = 0
+        self.write_backlog_tokens_ = 0
+        self._backlog_cap_hits = 0
+
+    def is_idle(self) -> bool:
+        """No queued writes, staged prefetches, or storage writes in flight
+        (all of which hold host staging or would re-trigger IO)."""
+        return not (
+            self.pending_write_queue or self.staged_prefetches or self.ongoing_backup
+        )
+
+    # ---- backup pipeline (device -> staging -> storage) ----
+
+    def _backup_parent_covered(self, node: UnifiedTreeNode) -> bool:
+        """Only admit a node whose parent is stored/in-flight: writing above
+        a dropped parent creates a permanent longest-prefix hole."""
+        parent = node.parent
+        if (
+            parent is self._cache.root_node
+            or parent.id in self.inflight_backup_node_ids
+        ):
+            return True
+        last_hash = parent.get_last_hash_value()
+        return last_hash is not None and self._cache.storage_existence_cache.contains(
+            PoolName.KV, last_hash
+        )
+
+    def _log_backup_dropped(self, num_tokens: int) -> None:
+        cache = self._cache
+        if cache.enable_storage_metrics and cache.storage_metrics_collector is not None:
+            cache.storage_metrics_collector.log_backup_dropped_tokens(num_tokens)
+
+    def enqueue_backup_intent(self, node: UnifiedTreeNode) -> None:
+        """Snapshot a backup intent and commit it to the write queue.
+        Admission gates: belief skip, parent-cover, backlog cap, oversize.
+        Drops are silent; the node re-triggers on a later hit."""
+        if not self._cache.enable_storage or not node.hash_value:
+            return
+        if node.id in self.inflight_backup_node_ids:
+            return
+        # Admission cover: beliefs plus content past its D2H launch. The
+        # launched cover keeps republished content (fill inserts under new
+        # node ids) from re-writing while the original write drains.
+        if self._cache.storage_existence_cache.covers_all(
+            PoolName.KV, node.hash_value, extra_cover=self.inflight_backup_hashes
+        ):
+            return
+        intent_tokens = len(node.hash_value) * self._cache.page_size
+        if self.write_backlog_tokens_ >= self.write_backlog_cap:
+            # The cap sits at 2x the intrinsic live-backlog ceiling (see
+            # init_hicache), so reaching it means leaked accounting or a
+            # broken stale sweep — a bug, not load.
+            self._backlog_cap_hits += 1
+            if self._backlog_cap_hits <= 3 or self._backlog_cap_hits % 1000 == 0:
+                logger.error(
+                    "HiCache write backlog cap hit (occurrence %d): "
+                    "backlog=%d cap=%d queue=%d. Live backlog is bounded "
+                    "by the device pool span, so this indicates a "
+                    "stale-sweep or accounting leak.",
+                    self._backlog_cap_hits,
+                    self.write_backlog_tokens_,
+                    self.write_backlog_cap,
+                    len(self.pending_write_queue),
+                )
+            self._log_backup_dropped(intent_tokens)
+            return
+        # A span larger than any pool's whole staging capacity can never
+        # stage; admitting it would wedge the head-of-line queue forever.
+        if not self._backup_parent_covered(node) or self._backup_oversize(
+            node, intent_tokens
+        ):
+            self._log_backup_dropped(intent_tokens)
+            return
+
+        prefix_keys = (
+            node.get_prefix_hash_values(node.parent)
+            if self._cache.hicache_storage_pass_prefix_keys
+            else None
+        )
+        intent = _UnifiedBackupIntent(
+            node=node,
+            node_id=node.id,
+            hash_values=list(node.hash_value),
+            key=node.key,
+            prefix_keys=prefix_keys,
+        )
+        self.pending_write_queue.append(intent)
+        self.inflight_backup_node_ids.add(node.id)
+        self.write_backlog_tokens_ += intent_tokens
+
+    def _build_aux_staging_transfers(
+        self, node: UnifiedTreeNode
+    ) -> Optional[list[PoolTransfer]]:
+        """Keys-only aux transfers mirroring what BACKUP_STORAGE would write;
+        sizes the per-pool oversize gate (beliefs do not consult these)."""
+        transfers: list[PoolTransfer] = []
+        if ComponentType.SWA in self._cache.components:
+            cd = node.component_data[ComponentType.SWA]
+            if cd.value is not None:
+                num_pages = len(cd.value) // self._cache.page_size
+                if num_pages > 0:
+                    transfers.append(
+                        PoolTransfer(
+                            name=PoolName.SWA,
+                            keys=node.hash_value[-num_pages:],
+                            hit_policy=PoolHitPolicy.TRAILING_PAGES,
+                        )
+                    )
+        return transfers or None
+
+    def _backup_oversize(
+        self,
+        node: UnifiedTreeNode,
+        intent_tokens: int,
+        aux_xfers: Optional[list[PoolTransfer]] = None,
+    ) -> bool:
+        """True if any pool's staging need exceeds that pool's write-usable
+        capacity (total for KV, total minus the loads-priority margin for aux
+        pools — matching ``_aux_budget_blocked``'s admission ceiling): such an
+        intent could never stage and would wedge the FIFO head."""
+        cc = self._cache.cache_controller
+        if intent_tokens > cc.mem_pool_host.size:
+            return True
+        if aux_xfers is None:
+            aux_xfers = self._build_aux_staging_transfers(node)
+        for t in aux_xfers or ():
+            entry = cc.mem_pool_host.entry_map.get(t.name)
+            if entry is not None and (
+                len(t.keys) * entry.host_pool.page_size
+                > entry.host_pool.size - self._aux_loads_margin(entry.host_pool)
+            ):
+                return True
+        return False
+
+    def _aux_loads_margin(self, host_pool) -> int:
+        """Aux-pool tokens reserved for loads: at least one trailing window
+        (prepare_prefetch allocates its window here and a failed alloc
+        forfeits the whole prefetch), plus a 10% burst absorber mirroring
+        live_cap."""
+        return max(
+            self._swa_window_pages * host_pool.page_size,
+            host_pool.size // 10,
+        )
+
+    def _backup_intent_stale(self, intent: _UnifiedBackupIntent) -> bool:
+        # Arena-lookup failure = deleted, hash mismatch vs the enqueue-time
+        # snapshot = split, a None FULL device value = evicted. Stale
+        # intents drop silently; the node re-triggers on a later hit.
+        node = intent.node
+        try:
+            self._cache.tree_core.node_by_id(intent.node_id)
+        except KeyError:
+            return True
+        return (
+            node.component_data[BASE_COMPONENT_TYPE].value is None
+            or node.hash_value != intent.hash_values
+        )
+
+    def _sweep_stale_backup_intents(self) -> None:
+        """Cancel stale intents anywhere in the queue, not just at the head:
+        a dead intent would otherwise inflate the backlog accounting and
+        hold FIFO position ahead of live segments."""
+        if not self.pending_write_queue:
+            return
+        page_size = self._cache.page_size
+        survivors: deque[_UnifiedBackupIntent] = deque()
+        for intent in self.pending_write_queue:
+            if self._backup_intent_stale(intent):
+                self.inflight_backup_node_ids.discard(intent.node_id)
+                self.write_backlog_tokens_ -= len(intent.hash_values) * page_size
+                continue
+            survivors.append(intent)
+        self.pending_write_queue = survivors
+
+    def flush_pending_writes(self) -> None:
+        """Launch D2H transfers for admitted intents, head-of-line: device
+        locks and staging slots are taken only here, when capacity allows."""
+        if not self.pending_write_queue:
+            return
+        cc = self._cache.cache_controller
+        self._sweep_stale_backup_intents()
+        # Loads have priority (writes are deferrable): the write window is
+        # the pool minus prefetch occupancy minus a 10% margin, floored at
+        # the configured fraction.
+        pool_tokens = cc.mem_pool_host.size
+        live_cap = max(
+            int(HICACHE_WRITE_STAGING_POOL_FRACTION * pool_tokens),
+            pool_tokens - cc.prefetch_tokens_occupied - pool_tokens // 10,
+        )
+        while self.pending_write_queue:
+            intent = self.pending_write_queue[0]
+            intent_tokens = len(intent.hash_values) * self._cache.page_size
+            if not self._backup_parent_covered(intent.node) or self._backup_oversize(
+                intent.node, intent_tokens
+            ):
+                # Unwritable intent (dropped parent or unstageable size):
+                # cascade the drop down the chain rather than creating a
+                # permanent storage hole / stalling the head-of-line queue.
+                self.pending_write_queue.popleft()
+                self.inflight_backup_node_ids.discard(intent.node_id)
+                self.write_backlog_tokens_ -= intent_tokens
+                self._log_backup_dropped(intent_tokens)
+                continue
+            if self.write_staged_tokens_ >= live_cap:
+                # Yield to live fetch demand; retry next round.
+                break
+            if self._aux_budget_blocked(intent):
+                # An aux pool lacks staging headroom: yield at the gate
+                # instead of failing the alloc inside cc.write; acks free
+                # aux staging, retry next round.
+                break
+            if not self._launch_backup_intent(intent):
+                # Pool full of in-flight staging and nothing reclaimable
+                # (the tree never holds host values in buffer mode):
+                # defer, head-of-line; pending acks will free slots.
+                break
+            self.pending_write_queue.popleft()
+
+    def _launch_backup_intent(self, intent: _UnifiedBackupIntent) -> bool:
+        """Launch one admitted intent's D2H (staging alloc + device lock +
+        async copy); the caller removes it from pending_write_queue. Returns
+        False when staging cannot be allocated. From a successful launch the
+        intent always reaches its storage-ack, so its content joins the
+        LAUNCHED cover consulted by admission."""
+        cache = self._cache
+        cc = cache.cache_controller
+        node = intent.node
+        # Build aux transfers from the node's CURRENT state: a SWA span
+        # tombstoned since admission backs up FULL-only, as in cache mode.
+        device_value, comp_xfers = cache.tree_core.build_backup_spec(node.id)
+        aux_xfers = [x for xfers in comp_xfers.values() for x in xfers]
+        host_indices = cc.write(
+            device_value,
+            node_id=node.id,
+            extra_pools=aux_xfers or None,
+        )
+        if host_indices is None:
+            return False
+        _track_content_refs(self.inflight_backup_hashes, intent.hash_values)
+        # NOTE: no commit_backup — the node must never appear
+        # host-resident in buffer mode; staging slots live in the entry.
+        lock_params = cache.inc_lock_ref(node.id).to_dec_params()
+        self.ongoing_write_through[node.id] = _UnifiedBufferBackupEntry(
+            intent=intent,
+            host_indices=host_indices,
+            aux_xfers=aux_xfers,
+            lock_params=lock_params,
+        )
+        self.write_staged_tokens_ += len(host_indices)
+        self.write_backlog_tokens_ -= len(intent.hash_values) * cache.page_size
+        return True
+
+    def _aux_budget_blocked(self, intent: _UnifiedBackupIntent) -> bool:
+        """True when an aux pool cannot stage this intent right now (free
+        minus the loads-priority margin falls short of the need): defer at
+        the gate instead of failing the alloc inside cc.write and blocking
+        pure-KV intents behind an unallocatable head. The margin enforces
+        loads-have-priority on aux pools the way live_cap does on the KV
+        pool; avail already reflects prefetch-held slots, so no occupancy
+        subtraction here."""
+        aux = self._build_aux_staging_transfers(intent.node)
+        if not aux:
+            return False
+        cc = self._cache.cache_controller
+        for t in aux:
+            entry = cc.mem_pool_host.entry_map.get(t.name)
+            if entry is None:
+                continue
+            need = len(t.keys) * entry.host_pool.page_size
+            headroom = entry.host_pool.available_size() - self._aux_loads_margin(
+                entry.host_pool
+            )
+            if need > headroom:
+                return True
+        return False
+
+    def _aux_window_keys(
+        self, hash_values: list[str], transfer: PoolTransfer
+    ) -> Optional[list[str]]:
+        """Trailing KV page hashes keying an aux transfer's staged window
+        (one key per aux-pool page)."""
+        if transfer.host_indices is None or transfer.host_indices.numel() == 0:
+            return None
+        if transfer.indices_from_pool is not None:
+            return None  # sidecar rides another pool's slots; nothing to key
+        entry = self._cache.cache_controller.mem_pool_host.entry_map.get(transfer.name)
+        if entry is None:
+            return None
+        num_keys = len(transfer.host_indices) // entry.host_pool.page_size
+        if num_keys == 0 or num_keys > len(hash_values):
+            return None
+        return hash_values[-num_keys:]
+
+    def finish_backup_ack(self, ack_id: int) -> None:
+        """D2H confirmed: drop the device lock and enqueue the storage write
+        (which reads from the staging copy, so device eviction may proceed)."""
+        entry = self.ongoing_write_through.pop(ack_id)
+        intent = entry.intent
+        self._cache.dec_lock_ref(intent.node_id, entry.lock_params)
+
+        # Every aux pool writes a trailing snapshot keyed by the last KV page
+        # hashes it covers: the SWA window spans page_size-sized pages, the
+        # Mamba state is a single slot (host pool page_size 1 -> one key).
+        storage_xfers: list[PoolTransfer] = []
+        for staged in entry.aux_xfers:
+            keys = self._aux_window_keys(intent.hash_values, staged)
+            if keys is None:
+                continue
+            storage_xfers.append(
+                PoolTransfer(
+                    name=staged.name,
+                    host_indices=staged.host_indices,
+                    keys=keys,
+                    hit_policy=PoolHitPolicy.TRAILING_PAGES,
+                )
+            )
+        operation_id = self._cache.cache_controller.write_storage(
+            entry.host_indices,
+            intent.key.token_ids,
+            intent.hash_values,
+            intent.prefix_keys,
+            extra_pools=storage_xfers or None,
+        )
+        self.ongoing_backup[operation_id] = entry
+
+    def finish_storage_write_ack(self, operation_id: int) -> None:
+        """Storage write acked (rank-synced drain): free the entry's staging
+        outright. Existence entries are added unconditionally
+        (completed_tokens can diverge across ranks under backend failure) to
+        keep admission decisions TP-deterministic. No-op for operations this
+        pipeline does not own (e.g. acks for already-reset state)."""
+        entry = self.ongoing_backup.pop(operation_id, None)
+        if entry is None:
+            return
+        intent = entry.intent
+        self._cache.storage_existence_cache.add(PoolName.KV, intent.hash_values)
+        self._free_staging_now(entry.host_indices, entry.aux_xfers)
+        self.write_staged_tokens_ -= len(entry.host_indices)
+        self.inflight_backup_node_ids.discard(entry.intent.node_id)
+        _untrack_content_refs(self.inflight_backup_hashes, intent.hash_values)
+
+    def _free_staging_now(
+        self, host_indices: torch.Tensor, aux_xfers: list[PoolTransfer]
+    ) -> None:
+        """Synchronously free a staging span (KV + aux pools) on the
+        scheduler thread; buffer-mode acks/drops all run here, so frees
+        land before the tick's next gate reads pool availability."""
+        cc = self._cache.cache_controller
+        if host_indices is not None and host_indices.numel() > 0:
+            cc.mem_pool_host.free(host_indices)
+        for t in aux_xfers or ():
+            if (
+                t.host_indices is None
+                or t.host_indices.numel() == 0
+                or t.indices_from_pool is not None
+            ):
+                continue
+            entry = cc.mem_pool_host.entry_map.get(t.name)
+            if entry is not None:
+                entry.host_pool.free(t.host_indices)
+
+    # ---- load back pipeline (storage -> staging -> device) ----
+
+    def set_prefix_ctx(self, req_id: str, matched_prefix_tokens) -> None:
+        """Record the device-matched prefix at prefetch enqueue; consumed at
+        staging commit to build the full-span tree key."""
+        self._prefetch_prefix_ctx[req_id] = list(matched_prefix_tokens or [])
+
+    def pop_prefix_ctx(self, req_id: str) -> None:
+        self._prefetch_prefix_ctx.pop(req_id, None)
+
+    def has_staged(self, req_id: str) -> bool:
+        return req_id in self.staged_prefetches
+
+    @staticmethod
+    def _occupied_span(host_indices) -> int:
+        """Occupancy units a buffer-mode prefetch holds: granted at
+        hit-alloc, sized to the allocation (0 while still querying)."""
+        return len(host_indices) if host_indices is not None else 0
+
+    def stage_completed_prefetch(
+        self,
+        req_id: str,
+        num_tokens: int,
+        hash_value: list[str],
+    ) -> bool:
+        """Park the completed fetch as a held bounce; the scheduler surfaces
+        it as host_hit_length and the adder consumes it via init_load_back.
+        Always returns True (ready is a stable, revisited state)."""
+        cache = self._cache
+        (
+            _anchor,
+            prefetch_key,
+            host_indices,
+            operation,
+            _lock_params,
+            comp_xfers,
+        ) = cache.ongoing_prefetch.pop(req_id)
+        cc = cache.cache_controller
+        prefix_tokens = self._prefetch_prefix_ctx.pop(req_id, None)
+        aux_xfers = [x for xfers in comp_xfers.values() for x in xfers]
+
+        if num_tokens == 0 or prefix_tokens is None:
+            # Nothing usable fetched: recompute.
+            cc.append_host_mem_release(
+                host_indices[:num_tokens], extra_pools=aux_xfers or None
+            )
+            cc.prefetch_tokens_occupied -= self._occupied_span(host_indices)
+            cache.prefetch_loaded_tokens_by_reqid[req_id] = 0
+            return True
+
+        staged_pages = num_tokens // cache.page_size
+        staged_hashes = hash_value[:staged_pages]
+        staged_kv = host_indices[:num_tokens]
+        # Feed existence beliefs from the storage-fetched pages: the fetch
+        # itself is the evidence, so feeding is sound even if this staged
+        # prefetch is later dropped unconsumed.
+        cache.storage_existence_cache.add(PoolName.KV, list(staged_hashes))
+        occupied_tokens = self._occupied_span(host_indices)
+
+        self.staged_prefetches[req_id] = _StagedPrefetch(
+            req_id=req_id,
+            key_tokens=prefix_tokens + list(prefetch_key[:num_tokens].token_ids),
+            extra_key=prefetch_key.extra_key,
+            matched_len=len(prefix_tokens),
+            num_tokens=num_tokens,
+            occupied_tokens=occupied_tokens,
+            host_indices=staged_kv,
+            aux_xfers=aux_xfers,
+            hash_values=staged_hashes,
+            operation_id=operation.id,
+        )
+        cache.prefetch_loaded_tokens_by_reqid[req_id] = num_tokens
+        return True
+
+    def staged_prefetch_tokens(self, req_id: str) -> int:
+        """Tokens a staged prefetch would splice (0 = no hold); surfaced by the
+        scheduler as the request's host_hit_length."""
+        f = self.staged_prefetches.get(req_id)
+        return f.num_tokens if f is not None else 0
+
+    def staged_prefetch_swa_tokens(self, req_id: str) -> int:
+        """SWA device tokens consuming this staged prefetch will allocate (the
+        staged trailing window); surfaced as the request's swa_host_hit_length
+        so the adder's SWA gate charges the admission-time alloc."""
+        f = self.staged_prefetches.get(req_id)
+        if f is None:
+            return 0
+        return sum(
+            len(t.host_indices)
+            for t in f.aux_xfers
+            if t.name == PoolName.SWA and t.host_indices is not None
+        )
+
+    def init_load_back(self, params: InitLoadBackParams) -> tuple[torch.Tensor, NodeId]:
+        """Buffer-mode branch of init_load_back: consume the staged prefetch
+        at prefill admission — device alloc (evict-before-alloc), layer-gated
+        H2D, and a plain insert so downstream sees ordinary tree state.
+        Misaligned or alloc-failed holds drop; the request recomputes."""
+        cache = self._cache
+        req = params.req
+        assert req is not None
+        empty = cache.tree_core.empty_match_result.device_indices
+        unchanged = (empty, req.last_node)
+        f = self.staged_prefetches.pop(req.rid, None)
+        if f is None:
+            return unchanged
+        cc = cache.cache_controller
+
+        def _drop() -> tuple[torch.Tensor, NodeId]:
+            self._free_staging_now(f.host_indices, f.aux_xfers)
+            cc.prefetch_tokens_occupied -= f.occupied_tokens
+            return unchanged
+
+        # Splice-validity: the span only fits if the device prefix still
+        # ends exactly at the enqueue-time matched_len.
+        if len(req.prefix_indices) != f.matched_len:
+            # Prefix moved while held (leaf eviction or sibling extension):
+            # drop and recompute.
+            return _drop()
+
+        # Evict-before-alloc (mirrors _load_back_transfers): the budget gate
+        # counts evictable pages, but cc.load draws from free slots only.
+        if cache.supports_swa():
+            avail = cache.token_to_kv_pool_allocator.full_available_size()
+        else:
+            avail = cache.token_to_kv_pool_allocator.available_size()
+        if avail < f.num_tokens:
+            needed = f.num_tokens - avail
+            evicted = cache.evict(EvictParams(num_tokens=needed))
+            if evicted.num_tokens_evicted < needed:
+                # Genuinely no room (locked pages): recompute.
+                return _drop()
+
+        load_back_id = -(f.operation_id) - 1
+        device_indices = cc.load(
+            host_indices=f.host_indices,
+            node_id=load_back_id,
+            extra_pools=f.aux_xfers or None,
+        )
+        if device_indices is None:
+            # Transient allocator shortfall despite the evict: recompute
+            # (init_load_back's degrade contract).
+            return _drop()
+
+        swa_dev = next(
+            (
+                t.device_indices
+                for t in f.aux_xfers
+                if t.name == PoolName.SWA
+                and t.device_indices is not None
+                and t.device_indices.numel() > 0
+            ),
+            None,
+        )
+        if swa_dev is not None:
+            # Register the trailing window's FULL->SWA translation NOW: the
+            # admitted request's attention reads the window through this
+            # mapping during the layer-gated forward.
+            cache._apply_cache_action(
+                RebuildFullToSWAMapping([device_indices[-len(swa_dev) :]], [swa_dev])
+            )
+
+        # Publish via a plain insert under the admission lock choreography;
+        # the caller's request lock then pins the span (load_back pattern).
+        key = RadixKey(
+            array("q", f.key_tokens),
+            extra_key=f.extra_key,
+            is_bigram=cache.tree_core.is_eagle,
+        ).page_aligned(cache.page_size)
+        span_end = f.matched_len + f.num_tokens
+        cache.insert(
+            InsertParams(
+                key=key,
+                value=torch.cat([req.prefix_indices, device_indices]),
+                prev_prefix_len=f.matched_len,
+                swa_evicted_seqlen=(
+                    max(0, span_end - len(swa_dev)) if swa_dev is not None else 0
+                ),
+            )
+        )
+        self.ongoing_buffer_load_back[load_back_id] = _OngoingBufferLoadBack(
+            req_id=f.req_id,
+            num_tokens=f.num_tokens,
+            occupied_tokens=f.occupied_tokens,
+            aux_xfers=f.aux_xfers,
+            host_indices=f.host_indices,
+            hash_values=f.hash_values,
+        )
+        m = cache.match_prefix(MatchPrefixParams(key=key))
+        if len(m.device_indices) < span_end:
+            # The insert walk did not adopt the full span (should not happen
+            # for a locked prefix); the slots are tree-owned/evictable — do
+            # not splice, the request recomputes.
+            return unchanged
+        return device_indices, m.last_device_node
+
+    def try_finish_load_back(self, ack_id: int) -> bool:
+        """Fill ack: free the host bounce and return True when the ack id is
+        a buffer-mode load-back. The span was published at admission; the
+        ack never touches the tree (existence beliefs were fed from the
+        storage-fetched pages at staging commit)."""
+        f = self.ongoing_buffer_load_back.pop(ack_id, None)
+        if f is None:
+            return False
+        cache = self._cache
+        cc = cache.cache_controller
+
+        # The H2D consumed the bounce buffers; free them outright.
+        self._free_staging_now(f.host_indices, f.aux_xfers)
+
+        cc.prefetch_tokens_occupied -= f.occupied_tokens
+        logger.info(
+            "HiCache prefetch fill committed req=%s filled=%d occupied=%d",
+            f.req_id,
+            f.num_tokens,
+            cc.prefetch_tokens_occupied,
+        )
+        if cache.enable_storage_metrics and cache.storage_metrics_collector is not None:
+            cache.storage_metrics_collector.log_prefetched_tokens(f.num_tokens)
+        return True
+
+    def release_aborted_staged(self, rid: str) -> bool:
+        """Free an aborted request's staged prefetch (nothing device-side
+        exists yet — only the bounce). Returns True when a hold existed."""
+        staged = self.staged_prefetches.pop(rid, None)
+        if staged is None:
+            return False
+        self._free_staging_now(staged.host_indices, staged.aux_xfers)
+        self._cache.cache_controller.prefetch_tokens_occupied -= staged.occupied_tokens
+        return True

python/sglang/srt/mem_cache/buffer_mode/storage_existence_cache.py

@@ -0,0 +1,89 @@
+"""Local existence cache for HiCache buffer_only mode.
+
+In buffer mode host memory holds no persistent copy, so without a local
+existence signal every re-insert of a hot prefix re-stages and re-writes to
+L3 storage. This cache is that signal: a bounded LRU of (pool, page-hash)
+entries *believed* present in storage.
+
+Semantics are advisory, not authoritative:
+
+- A hit skips the redundant D2H + storage write.
+- A stale positive (backend evicted the data) costs skipped write-backs until
+  a prefetch hit-query shortfall invalidates the entries; the next insert
+  then writes the data back. Never a correctness issue — at worst one cold
+  recompute, the same as any cache miss.
+- A miss (entry LRU-evicted or never seen) just costs one redundant write
+  (idempotent: storage keys are content-addressed).
+
+Keys are the content-chained page hashes already computed at insert time, so
+lookups never hash anything and entries survive node deletion, splits, and
+recompute (same tokens => same chain). Page hashes are chained and the write
+path is prefix-contiguous (parent-cover gate), so a node's own page set is
+the only thing a caller needs to check.
+
+TP determinism: replicas stay identical because every mutation happens on the
+scheduler thread at lockstep points with cross-rank-reduced inputs
+(storage-ack drain, prefetch-hit drain, fill commit). Do not touch it from
+anywhere else.
+"""
+
+from __future__ import annotations
+
+from collections import OrderedDict
+from typing import Container, Iterable, Sequence
+
+# ~131K entries; at ~150-250 B/entry this is <= ~30 MB and covers roughly
+# 8M KV tokens at page size 64 (aux-pool entries included). Coverage per MB
+# scales with page size — small-page configs simply remember fewer tokens.
+HICACHE_EXISTENCE_CACHE_MAX_ENTRIES = 128 * 1024
+
+
+class StorageExistenceCache:
+    def __init__(self, max_entries: int = HICACHE_EXISTENCE_CACHE_MAX_ENTRIES):
+        self.max_entries = max_entries
+        self._entries: OrderedDict[tuple[str, str], None] = OrderedDict()
+
+    def __len__(self) -> int:
+        return len(self._entries)
+
+    def add(self, pool: str, hashes: Iterable[str]) -> None:
+        entries = self._entries
+        for h in hashes:
+            entries[(pool, h)] = None
+            entries.move_to_end((pool, h))
+        while len(entries) > self.max_entries:
+            entries.popitem(last=False)
+
+    def contains(self, pool: str, page_hash: str) -> bool:
+        entries = self._entries
+        if (pool, page_hash) not in entries:
+            return False
+        entries.move_to_end((pool, page_hash))
+        return True
+
+    def contains_all(self, pool: str, hashes: Iterable[str]) -> bool:
+        return all(self.contains(pool, h) for h in hashes)
+
+    def covers_all(
+        self,
+        pool: str,
+        hashes: Iterable[str],
+        extra_cover: Container[str] = frozenset(),
+    ) -> bool:
+        """True when every page is believed stored or sits in
+        ``extra_cover`` (e.g. content past its D2H launch, which always
+        reaches its storage-ack). LRU-touches the believed entries."""
+        return all(self.contains(pool, h) or h in extra_cover for h in hashes)
+
+    def invalidate_beyond(
+        self, pool: str, hashes: Sequence[str], keep_pages: int
+    ) -> None:
+        """Ground-truth heal from a prefetch hit query: discard beliefs
+        beyond the leading ``keep_pages`` of a hash chain (the folded
+        usable cut). The next insert re-writes the discarded span, closing
+        stale positives and aux holes at the cut."""
+        for h in hashes[keep_pages:]:
+            self._entries.pop((pool, h), None)
+
+    def clear(self) -> None:
+        self._entries.clear()

python/sglang/srt/mem_cache/hiradix_cache.py

@@ -1775,6 +1775,8 @@ class HiRadixCache(RadixCache):
         new_input_tokens: List[int],
         last_hash: Optional[str] = None,
         prefix_keys: Optional[List[str]] = None,
+        # Scheduler-call parity with UnifiedRadixCache; unused in cache mode.
+        matched_prefix_tokens: Optional[List[int]] = None,
     ):
         prefetch_key = RadixKey(
             new_input_tokens,

python/sglang/srt/mem_cache/hybrid_cache/hybrid_cache_controller.py

@@ -112,6 +112,7 @@ class HybridCacheController(BaseHiCacheController):
         storage_backend_extra_config: Optional[dict] = None,
         transfer_layer_num: Optional[int] = None,
         enable_storage_metrics: bool = False,
+        host_memory_mode: str = "cache",
     ):
         startup_storage_backend = storage_backend
         self.extra_host_mem_release_queues: dict[PoolName, Queue[torch.Tensor]] = {}
@@ -131,6 +132,7 @@ class HybridCacheController(BaseHiCacheController):
             model_name=model_name,
             storage_backend_extra_config=storage_backend_extra_config,
             enable_storage_metrics=enable_storage_metrics,
+            host_memory_mode=host_memory_mode,
         )
         # Override layer_num: hybrid models transfer all layers (For example, Linear Model (KV + Mamba)),
         # not just the full attention layers reported by full_kv_pool.
@@ -587,6 +589,7 @@ class HybridCacheController(BaseHiCacheController):
         hash_value = self.get_hash_str(
             operation.token_ids, operation.last_hash, page_size=self.page_size
         )
+        operation.all_hash_values = hash_value
 
         extra_info = HiCacheStorageExtraInfo(
             prefix_keys=operation.prefix_keys.copy() if operation.prefix_keys else None
@@ -780,6 +783,28 @@ class HybridCacheController(BaseHiCacheController):
                 continue
             trailing_n = len(transfer.keys) if transfer.keys else 1
             transfer.keys = all_hashes[max(0, kv_hit_pages - trailing_n) : kv_hit_pages]
+            if transfer.host_indices is None:
+                continue
+            entry = self.mem_pool_host.entry_map.get(transfer.name)
+            pool_page_size = (
+                entry.host_pool.page_size if entry is not None else self.page_size
+            )
+            needed = len(transfer.keys) * pool_page_size
+            if transfer.host_indices.numel() > needed:
+                # The hit undershot the pre-allocated window buffer. Backends
+                # fetch keys zipped against the buffer head, so shrink the
+                # transfer to match and release the tail now — otherwise the
+                # length mismatch makes batch_get_v2 fetch nothing and the
+                # whole window is silently lost downstream.
+                self.append_host_mem_release(
+                    extra_pools=[
+                        PoolTransfer(
+                            name=transfer.name,
+                            host_indices=transfer.host_indices[needed:],
+                        )
+                    ]
+                )
+                transfer.host_indices = transfer.host_indices[:needed]
 
     def _resolve_pool_transfers_allocation(
         self,

python/sglang/srt/mem_cache/hybrid_cache/hybrid_pool_assembler.py

@@ -307,6 +307,7 @@ def build_kv_only_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     if params.mtp_draft_device_pools:
         cache_controller.set_mtp_draft_pools(params.mtp_draft_device_pools)
@@ -377,6 +378,7 @@ def build_hybrid_swa_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     if mtp_swa_device_pools:
         cache_controller.set_mtp_draft_pools(mtp_swa_device_pools)
@@ -664,6 +666,7 @@ def build_deepseek_v4_hicache_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     if mtp_swa_device_buffers:
         cache_controller.set_mtp_draft_pools(mtp_swa_device_buffers)
@@ -759,6 +762,7 @@ def build_hybrid_mamba_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     if mtp_draft_device_pools:
         cache_controller.set_mtp_draft_pools(mtp_draft_device_pools)
@@ -874,6 +878,7 @@ def build_hybrid_mamba_swa_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     return host_pool_group, cache_controller
 
@@ -951,6 +956,7 @@ def build_anchor_sidecar_stack(
         storage_backend_extra_config=storage_backend_extra_config,
         transfer_layer_num=transfer_layer_num,
         enable_storage_metrics=enable_storage_metrics,
+        host_memory_mode=server_args.hicache_host_memory_mode,
     )
     if mtp_draft_device_pools:
         cache_controller.set_mtp_draft_pools(mtp_draft_device_pools)

python/sglang/srt/mem_cache/pool_host/mamba.py

@@ -129,7 +129,7 @@ class MambaPoolHost(HostKVCache):
             for conv_state in device_pool.mamba_cache.conv
         ]
 
-        self.init_kv_buffer()
+        self.kv_buffer = self.init_kv_buffer()
         self._init_write_back_staging_buffers()
         self.lock = threading.RLock()
         self.clear()
@@ -201,6 +201,12 @@ class MambaPoolHost(HostKVCache):
                         allocator=self.allocator,
                     )
                 )
+        # destroy() unregisters via kv_buffer; without this list the pinned
+        # registrations leak past the buffers' mmap. 0-element buffers
+        # (conv-only models' temporal state) were never registered.
+        return [
+            buf for buf in (self.temporal_buffer, *self.conv_buffer) if buf.numel() > 0
+        ]
 
     def _init_write_back_staging_buffers(self):
         self.temporal_staging_buffer = None

python/sglang/srt/mem_cache/registry.py

@@ -237,6 +237,16 @@ def create_tree_cache(ctx: TreeCacheBuildContext) -> BasePrefixCache:
         cache = default_radix_cache_factory(ctx)
         source = "default"
 
+    if (
+        ctx.server_args.enable_hierarchical_cache
+        and ctx.server_args.hicache_host_memory_mode == "buffer_only"
+        and type(cache).__name__ != "UnifiedRadixCache"
+    ):
+        raise ValueError(
+            "--hicache-host-memory-mode buffer_only is only implemented for "
+            f"the unified radix tree; this model selected {type(cache).__name__}."
+        )
+
     if ctx.server_args.enable_session_radix_cache and not getattr(
         cache, "enable_session_radix_cache", False
     ):

python/sglang/srt/mem_cache/storage/backend_factory.py

@@ -158,7 +158,7 @@ class StorageBackendFactory:
         mem_pool_host: Any,
     ) -> HiCacheStorage:
         """Create built-in backend with original initialization logic."""
-        if backend_name == "file":
+        if backend_name in ("file", "sim"):
             return backend_class(storage_config)
         elif backend_name == "nixl":
             return backend_class(storage_config)
@@ -198,6 +198,10 @@ StorageBackendFactory.register_backend(
     "file", "sglang.srt.mem_cache.hicache_storage", "HiCacheFile"
 )
 
+StorageBackendFactory.register_backend(
+    "sim", "sglang.srt.mem_cache.storage.sim_storage", "SimHiCacheStorage"
+)
+
 StorageBackendFactory.register_backend(
     "nixl",
     "sglang.srt.mem_cache.storage.nixl.hicache_nixl",

python/sglang/srt/mem_cache/storage/sim_storage.py

@@ -0,0 +1,254 @@
+"""Deterministic no-IO storage simulator for HiCache benchmarking.
+
+Stores KEYS ONLY (served KV is garbage — benchmark-only, for ignore_eos
+workloads where nothing reads the generated text) and sleeps
+``bytes / bandwidth + op latency`` on the calling backup/prefetch thread,
+so pipeline dynamics are preserved while the medium is exactly
+reproducible. Bandwidth is per scheduler rank (each rank ships its own
+shard). extra_config knobs: ``sim_write_gbps`` (default 5.0; <=0 =
+infinite), ``sim_read_gbps`` (default = write), ``sim_op_latency_us``
+(default 100, applied to exists queries too).
+"""
+
+from __future__ import annotations
+
+import logging
+import threading
+import time
+from typing import Any, List, Optional
+
+import torch
+
+from sglang.srt.mem_cache.hicache_storage import (
+    HiCacheStorage,
+    HiCacheStorageConfig,
+    HiCacheStorageExtraInfo,
+    PoolHitPolicy,
+    PoolName,
+    PoolTransfer,
+    PoolTransferResult,
+)
+
+logger = logging.getLogger(__name__)
+
+
+class SimHiCacheStorage(HiCacheStorage):
+    def __init__(self, storage_config: HiCacheStorageConfig):
+        extra = storage_config.extra_config or {}
+        self.write_gbps = float(extra.get("sim_write_gbps", 5.0))
+        self.read_gbps = float(extra.get("sim_read_gbps", self.write_gbps))
+        self.op_latency_s = float(extra.get("sim_op_latency_us", 100.0)) * 1e-6
+        # Scoped key -> True. Keys only; there are no bytes to store.
+        self._keys: set[str] = set()
+        self._lock = threading.Lock()
+        logger.info(
+            "SimHiCacheStorage: write=%.2f GB/s read=%.2f GB/s latency=%.0fus "
+            "(per rank; <=0 GB/s = infinite)",
+            self.write_gbps,
+            self.read_gbps,
+            self.op_latency_s * 1e6,
+        )
+
+    # ---- timing model ----
+
+    def _sleep_io(self, num_bytes: int, gbps: float) -> None:
+        delay = self.op_latency_s
+        if gbps > 0:
+            delay += num_bytes / (gbps * 1e9)
+        if delay > 0:
+            time.sleep(delay)
+
+    def _pool_bytes(self, name: PoolName, num_slots: int) -> int:
+        return num_slots * self.registered_pools[name].size_per_token
+
+    @staticmethod
+    def _scoped(name: PoolName, key: str) -> str:
+        return key if name == PoolName.KV else f"{key}.{name}"
+
+    # ---- single-key surface (generic controller paths) ----
+
+    def get(
+        self,
+        key: str,
+        target_location: Optional[Any] = None,
+        target_sizes: Optional[Any] = None,
+    ) -> torch.Tensor | None:
+        with self._lock:
+            present = key in self._keys
+        return target_location if present else None
+
+    def set(
+        self,
+        key: str,
+        value: Optional[Any] = None,
+        target_location: Optional[Any] = None,
+        target_sizes: Optional[Any] = None,
+    ) -> bool:
+        with self._lock:
+            self._keys.add(key)
+        return True
+
+    def exists(self, key: str) -> bool:
+        with self._lock:
+            return key in self._keys
+
+    # ---- batch v0/v1 (generic page funcs) ----
+
+    def batch_get(
+        self,
+        keys: List[str],
+        target_locations: Optional[Any] = None,
+        target_sizes: Optional[Any] = None,
+    ) -> List[torch.Tensor | None]:
+        locations = target_locations or [None] * len(keys)
+        with self._lock:
+            present = [k in self._keys for k in keys]
+        num_bytes = sum(
+            loc.numel() * loc.element_size()
+            for loc, hit in zip(locations, present)
+            if hit and loc is not None
+        )
+        self._sleep_io(num_bytes, self.read_gbps)
+        return [loc if hit else None for loc, hit in zip(locations, present)]
+
+    def batch_set(
+        self,
+        keys: List[str],
+        values: Optional[Any] = None,
+        target_locations: Optional[Any] = None,
+        target_sizes: Optional[Any] = None,
+    ) -> bool:
+        num_bytes = sum(v.numel() * v.element_size() for v in values or ())
+        self._sleep_io(num_bytes, self.write_gbps)
+        with self._lock:
+            self._keys.update(keys)
+        return True
+
+    def batch_get_v1(
+        self,
+        keys: List[str],
+        host_indices: torch.Tensor,
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> List[bool]:
+        with self._lock:
+            present = [k in self._keys for k in keys]
+        hit_slots = (len(host_indices) // max(1, len(keys))) * sum(present)
+        self._sleep_io(hit_slots * self.mem_pool_host.size_per_token, self.read_gbps)
+        return present
+
+    def batch_set_v1(
+        self,
+        keys: List[str],
+        host_indices: torch.Tensor,
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> List[bool]:
+        self._sleep_io(
+            len(host_indices) * self.mem_pool_host.size_per_token, self.write_gbps
+        )
+        with self._lock:
+            self._keys.update(keys)
+        return [True] * len(keys)
+
+    # ---- batch v2 (hybrid multi-pool paths) ----
+
+    def batch_exists(
+        self,
+        keys: List[str],
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> int:
+        self._sleep_io(0, self.read_gbps)
+        with self._lock:
+            for i, key in enumerate(keys):
+                if key not in self._keys:
+                    return i
+        return len(keys)
+
+    def batch_exists_v2(
+        self,
+        keys: List[str],
+        pool_transfers: Optional[List[PoolTransfer]] = None,
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> PoolTransferResult:
+        """Same fold semantics as HiCacheFile.batch_exists_v2, over the
+        in-memory key set."""
+        self._sleep_io(0, self.read_gbps)
+        with self._lock:
+            snapshot = self._keys.copy()
+
+        kv_pages = next(
+            (i for i in range(len(keys)) if keys[i] not in snapshot), len(keys)
+        )
+        hit_count: dict[str, int] = {PoolName.KV: kv_pages} if kv_pages else {}
+        final_pages = kv_pages
+
+        for transfer in pool_transfers or []:
+            if final_pages == 0:
+                break
+            name = transfer.name
+            if transfer.hit_policy == PoolHitPolicy.ALL_PAGES:
+                boundary = next(
+                    (
+                        i
+                        for i in range(kv_pages)
+                        if self._scoped(name, keys[i]) not in snapshot
+                    ),
+                    kv_pages,
+                )
+            else:  # trailing_pages
+                trailing = max(1, len(transfer.keys) if transfer.keys else 1)
+                boundary = 0
+                for prefix_len in range(kv_pages, 0, -1):
+                    if all(
+                        self._scoped(name, keys[i]) in snapshot
+                        for i in range(max(0, prefix_len - trailing), prefix_len)
+                    ):
+                        boundary = prefix_len
+                        break
+            if boundary:
+                hit_count[name] = boundary
+            final_pages = min(final_pages, boundary)
+
+        return PoolTransferResult(final_pages, hit_count)
+
+    def _batch_v2(
+        self, transfers: List[PoolTransfer], gbps: float, record: bool
+    ) -> dict[str, List[bool]]:
+        results: dict[str, List[bool]] = {}
+        num_bytes = 0
+        for t in transfers:
+            t_keys = t.keys or []
+            if t.host_indices is not None:
+                num_bytes += self._pool_bytes(t.name, len(t.host_indices))
+            scoped = [self._scoped(t.name, k) for k in t_keys]
+            with self._lock:
+                if record:
+                    self._keys.update(scoped)
+                    results[t.name] = [True] * len(t_keys)
+                else:
+                    results[t.name] = [k in self._keys for k in scoped]
+        self._sleep_io(num_bytes, gbps)
+        return results
+
+    def batch_get_v2(
+        self,
+        transfers: List[PoolTransfer],
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> dict[str, List[bool]]:
+        return self._batch_v2(transfers, self.read_gbps, record=False)
+
+    def batch_set_v2(
+        self,
+        transfers: List[PoolTransfer],
+        extra_info: Optional[HiCacheStorageExtraInfo] = None,
+    ) -> dict[str, List[bool]]:
+        return self._batch_v2(transfers, self.write_gbps, record=True)
+
+    # ---- misc ----
+
+    def clear(self) -> bool:
+        with self._lock:
+            self._keys.clear()
+        return True
+
+    def get_stats(self):
+        return None

python/sglang/srt/mem_cache/unified_cache/components/swa_component.py

@@ -71,6 +71,9 @@ class SWAComponent(TreeComponent):
         super().__init__(cache, params)
         self._session_leaf_covered_len: dict[str, dict[UnifiedTreeNode, int]] = {}
         self.sliding_window_size = params.sliding_window_size
+        self.full_window_pages = (
+            self.sliding_window_size + params.page_size - 1
+        ) // params.page_size
         # HiCache state: set to host SWA pool when HiCache enabled
         self._swa_kv_pool_host = None
 
@@ -770,12 +773,27 @@ class SWAComponent(TreeComponent):
         # unified_kv keeps SWA as a device-only ring -- nothing to prefetch into.
         if self._swa_kv_pool_host is None:
             return PreparePrefetchResult()
-        sw_pages = (
+        sw_pages = self.full_window_pages
-            self.cache.sliding_window_size + self.cache.page_size - 1
+        if sw_pages == 0:
-        ) // self.cache.page_size
-        if sw_pages == 0 or prefetch_tokens // self.cache.page_size < sw_pages:
             return PreparePrefetchResult()
-        num_tokens = sw_pages * self.cache.page_size
+        prefetch_pages = prefetch_tokens // self.cache.page_size
+        if prefetch_pages >= sw_pages:
+            num_pages = sw_pages
+        elif prefetch_pages <= 0:
+            return PreparePrefetchResult()
+        elif (
+            self.tree_core.is_root(node_id)
+            or self.cache.host_memory_mode == "buffer_only"
+        ):
+            # Sub-window fetch: at root the sequence IS its window; mid-tree
+            # (buffer mode) the window head is the device prefix's own ring
+            # state, so only the suffix needs fetching.
+            num_pages = prefetch_pages
+        else:
+            # Cache-mode graft: a mid-tree window head is not
+            # device-guaranteed, require a full window.
+            return PreparePrefetchResult()
+        num_tokens = num_pages * self.cache.page_size
         host_indices = self._swa_kv_pool_host.alloc(num_tokens)
         if host_indices is None:
             self.cache.evict_host(num_tokens, ComponentType.SWA)
@@ -869,12 +887,14 @@ class SWAComponent(TreeComponent):
 
         if phase == CacheTransferPhase.PREFETCH:
             assert host_indices is not None
-            sw_pages = host_indices.numel() // self.tree_core.page_size
+            # Keys are unknowable at build time; placeholders carry the
+            # count, _sync_trailing_keys fills the real trailing hashes.
+            num_pages = host_indices.numel() // self.tree_core.page_size
             return [
                 PoolTransfer(
                     name=PoolName.SWA,
                     host_indices=host_indices,
-                    keys=["__placeholder__"] * sw_pages,
+                    keys=["__placeholder__"] * num_pages,
                     hit_policy=PoolHitPolicy.TRAILING_PAGES,
                 )
             ]
@@ -997,6 +1017,13 @@ class SWAComponent(TreeComponent):
             and insert_result.inserted_host_node is not None
             else None
         )
+        if anchor is not self.tree_core.root_node:
+            # Cache-mode graft commit only (buffer fills never reach here):
+            # a hit-shrunk window mid-tree is missing its head — drop it.
+            # Root anchors are complete windows of their own.
+            if window_require_pages < self.full_window_pages:
+                self._release_swa_host(host_indices, cache_actions)
+                return
         if (
             target is None
             or window_require_pages == 0

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -25,6 +25,13 @@ from sglang.srt.mem_cache.base_prefix_cache import (
     MatchPrefixParams,
     MatchResult,
 )
+from sglang.srt.mem_cache.buffer_mode.pipeline import (
+    BufferModePipeline,
+    validate_buffer_only_stack,
+)
+from sglang.srt.mem_cache.buffer_mode.storage_existence_cache import (
+    StorageExistenceCache,
+)
 from sglang.srt.mem_cache.common import RetractionBackup
 from sglang.srt.mem_cache.hicache_storage import (
     PoolHitPolicy,
@@ -71,6 +78,7 @@ from sglang.srt.mem_cache.unified_cache.unified_tree_core import (  # noqa: F401
 )
 from sglang.srt.observability.metrics_collector import (
     STAT_LOGGER_ROLE_STORAGE,
+    StorageMetrics,
     StorageMetricsCollector,
     resolve_collector_class,
 )
@@ -227,6 +235,30 @@ class UnifiedRadixCache(BasePrefixCache):
         self.prefetch_timeout_base = 1.0
         self.prefetch_timeout_per_page = 0.25
         self.hicache_storage_pass_prefix_keys = False
+        # Buffer-only host memory mode (host RAM as transient GPU↔storage
+        # staging, not an L2 tier); resolved in init_hicache, which also
+        # constructs the pipeline collaborator (None = cache mode).
+        self.host_memory_mode = "cache"
+        self.buffer_pipeline: Optional[BufferModePipeline] = None
+        # Write-side dedupe: beliefs about what storage already holds, so
+        # re-inserts of hot prefixes skip the redundant backup.
+        self.storage_existence_cache = StorageExistenceCache()
+        # Cumulative prefetch-outcome counters, exported through the
+        # log_storage_metrics flow.
+        self._prefetch_outcome_stats: dict[str, float] = {
+            "attempts": 0,
+            "issued": 0,
+            "declined_too_short": 0,
+            "declined_rate_limited": 0,
+            "revoked_insufficient": 0,
+            "revoked_full_miss": 0,
+            "l3_demand_requests": 0,
+            "l3_miss_tokens": 0,
+            "l1l2_miss_tokens": 0,
+            "l3_demand_total_tokens": 0,
+            "l3_sum_rate_all": 0.0,
+            "l3_sum_rate_main_weighted": 0.0,
+        }
 
         self.reset()
         logger.info(
@@ -316,6 +348,8 @@ class UnifiedRadixCache(BasePrefixCache):
         self.prefetch_loaded_tokens_by_reqid: dict[str, int] = {}
         self.ongoing_prefetch: dict[str, _OngoingPrefetch] = {}
         self.ongoing_backup: dict[int, tuple[NodeId, DecLockRefParams]] = {}
+        if self.buffer_pipeline is not None:
+            self.buffer_pipeline.reset()
 
         if self.cache_controller is not None:
             self.cache_controller.reset()
@@ -326,6 +360,20 @@ class UnifiedRadixCache(BasePrefixCache):
 
     def init_hicache(self, server_args: ServerArgs, params: CacheInitParams) -> None:
         """Initialize HiCache infrastructure."""
+        self.host_memory_mode = server_args.hicache_host_memory_mode
+        if self.host_memory_mode == "buffer_only":
+            # FULL and FULL+SWA only: Mamba has no state-handoff channel on
+            # the admission-time load-back read path and is not layer-gated.
+            # Lifting the fence also needs the admission charge: a staged
+            # state slot is request-pinned at consumption and must ride
+            # req.mamba_host_hit_length the way the SWA window does.
+            supported = {ComponentType.FULL, ComponentType.SWA}
+            if not set(self.tree_components) <= supported:
+                raise ValueError(
+                    "--hicache-host-memory-mode buffer_only supports only "
+                    "FULL/SWA unified trees; got components "
+                    f"{sorted(ct.name for ct in self.tree_components)}."
+                )
         from sglang.srt.mem_cache.hybrid_cache.hybrid_pool_assembler import (
             attach_hybrid_pool_to_unified_cache,
         )
@@ -368,6 +416,28 @@ class UnifiedRadixCache(BasePrefixCache):
                 swa = self.components[ComponentType.SWA]
                 self.tree_core.has_swa_host_pool = swa._swa_kv_pool_host is not None
 
+        if self.host_memory_mode == "buffer_only":
+            swa = self.components.get(ComponentType.SWA)
+            validate_buffer_only_stack(
+                sidecar_pool_specs=self.sidecar_pool_specs, swa_component=swa
+            )
+            self.buffer_pipeline = BufferModePipeline(
+                cache=self,
+                swa_window_pages=(
+                    swa.full_window_pages
+                    if swa is not None and self.tree_core.has_swa_host_pool
+                    else 0
+                ),
+                # Leak backstop only: live queued tokens are intrinsically
+                # bounded by the FULL device pool (one intent per node, stale
+                # intents swept per tick), so a cap that binds on live
+                # content would drop-newest and punch storage holes.
+                write_backlog_cap=2 * self.token_to_kv_pool_allocator.size_full,
+            )
+            self.cache_controller.host_write_staged_tokens_fn = (
+                lambda: self.buffer_pipeline.write_staged_tokens_
+            )
+
         # State initialization
         self.write_through_threshold = (
             1 if server_args.hicache_write_policy == "write_through" else 2
@@ -541,6 +611,9 @@ class UnifiedRadixCache(BasePrefixCache):
         request_by_type: dict[ComponentType, int],
         tracker: dict[ComponentType, int],
     ) -> None:
+        # Buffer mode: eviction always wins over queued backup intents — a
+        # destroyed victim's intent is stale-swept and the content rewrites
+        # after its recompute.
         for ct in self.tree_components:
             request_cnt = request_by_type[ct]
             # Skip eviction walk if request is already met
@@ -926,6 +999,10 @@ class UnifiedRadixCache(BasePrefixCache):
         self, num_tokens: int, component_type: ComponentType = BASE_COMPONENT_TYPE
     ) -> int:
         """Evict host resources for a specific component to free host pool space."""
+        if self.host_memory_mode == "buffer_only":
+            # The tree never holds host values in buffer mode, and staging
+            # is operation-owned (freed at each ack): nothing is evictable.
+            return 0
         result = self.tree_core.drive_host_eviction(component_type, num_tokens)
         self._free_values(result.device_frees, result.host_frees)
         return result.tracker.get(component_type, 0)
@@ -1170,6 +1247,16 @@ class UnifiedRadixCache(BasePrefixCache):
         self, action: BackupKV, write_back: bool = False
     ) -> int:
         """Run a backup action top-down, stopping at the first failed backup."""
+        if self.buffer_pipeline is not None:
+            # Buffer mode bypasses the host-backup contiguity below: nothing
+            # is ever host-backuped here. Contiguity comes from end-to-end
+            # FIFO ordering instead (BackupKV chains are parent-before-child
+            # and every pipeline stage drains in order).
+            for node_id in action.node_ids:
+                self.buffer_pipeline.enqueue_backup_intent(
+                    self.tree_core.node_by_id(node_id)
+                )
+            return 0
         written = 0
         for node_id in action.node_ids:
             device_value, comp_xfers = self.tree_core.build_backup_spec(node_id)
@@ -1249,6 +1336,10 @@ class UnifiedRadixCache(BasePrefixCache):
         )
 
     def _finish_write_through_ack(self, ack_id: int) -> None:
+        if self.buffer_pipeline is not None:
+            self.buffer_pipeline.finish_backup_ack(ack_id)
+            return
+
         lock_node_id, lock_params, publish_node_ids = self.ongoing_write_through.pop(
             ack_id
         )
@@ -1468,10 +1559,12 @@ class UnifiedRadixCache(BasePrefixCache):
         new_input_tokens: list[int],
         last_hash: Optional[str] = None,
         prefix_keys: Optional[list[str]] = None,
+        matched_prefix_tokens: Optional[list[int]] = None,
     ) -> None:
         if not self.enable_storage or self.cache_controller is None:
             return
 
+        buffer_mode = self.host_memory_mode == "buffer_only"
         extra_key, cache_salt = self.tree_core.prefetch_anchor_info(last_host_node_id)
         prefetch_key = RadixKey(
             new_input_tokens,
@@ -1480,13 +1573,30 @@ class UnifiedRadixCache(BasePrefixCache):
             cache_salt=cache_salt,
         ).page_aligned(self.page_size)
         prefetch_length = len(prefetch_key)
-        if (
+        stats = self._prefetch_outcome_stats
-            prefetch_length < self.prefetch_threshold
+        if prefetch_length > 0:
-            or self.cache_controller.prefetch_rate_limited()
+            stats["attempts"] += 1
+        if prefetch_length < self.prefetch_threshold:
+            if prefetch_length > 0:
+                stats["declined_too_short"] += 1
+            return
+        if not buffer_mode and self.cache_controller.prefetch_rate_limited():
+            stats["declined_rate_limited"] += 1
+            return
+        if req_id in self.ongoing_prefetch or (
+            buffer_mode and self.buffer_pipeline.has_staged(req_id)
         ):
+            # A fetch (or an unconsumed hold) already exists for this rid;
+            # overwriting would leak its staging slots.
             return
 
-        anchor_lock_params = self.inc_host_lock_ref(last_host_node_id).to_dec_params()
+        # Buffer mode holds no tree state during the fetch: buffers are
+        # operation-owned, so the anchor needs no pin.
+        anchor_lock_params = (
+            None
+            if buffer_mode
+            else self.inc_host_lock_ref(last_host_node_id).to_dec_params()
+        )
         comp_xfers: dict[ComponentType, list[PoolTransfer]] = {}
         alloc_failed = False
         for ct in self.tree_components:
@@ -1517,10 +1627,21 @@ class UnifiedRadixCache(BasePrefixCache):
             CacheTransferPhase.PREFETCH, kv_xfer, comp_xfers
         )
         if alloc_failed:
+            # The whole storage fetch is forfeited over one aux staging
+            # alloc (e.g. a single SWA window) — count it, or write-burst
+            # starvation of the aux pool reads as generic hit-rate loss.
+            if (
+                self.enable_storage_metrics
+                and self.storage_metrics_collector is not None
+            ):
+                self.storage_metrics_collector.log_prefetch_aux_alloc_failed_tokens(
+                    len(prefetch_key)
+                )
             self.cache_controller.append_host_mem_release(
                 extra_pools=[x for xfers in comp_xfers.values() for x in xfers],
             )
-            self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
+            if anchor_lock_params is not None:
+                self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
             return
 
         aux_xfers = [x for xfers in comp_xfers.values() for x in xfers]
@@ -1532,6 +1653,13 @@ class UnifiedRadixCache(BasePrefixCache):
             prefix_keys,
             extra_pools=aux_xfers or None,
         )
+        stats["issued"] += 1
+        # Snapshots for the L3 miss accounting at the query outcome (the
+        # hit/revoke drains): requested span and total prompt length.
+        operation.stats_requested_tokens = prefetch_length
+        operation.stats_total_tokens = prefetch_length + len(
+            matched_prefix_tokens or []
+        )
         self.ongoing_prefetch[req_id] = _OngoingPrefetch(
             last_host_node_id,
             prefetch_key,
@@ -1540,7 +1668,12 @@ class UnifiedRadixCache(BasePrefixCache):
             anchor_lock_params,
             comp_xfers,
         )
-        self.cache_controller.prefetch_tokens_occupied += len(prefetch_key)
+        if buffer_mode:
+            self.buffer_pipeline.set_prefix_ctx(req_id, matched_prefix_tokens)
+        else:
+            # Cache mode reserves the requested span up front; buffer mode
+            # grants occupancy later at hit-alloc time, sized to the hit.
+            self.cache_controller.prefetch_tokens_occupied += len(prefetch_key)
 
     def _prefetch_timeout_check_linear_func(self, operation: PrefetchOperation) -> bool:
         return (
@@ -1622,6 +1755,16 @@ class UnifiedRadixCache(BasePrefixCache):
             # Hybrid all-or-nothing check failed; result already discarded.
             return True
 
+        if self.buffer_pipeline is not None:
+            # No graft: release the rank-local tail beyond the synced usable
+            # length, then park the bounce for admission-time consumption.
+            self.cache_controller.append_host_mem_release(
+                host_indices[min_completed_tokens:completed_tokens]
+            )
+            return self.buffer_pipeline.stage_completed_prefetch(
+                req_id, min_completed_tokens, hash_value
+            )
+
         fetched_key = prefetch_key[:min_completed_tokens]
         insert_result = self.tree_core.insert_host(
             last_host_node_id,
@@ -1751,9 +1894,14 @@ class UnifiedRadixCache(BasePrefixCache):
                 host_indices=host_indices[:completed_tokens],
                 extra_pools=pool_transfers,
             )
-            self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
+            if anchor_lock_params is not None:
+                self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
             del self.ongoing_prefetch[req_id]
-            self.cache_controller.prefetch_tokens_occupied -= len(prefetch_key)
+            if self.buffer_pipeline is not None:
+                self.buffer_pipeline.pop_prefix_ctx(req_id)
+            self.cache_controller.prefetch_tokens_occupied -= (
+                self._prefetch_occupied_span(prefetch_key, host_indices)
+            )
             self.prefetch_loaded_tokens_by_reqid[req_id] = 0
             logger.warning(
                 "HiCache hybrid prefetch discarded req=%s completed=%d requested=%d",
@@ -1773,8 +1921,27 @@ class UnifiedRadixCache(BasePrefixCache):
     def pop_prefetch_loaded_tokens(self, req_id: str) -> int:
         return self.prefetch_loaded_tokens_by_reqid.pop(req_id, 0)
 
+    def staged_prefetch_tokens(self, req_id: str) -> int:
+        """Tokens a staged buffer-mode prefetch would splice (0 = no hold);
+        surfaced by the scheduler as the request's host_hit_length."""
+        if self.buffer_pipeline is None:
+            return 0
+        return self.buffer_pipeline.staged_prefetch_tokens(req_id)
+
+    def staged_prefetch_swa_tokens(self, req_id: str) -> int:
+        """SWA device tokens consuming a staged buffer-mode prefetch will
+        allocate; surfaced as the request's swa_host_hit_length."""
+        if self.buffer_pipeline is None:
+            return 0
+        return self.buffer_pipeline.staged_prefetch_swa_tokens(req_id)
+
     def release_aborted_request(self, rid: str) -> None:
         self.prefetch_loaded_tokens_by_reqid.pop(rid, None)
+        if (
+            self.buffer_pipeline is not None
+            and self.buffer_pipeline.release_aborted_staged(rid)
+        ):
+            return
         if rid not in self.ongoing_prefetch:
             return
 
@@ -1793,13 +1960,73 @@ class UnifiedRadixCache(BasePrefixCache):
 
         completed_tokens, _ = self.cache_controller.terminate_prefetch(operation)
         self._barrier_attn_groups()
-        self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
+        if anchor_lock_params is not None:
+            self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
         del self.ongoing_prefetch[rid]
+        if self.buffer_pipeline is not None:
+            self.buffer_pipeline.pop_prefix_ctx(rid)
         self.cache_controller.append_host_mem_release(
             host_indices=host_indices[:completed_tokens],
             extra_pools=[x for xfers in comp_xfers.values() for x in xfers],
         )
-        self.cache_controller.prefetch_tokens_occupied -= len(prefetch_key)
+        # Buffer mode granted occupancy at hit-alloc, sized to the bounce;
+        # cache mode reserved the requested span at enqueue.
+        self.cache_controller.prefetch_tokens_occupied -= self._prefetch_occupied_span(
+            prefetch_key, host_indices
+        )
+
+    def _invalidate_absent_from_hit_query(self, operation) -> None:
+        """Drop KV beliefs beyond the folded usable cut (rank-synced): the
+        next insert then re-writes the node (all pools), healing stale
+        positives and aux holes at the cut through one FULL check."""
+        if self.host_memory_mode != "buffer_only":
+            return
+        chain = operation.all_hash_values
+        if chain is None:
+            return
+        self.storage_existence_cache.invalidate_beyond(
+            PoolName.KV, chain, keep_pages=operation.storage_hit_count // self.page_size
+        )
+
+    def _account_prefetch_outcome(self, operation, revoked: bool) -> None:
+        """Feed the cumulative prefetch-outcome counters at the (rank-synced)
+        query outcome: T = prompt tokens, L = requested, m = L3-miss."""
+        requested = operation.stats_requested_tokens
+        if requested <= 0:
+            return
+        stats = self._prefetch_outcome_stats
+        hit = max(0, min(operation.storage_hit_count, requested))
+        if revoked:
+            if hit > 0:
+                stats["revoked_insufficient"] += 1
+            else:
+                stats["revoked_full_miss"] += 1
+        miss = requested - hit
+        total = max(operation.stats_total_tokens, requested, 1)
+        stats["l3_demand_requests"] += 1
+        stats["l1l2_miss_tokens"] += requested
+        stats["l3_miss_tokens"] += miss
+        stats["l3_demand_total_tokens"] += total
+        stats["l3_sum_rate_all"] += miss / total
+        stats["l3_sum_rate_main_weighted"] += (miss / requested) * total
+
+    def prefetch_outcome_stats_snapshot(self) -> dict:
+        """Cumulative counters + instantaneous occupancy, in the schema
+        log_prefetch_stats consumers expect."""
+        cc = self.cache_controller
+        cap = max(cc.prefetch_capacity_limit, 1)
+        return {
+            **self._prefetch_outcome_stats,
+            "occupancy_ratio": cc.prefetch_tokens_occupied / cap,
+        }
+
+    def _prefetch_occupied_span(self, prefetch_key, host_indices) -> int:
+        """Occupancy units held by a prefetch: cache mode reserves the
+        requested span at enqueue; buffer mode grants at hit-alloc, sized
+        to the allocation (0 while still querying / parked)."""
+        if self.host_memory_mode == "buffer_only":
+            return len(host_indices) if host_indices is not None else 0
+        return len(prefetch_key)
 
     def _revoke_pending_prefetch(self, req_id: str) -> None:
         info = self.ongoing_prefetch.pop(req_id, None)
@@ -1809,17 +2036,27 @@ class UnifiedRadixCache(BasePrefixCache):
             last_host_node_id,
             prefetch_key,
             _host_indices,
-            _operation,
+            operation,
             anchor_lock_params,
             comp_xfers,
         ) = info
+        self._invalidate_absent_from_hit_query(operation)
+        if self.buffer_pipeline is not None:
+            self.buffer_pipeline.pop_prefix_ctx(req_id)
         cc = self.cache_controller
         cc.append_host_mem_release(
             extra_pools=[x for xfers in comp_xfers.values() for x in xfers]
         )
-        self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
+        if anchor_lock_params is not None:
+            self.dec_host_lock_ref(last_host_node_id, anchor_lock_params)
+        # Every revoke path runs before the bounce alloc, so buffer mode
+        # holds no occupancy here; post-alloc aborts go through
+        # release_aborted_request instead.
+        assert _host_indices is None or self.host_memory_mode != "buffer_only"
         cc.prefetch_tokens_occupied = max(
-            0, cc.prefetch_tokens_occupied - len(prefetch_key)
+            0,
+            cc.prefetch_tokens_occupied
+            - self._prefetch_occupied_span(prefetch_key, _host_indices),
         )
 
     def _drain_storage_control_queues_impl(
@@ -1842,56 +2079,100 @@ class UnifiedRadixCache(BasePrefixCache):
                 drained += 1
                 yield item
 
+        buffer_mode = self.host_memory_mode == "buffer_only"
+
+        def _try_alloc_storage_hit(operation) -> bool:
+            """Allocate the hit-sized bounce and launch the transfer.
+            Returns False when staging pressure defers the allocation
+            (buffer mode parks and retries; cache mode revokes)."""
+            req_id = operation.request_id
+            info = self.ongoing_prefetch.get(req_id)
+            if info is None:
+                return True  # aborted/cleaned; nothing to retry
+            if operation.is_terminated():
+                self._revoke_pending_prefetch(req_id)
+                return True
+
+            if buffer_mode and cc.prefetch_rate_limited():
+                # Pool is load-saturated: hold the KNOWN hit until staged
+                # prefetches ahead of us are consumed. The op stays in
+                # ongoing_prefetch, so wait_complete keeps gating admission.
+                return False
+            alloc_len = operation.storage_hit_count
+            host_indices = cc.mem_pool_host.alloc(alloc_len)
+            if host_indices is None:
+                self.evict_host(alloc_len)
+                host_indices = cc.mem_pool_host.alloc(alloc_len)
+            if host_indices is None and not buffer_mode:
+                # Memory-pressure fallback: a shorter page-aligned prefix.
+                # (Cache mode only — buffer mode parks for the full hit.)
+                available_size = cc.mem_pool_host.available_size()
+                alloc_len = min(
+                    operation.storage_hit_count,
+                    available_size - (available_size % self.page_size),
+                )
+                if alloc_len >= self.prefetch_threshold:
+                    host_indices = cc.mem_pool_host.alloc(alloc_len)
+            if host_indices is None:
+                if buffer_mode:
+                    return False
+                self._revoke_pending_prefetch(req_id)
+                return True
+
+            operation.storage_hit_count = alloc_len
+            operation.hash_value = operation.hash_value[: alloc_len // self.page_size]
+            operation.host_indices = host_indices
+            self.ongoing_prefetch[req_id] = info._replace(host_indices=host_indices)
+            if buffer_mode:
+                cc.prefetch_tokens_occupied += alloc_len
+            cc.prefetch_buffer.put(operation)
+            return True
+
         def _drain_and_alloc_storage_hit():
+            # Parked hits first (FIFO fairness with retries; buffer only).
+            if buffer_mode:
+                parked = self.buffer_pipeline.pending_hit_allocs
+                while parked:
+                    if not _try_alloc_storage_hit(parked[0]):
+                        break
+                    parked.popleft()
             for operation in _drain_queue(cc.prefetch_hit_queue, n_storage_hit):
                 req_id = operation.request_id
                 info = self.ongoing_prefetch.get(req_id)
                 if info is None:
-                    # request already aborted/cleaned up, skip
+                    # Request already aborted/cleaned up; still flush the
+                    # query's absent-hash feedback.
+                    self._invalidate_absent_from_hit_query(operation)
                     continue
                 if operation.is_terminated():
-                    # request was aborted while the storage query was in flight
+                    # Aborted while the storage query was in flight.
                     self._revoke_pending_prefetch(req_id)
                     continue
                 if operation.storage_hit_count < self.prefetch_threshold:
-                    # not to prefetch if not enough benefits
+                    # Below-threshold hit: classify + feed the L3 miss
+                    # accounting, then revoke (not enough benefit).
+                    self._account_prefetch_outcome(operation, revoked=True)
                     self._revoke_pending_prefetch(req_id)
                     continue
-
+                self._invalidate_absent_from_hit_query(operation)
-                alloc_len = operation.storage_hit_count
+                self._account_prefetch_outcome(operation, revoked=False)
-                host_indices = cc.mem_pool_host.alloc(alloc_len)
+                if not _try_alloc_storage_hit(operation):
-                if host_indices is None:
+                    # Counted once at first parking, not per retry tick.
-                    self.evict_host(alloc_len)
+                    self._prefetch_outcome_stats["declined_rate_limited"] += 1
-                    host_indices = cc.mem_pool_host.alloc(alloc_len)
+                    self.buffer_pipeline.pending_hit_allocs.append(operation)
-                if host_indices is None:
-                    # Memory-pressure fallback: a shorter page-aligned prefix.
-                    available_size = cc.mem_pool_host.available_size()
-                    alloc_len = min(
-                        operation.storage_hit_count,
-                        available_size - (available_size % self.page_size),
-                    )
-                    if alloc_len >= self.prefetch_threshold:
-                        host_indices = cc.mem_pool_host.alloc(alloc_len)
-                if host_indices is None:
-                    self._revoke_pending_prefetch(req_id)
-                    continue
-
-                operation.storage_hit_count = alloc_len
-                operation.hash_value = operation.hash_value[
-                    : alloc_len // self.page_size
-                ]
-                operation.host_indices = host_indices
-                self.ongoing_prefetch[req_id] = info._replace(host_indices=host_indices)
-                cc.prefetch_buffer.put(operation)
 
         def _drain_backup():
             drained = 0
             for operation in _drain_queue(cc.ack_backup_queue, n_backup):
                 drained += 1
-                entry = self.ongoing_backup.pop(operation.id, None)
+                if buffer_mode:
-                if entry is not None:
+                    # Storage write acked: free the staging.
-                    node_id, lock_params = entry
+                    self.buffer_pipeline.finish_storage_write_ack(operation.id)
-                    self.dec_host_lock_ref(node_id, lock_params)
+                else:
+                    entry = self.ongoing_backup.pop(operation.id, None)
+                    if entry is not None:
+                        node_id, lock_params = entry
+                        self.dec_host_lock_ref(node_id, lock_params)
                 if (
                     log_metrics
                     and self.enable_storage_metrics
@@ -2054,6 +2335,9 @@ class UnifiedRadixCache(BasePrefixCache):
             logger.error("Failed to clear hierarchical cache storage backend: %s", e)
             return False
         if ok:
+            # L3 is empty now: every storage-presence belief is stale, and a
+            # retained positive would skip that page's backup forever.
+            self.storage_existence_cache.clear()
             logger.info("Hierarchical cache storage backend cleared successfully!")
         return ok
 
@@ -2205,6 +2489,11 @@ class UnifiedRadixCache(BasePrefixCache):
             ack = cc.ack_load_queue.pop(0)
             ack.finish_event.synchronize()
             for ack_id in ack.node_ids:
+                if (
+                    self.buffer_pipeline is not None
+                    and self.buffer_pipeline.try_finish_load_back(ack_id)
+                ):
+                    continue
                 node, lock_params, host_lock_params = self.ongoing_load_back.pop(ack_id)
                 self.dec_lock_ref(node, lock_params)
                 self.dec_host_lock_ref(node, host_lock_params)
@@ -2233,7 +2522,10 @@ class UnifiedRadixCache(BasePrefixCache):
         params: InitLoadBackParams,
     ) -> tuple[torch.Tensor, NodeId]:
         """Prepare KV cache loading from host to device.
-        Returns (device_indices, last_node) tuple."""
+        Returns (device_indices, last_node). Buffer mode dispatches to the
+        staged-prefetch consumption (BufferModePipeline.init_load_back)."""
+        if self.buffer_pipeline is not None:
+            return self.buffer_pipeline.init_load_back(params)
         best_match_node_id = params.best_match_node
         mem_quota = params.mem_quota
         req = params.req
@@ -2306,10 +2598,16 @@ class UnifiedRadixCache(BasePrefixCache):
                     extra_release_counts=extra_release_counts,
                     log_metrics=True,
                 )
+        if self.buffer_pipeline is not None:
+            self.buffer_pipeline.flush_pending_writes()
         if self.enable_storage_metrics and self.storage_metrics_collector is not None:
-            self.storage_metrics_collector.log_storage_metrics(
+            storage_metrics = self.cache_controller.storage_backend.get_stats()
-                self.cache_controller.storage_backend.get_stats()
+            if storage_metrics is None:
-            )
+                # Backends without native stats (e.g. file) still carry the
+                # controller-side prefetch outcome counters.
+                storage_metrics = StorageMetrics()
+            storage_metrics.prefetch_stats = self.prefetch_outcome_stats_snapshot()
+            self.storage_metrics_collector.log_storage_metrics(storage_metrics)
 
     def ready_to_load_host_cache(self) -> int:
         """Notify the cache controller to start the KV cache loading."""
@@ -2464,9 +2762,15 @@ class UnifiedRadixCache(BasePrefixCache):
 
         # Pass ongoing ops as lightweight (id, node_id) pairs so the tree core
         # can resolve + validate them without reaching into Controller state.
-        ongoing_write_through = [
+        if self.buffer_pipeline is not None:
-            (nid, wt.node_id) for nid, wt in self.ongoing_write_through.items()
+            ongoing_write_through = [
-        ]
+                (nid, entry.intent.node_id)
+                for nid, entry in self.buffer_pipeline.ongoing_write_through.items()
+            ]
+        else:
+            ongoing_write_through = [
+                (nid, wt.node_id) for nid, wt in self.ongoing_write_through.items()
+            ]
         ongoing_load_back = [
             (nid, lb.node_id) for nid, lb in self.ongoing_load_back.items()
         ]

python/sglang/srt/observability/metrics_collector.py

@@ -1852,9 +1852,11 @@ class StorageMetricsCollector(_StatLoggerDIMixin):
         labels: Dict[str, str],
     ):
         from prometheus_client import Counter as _PromCounter
+        from prometheus_client import Gauge as _PromGauge
         from prometheus_client import Histogram as _PromHistogram
 
         Counter = self._counter_cls or _PromCounter
+        Gauge = self._gauge_cls or _PromGauge
         Histogram = self._histogram_cls or _PromHistogram
 
         self.labels = labels
@@ -1871,6 +1873,21 @@ class StorageMetricsCollector(_StatLoggerDIMixin):
             labelnames=labels.keys(),
         )
 
+        self.backup_dropped_tokens_total = Counter(
+            name="sglang:hicache_backup_dropped_tokens_total",
+            documentation="Buffer-mode backup tokens dropped by write-path rate "
+            "limiting (backlog cap or dropped-parent cascade).",
+            labelnames=labels.keys(),
+        )
+
+        self.prefetch_aux_alloc_failed_tokens_total = Counter(
+            name="sglang:hicache_prefetch_aux_alloc_failed_tokens_total",
+            documentation="Prefetch tokens abandoned because an aux pool "
+            "(e.g. the SWA trailing window) could not allocate host staging "
+            "— the whole storage fetch is forfeited, not just the aux part.",
+            labelnames=labels.keys(),
+        )
+
         bucket_io = [
             1,
             5,
@@ -1925,6 +1942,16 @@ class StorageMetricsCollector(_StatLoggerDIMixin):
         if backuped_tokens > 0:
             self.backuped_tokens_total.labels(**self.labels).inc(backuped_tokens)
 
+    def log_backup_dropped_tokens(self, dropped_tokens: int):
+        if dropped_tokens > 0:
+            self.backup_dropped_tokens_total.labels(**self.labels).inc(dropped_tokens)
+
+    def log_prefetch_aux_alloc_failed_tokens(self, num_tokens: int):
+        if num_tokens > 0:
+            self.prefetch_aux_alloc_failed_tokens_total.labels(**self.labels).inc(
+                num_tokens
+            )
+
     def _log_histogram(self, histogram, data: Union[int, float]):
         histogram.labels(**self.labels).observe(data)
 

python/sglang/srt/server_args.py

@@ -2668,14 +2668,22 @@ class ServerArgs:
     enable_hierarchical_cache: A[bool, "Enable hierarchical cache", NS("memory")] = (
         False
     )
+    hicache_host_memory_mode: A[
+        str,
+        Arg(
+            help="Whether host memory is a persistent HiCache tier (cache) or a transient staging buffer between GPU and the storage backend (buffer_only). buffer_only requires --hicache-storage-backend.",
+            choices=["cache", "buffer_only"],
+        ),
+        NS("memory"),
+    ] = "cache"
     hicache_ratio: A[
         Optional[float],
-        "The ratio of the size of host KV cache memory pool to the size of device pool. Defaults to 2.0, or 1.0 for host-pool decode retraction.",
+        "The ratio of the size of host KV cache memory pool to the size of device pool. Defaults to 2.0 in cache mode, 1.2 in buffer_only mode, or 1.0 for host-pool decode retraction.",
         NS("memory"),
     ] = None
     hicache_size: A[
         int,
-        "The size of host KV cache memory pool in gigabytes, which will override the hicache_ratio if set.",
+        "The size of host KV cache memory pool in gigabytes. Overrides --hicache-ratio in either host memory mode.",
         NS("memory"),
     ] = 0
     hicache_write_policy: A[
@@ -2714,6 +2722,7 @@ class ServerArgs:
             help="The storage backend for hierarchical KV cache. Built-in backends: file, mooncake, hf3fs, nixl, aibrix. For dynamic backend, use --hicache-storage-backend-extra-config to specify: backend_name (custom name), module_path (Python module path), class_name (backend class name).",
             choices=[
                 "file",
+                "sim",
                 "mooncake",
                 "hf3fs",
                 "nixl",
@@ -7393,8 +7402,20 @@ class ServerArgs:
             )
 
     def _handle_hicache_ratio_default(self):
+        """Default the host/device ratio per host memory mode.
+
+        Runs before the dummy-model boundary: direct HostKVCache consumers
+        (unit fixtures, dummy-model launches) must never see a None ratio.
+        buffer_only stages in flight rather than retaining, so it needs only
+        enough to cover the write backlog plus parked prefetches.
+
+        A decode server keeps the ratio unset here: kv_cache_builder resolves
+        it against the retraction-backup backend (1.0 for host_pool, else 2.0).
+        """
         if self.hicache_ratio is None and self.disaggregation_mode != "decode":
-            self.hicache_ratio = 2.0
+            self.hicache_ratio = (
+                1.2 if self.hicache_host_memory_mode == "buffer_only" else 2.0
+            )
 
     def _handle_hicache(self):
         """Normalize hicache-related knobs into a valid runtime configuration.
@@ -7414,6 +7435,8 @@ class ServerArgs:
         ):
             return
 
+        self._validate_hicache_host_memory_mode()
+
         # Step 1: Initial layout-io compatibility normalization.
         self._resolve_layout_io_compatibility()
 
@@ -7423,6 +7446,51 @@ class ServerArgs:
         # Step 3: DCP compatibility for the L2 (device<->host) path.
         self._resolve_hicache_dcp_compatibility()
 
+    def _validate_hicache_host_memory_mode(self):
+        if self.hicache_host_memory_mode not in ("cache", "buffer_only"):
+            raise ValueError(
+                "hicache_host_memory_mode must be 'cache' or 'buffer_only', "
+                f"got {self.hicache_host_memory_mode!r}"
+            )
+
+        # Both modes are defaulted upstream (a decode server resolves the
+        # ratio later, in kv_cache_builder), so this fires only if that
+        # defaulting regresses -- never build an unsized host pool.
+        if (
+            self.hicache_size <= 0
+            and self.hicache_ratio is None
+            and self.disaggregation_mode != "decode"
+        ):
+            raise ValueError(
+                f"--hicache-host-memory-mode {self.hicache_host_memory_mode} "
+                "requires a host pool size: pass --hicache-size or "
+                "--hicache-ratio."
+            )
+
+        if self.hicache_host_memory_mode == "cache":
+            return
+
+        if self.hicache_storage_backend is None:
+            raise ValueError(
+                "--hicache-host-memory-mode buffer_only requires a storage backend "
+                "(--hicache-storage-backend): host memory is only a staging buffer "
+                "and all cached data lives in storage."
+            )
+        if self.hicache_write_policy == "write_back":
+            raise ValueError(
+                "--hicache-host-memory-mode buffer_only does not support "
+                "--hicache-write-policy write_back; use write_through or "
+                "write_through_selective."
+            )
+        if self.disaggregation_mode == "decode":
+            raise ValueError(
+                "--hicache-host-memory-mode buffer_only is not supported on "
+                "decode instances: the decode-side prefetch and offload paths "
+                "bypass the buffer-mode pipeline, fetching without its prefix "
+                "context and never consuming its staged holds. Prefill "
+                "instances share the standard scheduler path and are supported."
+            )
+
     def _resolve_hicache_dcp_compatibility(self):
         if self.dcp_size <= 1 or not self.enable_hierarchical_cache:
             return

test/registered/unit/mem_cache/test_unified_radix_cache_unittest.py

@@ -92,8 +92,8 @@ from sglang.srt.utils import get_device
 from sglang.test.ci.ci_register import register_amd_ci, register_cuda_ci
 from sglang.test.test_utils import CustomTestCase
 
-register_cuda_ci(est_time=16, stage="base-b", runner_config="1-gpu-small")
+register_cuda_ci(est_time=50, stage="base-b", runner_config="1-gpu-small")
-register_amd_ci(est_time=16, suite="stage-b-test-1-gpu-small-amd")
+register_amd_ci(est_time=50, suite="stage-b-test-1-gpu-small-amd")
 
 
 @dataclass(frozen=True)
@@ -595,39 +595,20 @@ class TestUnifiedRadixCacheKVEvents(CustomTestCase):
         return leaf
 
     def _init_hicache(self, cache, *, write_policy: str = "write_through"):
-        import sglang.srt.mem_cache.hybrid_cache.hybrid_pool_assembler as assembler
+        # Production config: kernel IO backend + page_first layout with
-
+        # PINNED host pools (kernel GPU DMA requires them). Pools track their
-        # Wrap the host-pool factory (not MHATokenToKVPoolHost directly)
+        # cudaHostRegister'd pointers and unregister on destroy()/GC, so the
-        # because the assembler picks between MHATokenToKVPoolHost and
+        # many fixtures sharing this process cannot collide on recycled
-        # AsymmetricMHATokenToKVPoolHost via get_mha_host_pool_cls(device_pool).
+        # address ranges (rc=712).
-        orig_get_mha_host_pool_cls = assembler.get_mha_host_pool_cls
-
-        def get_mha_host_pool_cls_wrapper(device_pool):
-            host_pool_cls = orig_get_mha_host_pool_cls(device_pool)
-
-            def kv_host_pool_wrapper(*args, **kwargs):
-                kwargs["pin_memory"] = False
-                return host_pool_cls(*args, **kwargs)
-
-            return kv_host_pool_wrapper
-
-        patcher = mock.patch.object(
-            assembler,
-            "get_mha_host_pool_cls",
-            side_effect=get_mha_host_pool_cls_wrapper,
-        )
-        patcher.start()
-        self.addCleanup(patcher.stop)
-
         server_args = ServerArgs(
             model_path="dummy",
             page_size=self.cfg.page_size,
-            hicache_io_backend="direct",
+            hicache_io_backend="kernel",
-            hicache_mem_layout="page_first_direct",
             hicache_write_policy=write_policy,
         )
         set_global_server_args_for_scheduler(server_args)
         cache.init_hicache(server_args, cache.cache_init_params)
+        self.addCleanup(cache.release_host_resources)
         cache.write_through_threshold = 1 << 30
         cache.load_back_threshold = 0
 
@@ -2468,28 +2449,87 @@ class UnifiedRadixCacheSuite:
         for n in self._path_chain(cache, node):
             cache.write_backup_storage(n.id)
 
+    def _ongoing_l3_backups(self, cache):
+        """Storage writes in flight (buffer mode tracks them on the pipeline)."""
+        if cache.buffer_pipeline is not None:
+            return cache.buffer_pipeline.ongoing_backup
+        return cache.ongoing_backup
+
     def _flush_l3_backups(self, cache, timeout: float = 10.0):
         """Wait for backup threads to finish, then drain acks (release locks)."""
         deadline = time.time() + timeout
-        while cache.ongoing_backup and time.time() < deadline:
+        while self._ongoing_l3_backups(cache) and time.time() < deadline:
             cache.drain_storage_control_queues()
-            if cache.ongoing_backup:
+            if self._ongoing_l3_backups(cache):
                 time.sleep(0.01)
         cache.drain_storage_control_queues()
-        self.assertFalse(cache.ongoing_backup, "L3 backups did not complete in time")
+        self.assertFalse(
+            self._ongoing_l3_backups(cache), "L3 backups did not complete in time"
+        )
 
     def _run_prefetch_to_completion(self, cache, req_id, timeout: float = 10.0):
         deadline = time.time() + timeout
         while time.time() < deadline:
             # Host memory is reserved (and IO started) by the scheduler-thread
-            # drain once the L3 hit count is known, so pump it like the real
+            # drain once the L3 hit count is known, and the buffer-mode fill
-            # scheduler loop does (check_hicache_events before progress checks).
+            # commits at its H2D ack in loading_check — pump the full event
-            cache.drain_storage_control_queues()
+            # round like the real scheduler loop does.
+            cache.check_hicache_events()
             if cache.check_prefetch_progress(req_id):
+                # Buffer mode parks a completed fetch as a staged prefetch;
+                # consume it like the PrefillAdder would so callers see the
+                # span tree-resident.
+                if (
+                    cache.buffer_pipeline is not None
+                    and cache.buffer_pipeline.has_staged(req_id)
+                ):
+                    self._consume_staged_prefetch(cache, req_id, timeout=timeout)
                 return
             time.sleep(0.01)
         self.fail(f"prefetch {req_id} did not complete in time")
 
+    def _consume_staged_prefetch(
+        self, cache, req_id, prefix_len=None, prefix_indices=None, timeout: float = 10.0
+    ):
+        """Simulate the PrefillAdder consuming a staged prefetch at admission:
+        init_load_back (buffer dispatch: device alloc + queued H2D), the batch
+        start_loading flush, then pump until the ack commit lands. Returns
+        the spliced device indices (empty on degrade)."""
+        from sglang.srt.mem_cache.base_prefix_cache import InitLoadBackParams
+
+        f = cache.buffer_pipeline.staged_prefetches[req_id]
+        if prefix_len is None:
+            prefix_len = f.matched_len
+        req = mock.Mock()
+        req.rid = req_id
+        if prefix_indices is not None:
+            # Spliceable mid-anchor consumption publishes value=cat(prefix,
+            # fill) — the real device prefix is required (zeros would insert
+            # bogus slots into the tree).
+            assert len(prefix_indices) == prefix_len
+            req.prefix_indices = prefix_indices
+        else:
+            req.prefix_indices = torch.zeros(
+                prefix_len,
+                dtype=torch.int64,
+                device=cache.tree_core.empty_match_result.device_indices.device,
+            )
+        req.last_node = cache.root_node.id
+        new_indices, _last_node = cache.init_load_back(
+            InitLoadBackParams(
+                best_match_node=None, host_hit_length=f.num_tokens, req=req
+            )
+        )
+        # Batch formation flushes the queued load into the batch's producer.
+        cache.ready_to_load_host_cache()
+        self._pump_hicache_until(
+            cache,
+            lambda: not cache.buffer_pipeline.ongoing_buffer_load_back,
+            "staged-prefetch consumption did not commit",
+            timeout=timeout,
+        )
+        return new_indices
+
     def _all_page_hashes(self, cache, node):
         hashes = []
         for n in self._path_chain(cache, node):
@@ -2606,6 +2646,494 @@ class UnifiedRadixCacheSuite:
         self.assertTrue(torch.equal(loaded_v, expected_v))
         cons.sanity_check()
 
+    # ================================================================
+    # Buffer-only host memory mode (host = transient staging, L3 = cache)
+    # ================================================================
+
+    def _init_buffer_hicache(
+        self,
+        cache,
+        storage_dir,
+        prefetch_policy: str = "wait_complete",
+        storage_extra: Optional[dict] = None,
+    ):
+        if self.cfg.has_mamba:
+            self.skipTest(
+                "buffer_only is FULL/SWA-only (no Mamba state-handoff channel "
+                "on the admission-time load-back read path)"
+            )
+        self._init_hicache(
+            cache,
+            storage_backend="file",
+            storage_dir=storage_dir,
+            prefetch_threshold=1,
+            host_memory_mode="buffer_only",
+            prefetch_policy=prefetch_policy,
+            storage_extra=storage_extra,
+        )
+
+    def _pump_hicache_until(self, cache, cond, msg, timeout: float = 10.0):
+        deadline = time.time() + timeout
+        while time.time() < deadline:
+            cache.check_hicache_events()
+            if cond():
+                return
+            time.sleep(0.01)
+        self.fail(msg)
+
+    def _host_avail_sizes(self, cache):
+        group = cache.cache_controller.mem_pool_host
+        return {entry.name: entry.host_pool.available_size() for entry in group.entries}
+
+    def _storage_exists_count(self, cache, page_hashes, pool_transfers=None):
+        """Ground-truth longest-prefix existence count from the backend,
+        folded across pools like the prefetch hit query."""
+        from sglang.srt.mem_cache.hicache_storage import HiCacheStorageExtraInfo
+
+        backend = cache.cache_controller.storage_backend
+        extra_info = HiCacheStorageExtraInfo(prefix_keys=None)
+        if pool_transfers:
+            result = backend.batch_exists_v2(page_hashes, pool_transfers, extra_info)
+            return min(result.kv_hit_pages, len(page_hashes))
+        return backend.batch_exists(page_hashes, extra_info)
+
+    def _buffer_backup_and_wait(self, cache, node):
+        # Parent-first over the whole path, mirroring the production trigger
+        # (_inc_hit_count fires per matched node on the insert walk): the SWA
+        # component may have split the leaf at the window boundary.
+        pipeline = cache.buffer_pipeline
+        for n in self._path_chain(cache, node):
+            pipeline.enqueue_backup_intent(n)
+            self.assertIn(n.id, pipeline.inflight_backup_node_ids)
+        self._pump_hicache_until(
+            cache,
+            lambda: not pipeline.inflight_backup_node_ids
+            and not pipeline.ongoing_backup,
+            "buffer backup pipeline did not drain",
+        )
+
+    def _produce_buffer_l3(self, storage_dir, seq, marker=None):
+        """Producer tree in buffer mode: insert seq and push it to L3."""
+        prod, prod_alloc, prod_rtp = build_fixture(self.cfg)
+        self._init_buffer_hicache(prod, storage_dir)
+        self._insert(prod, prod_alloc, prod_rtp, seq)
+        leaf = prod.resolve_node_handle(
+            prod.match_prefix(
+                MatchPrefixParams(key=RadixKey(array("q", seq)))
+            ).last_device_node
+        )
+        expected = None
+        if marker is not None:
+            m = prod.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq))))
+            self._fill_full_kv(prod_alloc, m.device_indices, marker=marker)
+            expected = self._snapshot_full_kv(prod_alloc, m.device_indices)
+        self._buffer_backup_and_wait(prod, leaf)
+        return leaf, expected
+
+    def _buffer_swa_seq(self, min_pages=4):
+        """Sequence long enough for SWA prefetch (one full window + 1)."""
+        num_pages = min_pages
+        if self.cfg.has_swa:
+            sw_pages = (
+                self.cfg.sliding_window_size + self.cfg.page_size - 1
+            ) // self.cfg.page_size
+            num_pages = max(num_pages, sw_pages + 1)
+        return self._make_seq(1, num_pages)
+
+    def test_buffer_only_write_path_roundtrip(self):
+        """Write path end to end: admission -> D2H staging -> storage write
+        -> free. Staging and locks fully released, pages stored under every
+        pool namespace, beliefs registered, re-hits skipped."""
+        self._skip_unsupported_hicache_test()
+        storage_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, storage_dir, ignore_errors=True)
+
+        cache, allocator, req_to_token_pool = build_fixture(self.cfg)
+        self._init_buffer_hicache(cache, storage_dir)
+        avail0 = self._host_avail_sizes(cache)
+
+        seq_a = self._make_seq(1, 2)
+        seq_ab = seq_a + self._make_seq(500, 2)
+        self._insert(cache, allocator, req_to_token_pool, seq_a)
+        self._insert(cache, allocator, req_to_token_pool, seq_ab)
+        leaf = cache.resolve_node_handle(
+            cache.match_prefix(
+                MatchPrefixParams(key=RadixKey(array("q", seq_ab)))
+            ).last_device_node
+        )
+        chain = self._path_chain(cache, leaf)
+
+        self._buffer_backup_and_wait(cache, leaf)
+        self.assertFalse(leaf.backuped)
+        self.assertEqual(leaf.component_data[ComponentType.FULL].lock_ref, 0)
+        self.assertEqual(self._host_avail_sizes(cache), avail0)
+        self.assertEqual(cache.buffer_pipeline.write_backlog_tokens_, 0)
+        page_hashes = self._all_page_hashes(cache, leaf)
+        self.assertEqual(
+            self._storage_exists_count(
+                cache,
+                page_hashes,
+                cache.buffer_pipeline._build_aux_staging_transfers(leaf),
+            ),
+            len(page_hashes),
+        )
+        for n in chain:
+            self.assertTrue(
+                cache.storage_existence_cache.contains_all(PoolName.KV, n.hash_value)
+            )
+        # Re-hit absorbed by the (FULL-focused) belief skip.
+        cache.buffer_pipeline.enqueue_backup_intent(leaf)
+        self.assertNotIn(leaf.id, cache.buffer_pipeline.inflight_backup_node_ids)
+        cache.sanity_check()
+
+    def test_buffer_only_read_path_roundtrip(self):
+        """Read path end to end: prefetch -> staged (host bounce only,
+        nothing device-side, unmatchable, stable readiness, counters fed) ->
+        admission-time load-back against a saturated-but-evictable pool
+        (evict-before-alloc) publishing pre-ack -> ack frees the bounce.
+        Data bytes match the producer's; no CPU-tier KV events anywhere;
+        declines feed the outcome counters."""
+        self._skip_unsupported_hicache_test()
+        storage_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, storage_dir, ignore_errors=True)
+
+        seq = self._buffer_swa_seq()
+        _, (expected_k, expected_v) = self._produce_buffer_l3(
+            storage_dir, seq, marker=7
+        )
+
+        cons, cons_alloc, cons_rtp = build_fixture(
+            self.cfg, enable_kv_cache_events=True
+        )
+        self._init_buffer_hicache(cons, storage_dir)
+        cons.take_events()
+        avail0 = self._host_avail_sizes(cons)
+        dev_avail0 = cons.token_to_kv_pool_allocator.available_size()
+        stats = cons._prefetch_outcome_stats
+
+        req_id = "buffer-read-roundtrip"
+        cons.prefetch_from_storage(
+            req_id, cons.root_node.id, array("q", seq), None, None
+        )
+        self.assertEqual((stats["attempts"], stats["issued"]), (1, 1))
+        self._pump_hicache_until(
+            cons,
+            lambda: cons.check_prefetch_progress(req_id)
+            and cons.buffer_pipeline.has_staged(req_id),
+            "prefetch did not stage",
+        )
+        # Staged: bounce occupies host staging; nothing device-side; span
+        # unmatchable; readiness stable; hit accounting reported once.
+        self.assertNotEqual(self._host_avail_sizes(cons), avail0)
+        self.assertEqual(cons.token_to_kv_pool_allocator.available_size(), dev_avail0)
+        self.assertEqual(
+            len(
+                cons.match_prefix(
+                    MatchPrefixParams(key=RadixKey(array("q", seq)))
+                ).device_indices
+            ),
+            0,
+        )
+        self.assertTrue(cons.check_prefetch_progress(req_id))
+        self.assertEqual(cons.pop_prefetch_loaded_tokens(req_id), len(seq))
+        self.assertEqual(stats["l3_demand_requests"], 1)
+        self.assertEqual(stats["l3_miss_tokens"], 0)
+
+        # Saturate the device pool with unrelated evictable spans: the
+        # load-back must evict, not degrade to recompute (run-2 regression).
+        def _avail():
+            if cons.supports_swa():
+                return cons.token_to_kv_pool_allocator.full_available_size()
+            return cons.token_to_kv_pool_allocator.available_size()
+
+        filler_base = 90000
+        while _avail() >= self.cfg.page_size:
+            pages = max(1, min(2048, _avail()) // self.cfg.page_size)
+            self._insert(cons, cons_alloc, cons_rtp, self._make_seq(filler_base, pages))
+            filler_base += 1000
+        self.assertLess(_avail(), len(seq), "pool not saturated")
+
+        # Consume WITHOUT pumping the ack: published (matchable, same slot
+        # ids) before any ack lands.
+        from sglang.srt.mem_cache.base_prefix_cache import InitLoadBackParams
+
+        held = cons.buffer_pipeline.staged_prefetches[req_id]
+        req = mock.Mock()
+        req.rid = req_id
+        req.last_node = cons.root_node.id
+        req.prefix_indices = torch.zeros(
+            held.matched_len,
+            dtype=torch.int64,
+            device=cons.tree_core.empty_match_result.device_indices.device,
+        )
+        spliced, _last = cons.init_load_back(
+            InitLoadBackParams(
+                best_match_node=None, host_hit_length=held.num_tokens, req=req
+            )
+        )
+        self.assertEqual(int(spliced.numel()), len(seq))
+        cons.ready_to_load_host_cache()
+        m = cons.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq))))
+        self.assertTrue(torch.equal(m.device_indices, spliced))
+
+        # Ack: bounce freed, beliefs fed, tree holds no host values, and the
+        # loaded KV bytes equal the producer's.
+        self._pump_hicache_until(
+            cons,
+            lambda: not cons.buffer_pipeline.ongoing_buffer_load_back
+            and self._host_avail_sizes(cons) == avail0,
+            "load-back ack did not free the bounce",
+        )
+        mc = cons.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq))))
+        self.assertEqual(mc.host_hit_length, 0)
+        self.assertEqual(len(mc.device_indices), len(seq))
+        leaf = cons.resolve_node_handle(mc.last_device_node)
+        for cur in self._path_chain(cons, leaf):
+            for cd in cur.component_data:
+                self.assertIsNone(cd.host_value)
+        self.assertTrue(
+            cons.storage_existence_cache.contains_all(
+                PoolName.KV, self._all_page_hashes(cons, leaf)
+            )
+        )
+        loaded_k, loaded_v = self._snapshot_full_kv(cons_alloc, mc.device_indices)
+        self.assertTrue(torch.equal(loaded_k, expected_k))
+        self.assertTrue(torch.equal(loaded_v, expected_v))
+        self.assertEqual(cons.cache_controller.prefetch_tokens_occupied, 0)
+        cpu_events = [
+            e
+            for e in cons.take_events()
+            if isinstance(e, (BlockStored, BlockRemoved))
+            and e.medium == StorageMedium.CPU
+        ]
+        self.assertEqual(cpu_events, [])
+
+        self.assertIn("occupancy_ratio", cons.prefetch_outcome_stats_snapshot())
+        cons.sanity_check()
+
+    def test_buffer_load_back_swa_window_charged_at_admission(self):
+        """Admission contract: a request the SWA budget gate accepts must be
+        allocatable at batch time (_swa_reserved_tokens: "an admitted request
+        cannot OOM"). Regression: buffer mode surfaced a staged prefetch as
+        host_hit_length only, so the gate never charged the SWA window that
+        consumption (init_load_back -> cc.load) allocates and the request
+        lock pins; with the rest of the SWA pool batch-held, the batch alloc
+        fell short by up to one window and raised the fail-loud prefill OOM
+        (prod scheduler crash, 2026-08-17)."""
+        self._skip_unsupported_hicache_test()
+        if not self.cfg.has_swa:
+            self.skipTest("SWA-specific admission accounting")
+        from sglang.srt.mem_cache.allocation import (
+            alloc_paged_token_slots_extend,
+            alloc_token_slots,
+        )
+        from sglang.srt.mem_cache.base_prefix_cache import InitLoadBackParams
+
+        storage_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, storage_dir, ignore_errors=True)
+        ps = self.cfg.page_size
+        window = self.cfg.sliding_window_size
+        seq = self._buffer_swa_seq()  # one full window + 1 page
+        self._produce_buffer_l3(storage_dir, seq)
+
+        cons, cons_alloc, _ = build_fixture(self.cfg)
+        self._init_buffer_hicache(cons, storage_dir)
+        req_id = "buffer-swa-admission-oom"
+        cons.prefetch_from_storage(
+            req_id, cons.root_node.id, array("q", seq), None, None
+        )
+        self._pump_hicache_until(
+            cons,
+            lambda: cons.check_prefetch_progress(req_id)
+            and cons.buffer_pipeline.has_staged(req_id),
+            "prefetch did not stage",
+        )
+
+        # Batch-held SWA (chunk allocs, decode windows) is neither free nor
+        # evictable: leave one token less than window + extend_need, enough
+        # for an un-charged gate to accept.
+        extend_need = 2 * ps + 1
+        max_new = 8
+        self.assertIsNotNone(
+            cons_alloc.swa_attn_allocator.alloc(
+                cons_alloc.swa_available_size() - (window + extend_need - 1)
+            )
+        )
+
+        # The adder's SWA gate for this request (_swa_budget_for_req).
+        surfaced_swa_hit = cons.staged_prefetch_swa_tokens(req_id)
+        reserved = (
+            max(extend_need - window, 0)
+            + min(extend_need + max_new, window)
+            + ps
+            + (surfaced_swa_hit + ps - 1) // ps * ps
+        )
+        budget = cons_alloc.swa_available_size() + cons.swa_evictable_size()
+
+        # Consume at admission (init_load_back + request lock).
+        held = cons.buffer_pipeline.staged_prefetches[req_id]
+        req = mock.Mock()
+        req.rid = req_id
+        req.last_node = cons.root_node.id
+        req.prefix_indices = torch.zeros(
+            held.matched_len,
+            dtype=torch.int64,
+            device=cons.tree_core.empty_match_result.device_indices.device,
+        )
+        spliced, last_node = cons.init_load_back(
+            InitLoadBackParams(
+                best_match_node=None, host_hit_length=held.num_tokens, req=req
+            )
+        )
+        self.assertEqual(int(spliced.numel()), len(seq), "load-back degraded")
+        cons.ready_to_load_host_cache()
+        cons.inc_lock_ref(last_node)  # _req_inc_lock_ref
+
+        self.assertEqual(cons.swa_evictable_size(), 0)  # window is protected
+        # FULL stays roomy: only SWA can fail below.
+        self.assertGreaterEqual(cons_alloc.full_available_size(), extend_need + ps)
+
+        if reserved <= budget:
+            try:
+                if ps == 1:
+                    alloc_token_slots(cons, extend_need)
+                else:  # paged batch path — the production crash site
+                    prefix_len = int(spliced.numel())
+                    prefix_t = torch.tensor(
+                        [prefix_len], dtype=torch.int64, device=spliced.device
+                    )
+                    seq_t = torch.tensor(
+                        [prefix_len + extend_need],
+                        dtype=torch.int64,
+                        device=spliced.device,
+                    )
+                    alloc_paged_token_slots_extend(
+                        cons,
+                        prefix_t,
+                        prefix_t.cpu(),
+                        seq_t,
+                        seq_t.cpu(),
+                        spliced[-1:],
+                        extend_need,
+                    )
+            except RuntimeError as e:
+                self.fail(
+                    f"gate admitted (reserved={reserved} <= budget={budget}) "
+                    f"but the batch alloc OOMed: {e}"
+                )
+        else:
+            # Rejection must come from the surfaced window charge.
+            self.assertGreaterEqual(surfaced_swa_hit, window)
+
+    def test_buffer_only_swa_window_semantics(self):
+        """SWA window handling across the three partial-window cases:
+        root-anchored sub-window sequence (the sequence IS its window),
+        mid-tree sub-window continuation (head = device ring state), and a
+        storage hit shorter than the requested window (shrunk, tail
+        released). Each was a zero-L3-reuse regression on Llama-4-Scout."""
+        self._skip_unsupported_hicache_test()
+        if not self.cfg.has_swa:
+            self.skipTest("requires an SWA component")
+        window = self.cfg.sliding_window_size
+        if window <= self.cfg.page_size:
+            self.skipTest("window fits in one page")
+        sw_pages = (window + self.cfg.page_size - 1) // self.cfg.page_size
+        storage_dir = tempfile.mkdtemp()
+        self.addCleanup(shutil.rmtree, storage_dir, ignore_errors=True)
+
+        # 1. Root-anchored sequence shorter than the window.
+        seq = self._make_seq(1, (window // self.cfg.page_size) - 1)
+        self._produce_buffer_l3(storage_dir, seq, marker=5)
+        cons, _, _ = build_fixture(self.cfg)
+        self._init_buffer_hicache(cons, storage_dir)
+        cons.prefetch_from_storage(
+            "short-req", cons.root_node.id, array("q", seq), None, None
+        )
+        self._run_prefetch_to_completion(cons, "short-req")
+        cons.drain_storage_control_queues()
+        mc = cons.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq))))
+        self.assertEqual(len(mc.device_indices), len(seq))
+        self.assertIsNotNone(
+            cons.resolve_node_handle(mc.last_device_node)
+            .component_data[ComponentType.SWA]
+            .value
+        )
+        cons.sanity_check()
+
+        # 2. Mid-tree continuation shorter than the window: the staged
+        # prefetch must carry an SWA transfer (not a KV-only degrade).
+        if sw_pages >= 2:
+            seq_a = self._make_seq(1, max(2, sw_pages))
+            seq_ab = seq_a + self._make_seq(900, sw_pages - 1)
+            self._produce_buffer_l3(storage_dir, seq_ab, marker=14)
+            cons2, cons2_alloc, cons2_rtp = build_fixture(self.cfg)
+            self._init_buffer_hicache(cons2, storage_dir)
+            avail2 = self._host_avail_sizes(cons2)
+            self._insert(cons2, cons2_alloc, cons2_rtp, seq_a)
+            m = cons2.match_prefix(MatchPrefixParams(key=RadixKey(array("q", seq_a))))
+            cons2.prefetch_from_storage(
+                "subwin-req",
+                m.last_device_node,
+                array("q", seq_ab[len(seq_a) :]),
+                cons2.get_last_hash_value(m.last_device_node),
+                None,
+                matched_prefix_tokens=list(seq_a),
+            )
+            self._pump_hicache_until(
+                cons2,
+                lambda: cons2.check_prefetch_progress("subwin-req")
+                and cons2.buffer_pipeline.has_staged("subwin-req"),
+                "sub-window prefetch did not stage",
+            )
+            self.assertTrue(
+                any(
+                    t.name == PoolName.SWA
+                    for t in cons2.buffer_pipeline.staged_prefetches[
+                        "subwin-req"
+                    ].aux_xfers
+                ),
+                "sub-window fetch degraded to KV-only",
+            )
+            spliced = self._consume_staged_prefetch(
+                cons2, "subwin-req", prefix_indices=m.device_indices
+            )
+            self.assertEqual(int(spliced.numel()), len(seq_ab) - len(seq_a))
+            self.assertEqual(
+                len(
+                    cons2.match_prefix(
+                        MatchPrefixParams(key=RadixKey(array("q", seq_ab)))
+                    ).device_indices
+                ),
+                len(seq_ab),
+            )
+            self.assertEqual(self._host_avail_sizes(cons2), avail2)
+            cons2.sanity_check()
+
+        # 3. Hit one page short of the requested window: the shrunk window
+        # is kept (its own trailing window) and the buffer tail released.
+        full = self._buffer_swa_seq()
+        stored = full[: window - self.cfg.page_size]
+        self._produce_buffer_l3(storage_dir, stored, marker=6)
+        cons3, _, _ = build_fixture(self.cfg)
+        self._init_buffer_hicache(cons3, storage_dir)
+        avail3 = self._host_avail_sizes(cons3)
+        cons3.prefetch_from_storage(
+            "partial-req", cons3.root_node.id, array("q", full), None, None
+        )
+        self._run_prefetch_to_completion(cons3, "partial-req")
+        cons3.drain_storage_control_queues()
+        self.assertEqual(
+            len(
+                cons3.match_prefix(
+                    MatchPrefixParams(key=RadixKey(array("q", stored)))
+                ).device_indices
+            ),
+            len(stored),
+            "partial hit lost its SWA window",
+        )
+        self.assertEqual(self._host_avail_sizes(cons3), avail3)
+        cons3.sanity_check()
+
     # ---------- TP consistency for SWA prefetch (all-or-nothing) ----------
 
     def _patch_tp_all_reduce(self, cache, drop_swa: bool):
@@ -2898,44 +3426,9 @@ class UnifiedRadixCacheSuite:
         storage_dir: Optional[str] = None,
         prefetch_threshold: Optional[int] = None,
         prefetch_policy: str = "wait_complete",
+        host_memory_mode: str = "cache",
+        storage_extra: Optional[dict] = None,
     ):
-        import sglang.srt.mem_cache.hybrid_cache.hybrid_pool_assembler as assembler
-
-        # See _init_hicache: wrap the factory rather than MHATokenToKVPoolHost
-        # directly so the pin_memory=False override applies to both
-        # MHATokenToKVPoolHost and AsymmetricMHATokenToKVPoolHost.
-        orig_get_mha_host_pool_cls = assembler.get_mha_host_pool_cls
-        orig_mamba_host_pool = assembler.MambaPoolHost
-
-        def get_mha_host_pool_cls_wrapper(device_pool):
-            host_pool_cls = orig_get_mha_host_pool_cls(device_pool)
-
-            def kv_host_pool_wrapper(*args, **kwargs):
-                kwargs["pin_memory"] = False
-                return host_pool_cls(*args, **kwargs)
-
-            return kv_host_pool_wrapper
-
-        def mamba_host_pool_wrapper(*args, **kwargs):
-            kwargs["pin_memory"] = False
-            return orig_mamba_host_pool(*args, **kwargs)
-
-        patchers = [
-            mock.patch.object(
-                assembler,
-                "get_mha_host_pool_cls",
-                side_effect=get_mha_host_pool_cls_wrapper,
-            ),
-            mock.patch.object(
-                assembler,
-                "MambaPoolHost",
-                side_effect=mamba_host_pool_wrapper,
-            ),
-        ]
-        for patcher in patchers:
-            patcher.start()
-            self.addCleanup(patcher.stop)
-
         storage_extra_config = None
         if storage_backend == "file":
             from sglang.srt.runtime_context import get_parallel
@@ -2957,22 +3450,26 @@ class UnifiedRadixCacheSuite:
             extra = {}
             if prefetch_threshold is not None:
                 extra["prefetch_threshold"] = prefetch_threshold
+            if storage_extra:
+                extra.update(storage_extra)
             storage_extra_config = json.dumps(extra) if extra else None
 
+        # See _init_hicache: production kernel IO backend, pinned pools.
         server_args = ServerArgs(
             model_path="dummy",
             page_size=self.cfg.page_size,
-            hicache_io_backend="direct",
+            hicache_io_backend="kernel",
-            hicache_mem_layout="page_first_direct",
             hicache_write_policy=write_policy,
             hicache_storage_backend=storage_backend,
             hicache_storage_backend_extra_config=storage_extra_config,
             hicache_storage_prefetch_policy=prefetch_policy,
+            hicache_host_memory_mode=host_memory_mode,
         )
         # See build_fixture for why _mamba_cache_chunk_size is preset.
         server_args._mamba_cache_chunk_size = max(FLA_CHUNK_SIZE, self.cfg.page_size)
         set_global_server_args_for_scheduler(server_args)
         cache.init_hicache(server_args, cache.cache_init_params)
+        self.addCleanup(cache.release_host_resources)
         cache.write_through_threshold = 1 << 30
         cache.load_back_threshold = 0
         if storage_backend is not None:
@@ -3020,8 +3517,8 @@ class UnifiedRadixCacheSuite:
             if node is not cache.root_node:
                 self._backup_node(cache, node)
 
-    def _load_back_node(self, cache, node):
+    def _load_back_node(self, cache, node, req=None):
-        loaded = cache.load_back(node.id)
+        loaded = cache.load_back(node.id, req=req)
         self.assertTrue(loaded)
         producer_id = cache.ready_to_load_host_cache()
         self.assertNotEqual(producer_id, -1)
@@ -3729,10 +4226,14 @@ class UnifiedRadixCacheSuite:
         cache, _, _ = self._build_hicache_fixture()
         sw = cache.sliding_window_size
         swa = cache.components[ComponentType.SWA]
-        # below a full window -> does not participate, no alloc
+        # zero-length prefetch -> does not participate, no alloc
-        prep = swa.prepare_prefetch(cache.root_node.id, prefetch_tokens=sw - 1)
+        prep = swa.prepare_prefetch(cache.root_node.id, prefetch_tokens=0)
         self.assertFalse(prep.alloc_failed)
         self.assertIsNone(prep.host_indices)
+        # below a full window at the ROOT anchor -> the whole sequence is its
+        # own trailing window (sub-window prompts stay reusable via storage)
+        prep = swa.prepare_prefetch(cache.root_node.id, prefetch_tokens=sw - 1)
+        self.assertEqual(int(prep.host_indices.numel()), sw - 1)
         # a full window available -> participates, allocs one window of host pages
         prep = swa.prepare_prefetch(cache.root_node.id, prefetch_tokens=sw)
         self.assertEqual(int(prep.host_indices.numel()), sw)
@@ -6293,6 +6794,7 @@ class TestPrefetchCommitOrdering(CustomTestCase):
         cache = mock.MagicMock()
         cache.page_size = 1
         cache.enable_storage_metrics = False
+        cache.buffer_pipeline = None  # cache-mode commit path
         walk_action = object()
         insert_result = mock.MagicMock()
         insert_result.cache_actions = [walk_action]