Fix unified SWA: size a non-owner's v2p by the id space it must address (#37560)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

python/sglang/srt/mem_cache/multi_ended_allocator.py

@@ -267,6 +267,7 @@ class MultiEndedAllocator(BaseTokenToKVPoolAllocator):
         sub_pool_name: str,
         device: str,
         is_id_owner: bool,
+        virtual_num_pages: Optional[int] = None,
         page_size: int = 1,
         shards_under_dcp: bool = False,
         need_sort: bool = False,
@@ -332,10 +333,15 @@ class MultiEndedAllocator(BaseTokenToKVPoolAllocator):
         ) // self.pool_page_size
         self.entry_bytes_per_page = self.entry_bytes * self.pool_page_size
 
-        # v2p / p2v sized by PAGES. Page 0 is the padding anchor; trailing row is
-        # the -1 sentinel.
+        # v2p is indexed by VIRTUAL page id, p2v by PHYSICAL page id. A
+        # non-owner consumes the owner's ids, so its v2p spans the owner's
+        # count; the two are unrelated and either can be the larger.
+        self.num_virtual_ids = (
+            self.num_pages if virtual_num_pages is None else virtual_num_pages
+        )
+        # Page 0 is the padding anchor; the trailing row is the -1 sentinel.
         self.virtual_to_physical = torch.full(
-            (self.num_pages + 1,),
+            (self.num_virtual_ids + 1,),
             -1,
             dtype=torch.int64,
             device=device,
@@ -346,8 +352,6 @@ class MultiEndedAllocator(BaseTokenToKVPoolAllocator):
             dtype=torch.int64,
             device=device,
         )
-        # Back-compat alias (count of virtual PAGES) consulted by is_slot_allocated.
-        self.num_virtual_ids = self.num_pages
 
         # Chain neighbours: `low_peer` toward byte 0, `high_peer` toward
         # `total_bytes`. Ends have one (`bind_peer`), float middles have both.
@@ -540,7 +544,7 @@ class MultiEndedAllocator(BaseTokenToKVPoolAllocator):
     def is_slot_allocated(self, slot: int) -> bool:
         """Whether the PAGE containing this virtual id is in use."""
         virt_page = slot // self.page_size
-        if virt_page < 0 or virt_page >= self.num_pages:
+        if virt_page < 0 or virt_page >= self.num_virtual_ids:
             return False
         return int(self.virtual_to_physical[virt_page].item()) != -1
 
@@ -3212,6 +3216,9 @@ class UnifiedSWATokenToKVPoolAllocator(SWATokenToKVPoolAllocator):
             need_sort=need_sort,
             forward_stream=forward_stream,
             lazy_compaction=lazy_compaction,
+            # swa binds the virtual pages full mints, so it must address
+            # full's whole id space.
+            virtual_num_pages=self.full_attn_allocator.num_virtual_ids,
         )
         self._wire_peers()
 

test/registered/attention/test_gemma4_unified_swa_virtual_ids.py

@@ -0,0 +1,100 @@
+# Copyright 2023-2026 SGLang Team
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""Unified memory on a hybrid sliding-window model, which nothing else covered.
+
+The unified SWA composite mints one virtual page id per allocation and binds it
+on both sides, so the swa sub-pool's `virtual_to_physical` is indexed by the
+FULL side's ids while it used to be sized by its own page count. A model with
+few full-attention layers and many sliding ones gives the id owner more pages,
+and the top of that id space then lands off the end of the swa table. On GPU
+the write is unchecked (`alloc_bind_inplace`'s `tl.store`), so the symptom
+surfaces later and elsewhere: a device-side index assert on the read in
+`_swa_write_loc_unified`, and a dead scheduler.
+
+The reachability of it is a KV-budget property, not just a model property: ids
+come off the front of the owner's free list and freed ids return to the back, so
+the cursor sweeps the owner's range over time and the failure needs cumulative
+churn past `swa.num_pages`. `--max-total-tokens 60000` puts that within one
+200-example GSM8K run; at this model's default budget the swa table is 3.8M
+entries and it would take dozens.
+
+Every argument below is load-bearing: a larger pool, or synthetic prompts in
+place of this eval, and the narrow table passes.
+
+    python -m pytest test/registered/attention/test_gemma4_unified_swa_virtual_ids.py -v
+"""
+
+import types
+import unittest
+
+from sglang.srt.utils import kill_process_tree
+from sglang.test.ci.ci_register import register_cuda_ci
+from sglang.test.run_eval import run_eval
+from sglang.test.test_utils import (
+    DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+    DEFAULT_URL_FOR_TEST,
+    CustomTestCase,
+    popen_launch_server,
+)
+
+register_cuda_ci(est_time=65, stage="base-b", runner_config="1-gpu-large")
+
+# Healthy 0.865-0.880 (static pool 0.870); a died-mid-run server scores
+# 0.05-0.21. 1 sigma over 200 examples is ~0.024.
+SCORE_THRESHOLD = 0.82
+
+
+class TestGemma4UnifiedSwaVirtualIds(CustomTestCase):
+    @classmethod
+    def setUpClass(cls):
+        cls.model = "google/gemma-4-E2B-it"
+        cls.base_url = DEFAULT_URL_FOR_TEST
+        cls.process = popen_launch_server(
+            cls.model,
+            cls.base_url,
+            timeout=DEFAULT_TIMEOUT_FOR_SERVER_LAUNCH,
+            other_args=[
+                "--attention-backend",
+                "triton",
+                "--enable-unified-memory",
+                "--disable-radix-cache",
+                "--mem-fraction-static",
+                "0.8",
+                # The trigger: this is what brings `swa.num_pages` within one
+                # eval's worth of churn.
+                "--max-total-tokens",
+                "60000",
+            ],
+        )
+
+    @classmethod
+    def tearDownClass(cls):
+        if hasattr(cls, "process") and cls.process:
+            kill_process_tree(cls.process.pid)
+
+    def test_gsm8k_survives_churn_past_the_swa_page_count(self):
+        metrics = run_eval(
+            types.SimpleNamespace(
+                base_url=self.base_url,
+                model=self.model,
+                eval_name="gsm8k",
+                num_examples=200,
+                num_threads=128,
+            )
+        )
+        self.assertGreaterEqual(metrics["score"], SCORE_THRESHOLD)
+
+
+if __name__ == "__main__":
+    unittest.main()

test/registered/unit/mem_cache/test_unified_swa_shared_virtual_ids.py

@@ -0,0 +1,119 @@
+# Copyright 2023-2026 SGLang Team
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# ==============================================================================
+"""The unified SWA composite mints one virtual page id and binds it on both
+sides, so the swa side's `virtual_to_physical` is indexed by the FULL side's
+ids. Its own page count says nothing about how wide that has to be: which side
+gets more pages out of a shared byte budget depends on the layer split, and a
+model with few full-attention layers and many sliding ones (gemma-4: 10 and 50)
+gives the full side an order of magnitude more.
+
+Reaching an id that high takes cumulative allocation, so a narrow table fails
+after churn rather than at once -- and on GPU it fails as a `tl.store` past the
+end of the table: an unchecked write, not a raised index.
+
+    python -m pytest test/registered/unit/mem_cache/test_unified_swa_shared_virtual_ids.py -v
+"""
+
+import unittest
+
+import torch
+from test_swa_locked_full_recover_unified import _DEV, _FakeUnifiedSWAKVPool
+
+from sglang.srt.mem_cache.multi_ended_allocator import (
+    UnifiedSWATokenToKVPoolAllocator,
+)
+from sglang.srt.mem_cache.unified_memory_pool import MHASubPoolSpec, UnifiedKVPool
+from sglang.test.ci.ci_register import register_cpu_ci
+
+register_cpu_ci(est_time=10, suite="base-a-test-cpu")
+
+
+def _build(n_full: int, n_swa: int, full_layers: int, swa_layers: int):
+    """A composite whose sides have different per-page byte costs. `full_layers
+    < swa_layers` is the gemma-4 shape: the cheap side is the id owner and ends
+    up with far more pages than the side that has to address them."""
+    full_spec = MHASubPoolSpec(
+        name="full",
+        layer_num=full_layers,
+        head_num=2,
+        head_dim=4,
+        store_dtype=torch.float16,
+        grow_direction="up",
+    )
+    swa_spec = MHASubPoolSpec(
+        name="swa",
+        layer_num=swa_layers,
+        head_num=2,
+        head_dim=4,
+        store_dtype=torch.float16,
+        grow_direction="down",
+    )
+    pool = UnifiedKVPool(
+        total_bytes=n_full * full_spec.entry_bytes() + n_swa * swa_spec.entry_bytes(),
+        sub_pool_specs=[full_spec, swa_spec],
+        device=_DEV,
+        enable_memory_saver=False,
+    )
+    return UnifiedSWATokenToKVPoolAllocator(
+        unified_buffer=pool,
+        kvcache=_FakeUnifiedSWAKVPool(pool),
+        device=_DEV,
+        full_max_total_num_tokens=n_full,
+        swa_max_total_num_tokens=n_swa,
+        need_sort=False,
+        forward_stream=None,
+    )
+
+
+class TestSharedVirtualIdSpace(unittest.TestCase):
+    def test_swa_table_spans_the_owners_id_space(self):
+        """Static form: the table has to be wide enough before any alloc runs."""
+        for full_layers, swa_layers in ((1, 5), (5, 1), (2, 2)):
+            with self.subTest(full_layers=full_layers, swa_layers=swa_layers):
+                alloc = _build(200, 20, full_layers, swa_layers)
+                owner = alloc.full_attn_allocator
+                swa = alloc.swa_attn_allocator
+                self.assertGreaterEqual(
+                    int(swa.virtual_to_physical.shape[0]),
+                    owner.num_virtual_ids + 1,
+                    "swa v2p cannot address every id the owner can mint",
+                )
+                # p2v stays this pool's own business: it is indexed by physical id.
+                self.assertEqual(
+                    int(swa.physical_to_virtual.shape[0]), swa.num_pages + 1
+                )
+
+    def test_churn_past_the_swa_page_count_binds_cleanly(self):
+        """Dynamic form: alloc/free until the owner's cursor passes the swa
+        side's page count, which is where the narrow table used to be written
+        off the end."""
+        alloc = _build(200, 20, full_layers=1, swa_layers=5)
+        swa_pages = alloc.swa_attn_allocator.num_pages
+        highest = 0
+        for _ in range(40):
+            v = alloc.alloc(4)
+            if v is None:
+                break
+            highest = max(highest, int(v.max()) // alloc.page_size)
+            alloc.free(v)
+        self.assertGreater(
+            highest,
+            swa_pages,
+            f"churn never reached past the swa side's {swa_pages} pages, so this "
+            "test would pass on a narrow table too",
+        )
+
+
+if __name__ == "__main__":
+    unittest.main()