[PD] Optimize paged allocator free-list release (#37146)

Co-authored-by: wangwenming.41 <wangwenming.41@jd.com>
Co-authored-by: hnyls2002 <lsyincs@gmail.com>
Co-authored-by: Liangsheng Yin <hnyls2002@gmail.com>

python/sglang/srt/hardware_backend/npu/allocator_npu.py

@@ -48,7 +48,7 @@ class NPUPagedTokenToKVPoolAllocator(PagedTokenToKVPoolAllocator):
             num_new_pages_item = num_new_pages_tensor.item()
         else:
             num_new_pages_item = num_new_pages
-        if self.need_sort and num_new_pages_item > len(self.free_pages):
+        if num_new_pages_item > len(self.free_pages):
             self.merge_and_sort_free()
 
         if num_new_pages_item > len(self.free_pages):
@@ -116,7 +116,6 @@ class NPUPagedTokenToKVPoolAllocator(PagedTokenToKVPoolAllocator):
 
         if num_new_pages > len(self.free_pages):
             self.merge_and_sort_free()
-
         if num_new_pages > len(self.free_pages):
             return None
 
@@ -144,11 +143,7 @@ class NPUPagedTokenToKVPoolAllocator(PagedTokenToKVPoolAllocator):
         if self.free_group is None:
             device = free_index.device
             free_page_indices = torch.unique(free_index.cpu() // self.page_size)
-            free_page_indices = free_page_indices.to(device)
+            self._release_page_ids(free_page_indices.to(device))
-            if self.need_sort:
-                self.release_pages = torch.cat((free_page_indices, self.release_pages))
-            else:
-                self.free_pages = torch.cat((free_page_indices, self.free_pages))
         else:
             self.free_group.append(self._copy_for_free_group(free_index))
 

python/sglang/srt/managers/scheduler_components/invariant_checker.py

@@ -171,14 +171,12 @@ class SchedulerInvariantChecker:
             self.req_to_token_pool.mamba_pool.size,
         )
         if leak:
-            # Page-level leak diagnosis for mamba. Allocator flavors without
+            # Pools without a page free list return None; skip the census rather
-            # page free-lists (free_pages is None) skip the page census — the
+            # than crash the watchdog thread that runs this dump.
-            # dump must never crash the watchdog thread that calls it.
+            free_pages = self.token_to_kv_pool_allocator.get_all_free_pages()
-            free_pages = self.token_to_kv_pool_allocator.free_pages
+            if free_pages is None:
-            release_pages = self.token_to_kv_pool_allocator.release_pages
-            if free_pages is None or release_pages is None:
                 return leak, msg
-            free_full_pages = set(free_pages.tolist() + release_pages.tolist())
+            free_full_pages = set(free_pages.tolist())
             cached_full_pages = set(self.tree_cache.all_values_flatten().tolist())
             full_page_msg = ""
             if (
@@ -386,18 +384,9 @@ class SchedulerInvariantChecker:
         if not sub_allocs:
             return
 
-        def _free_pages(a):
-            free = a.free_pages
-            release = getattr(a, "release_pages", None)
-            return (
-                torch.cat((free, release))
-                if release is not None and len(release) > 0
-                else free
-            )
-
         # Check B: every sub-pool's free set has no duplicate pages.
         for i, sub in enumerate(sub_allocs):
-            free = _free_pages(sub)
+            free = sub.get_all_free_pages()
             uniq = torch.unique(free)
             if uniq.numel() != free.numel():
                 raise_error_or_warn(
@@ -409,7 +398,7 @@ class SchedulerInvariantChecker:
 
         # Check A: owner pages (full-pool indices) must not be in the full free
         # set (sub_allocs[0] is the full pool, even on hybrid-SWA).
-        full_unique = torch.unique(_free_pages(sub_allocs[0]))
+        full_unique = torch.unique(sub_allocs[0].get_all_free_pages())
         stale = owner_pages[torch.isin(owner_pages, full_unique)]
         if stale.numel() > 0:
             raise_error_or_warn(

python/sglang/srt/mem_cache/allocator/base.py

@@ -93,6 +93,14 @@ class BaseTokenToKVPoolAllocator(abc.ABC):
     def get_kvcache(self):
         return self._kvcache
 
+    def get_all_free_pages(self):
+        # Debug / invariant census; None when the pool has no page free list.
+        if self.free_pages is None:
+            return None
+        if self.release_pages is None or len(self.release_pages) == 0:
+            return self.free_pages
+        return torch.cat((self.free_pages, self.release_pages))
+
     def free_group_begin(self):
         assert self.free_group is None, "free groups cannot be nested"
         self.free_group = []

python/sglang/srt/mem_cache/allocator/paged.py

@@ -146,6 +146,19 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
                 pass
         self.clear()
 
+    def available_size(self):
+        return (len(self.free_pages) + self.num_staged_pages) * self.page_size
+
+    def get_all_free_pages(self):
+        return torch.cat((self.free_pages, *self.staged_pages))
+
+    def merge_and_sort_free(self):
+        if not self.staged_pages:
+            return
+        self.free_pages, _ = torch.sort(self.get_all_free_pages())
+        self.staged_pages = []
+        self.num_staged_pages = 0
+
     def alloc(self, need_size: int):
         # page-aligned allocation, returning contiguous indices of pages
         if self.debug_mode:
@@ -154,7 +167,7 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
             ), "The allocation size should be page-aligned"
 
         num_pages = need_size // self.page_size
-        if self.need_sort and num_pages > len(self.free_pages):
+        if num_pages > len(self.free_pages):
             self.merge_and_sort_free()
         if num_pages > len(self.free_pages):
             return None
@@ -185,9 +198,7 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
             )
 
         bs = len(prefix_lens)
-        if self.need_sort and extend_num_tokens // self.page_size + bs + 1 > len(
+        if extend_num_tokens // self.page_size + bs + 1 > len(self.free_pages):
-            self.free_pages
-        ):
             self.merge_and_sort_free()
 
         out_indices = torch.empty(
@@ -231,7 +242,7 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
             )
 
         bs = len(seq_lens)
-        if self.need_sort and bs > len(self.free_pages):
+        if bs > len(self.free_pages):
             self.merge_and_sort_free()
 
         out_indices = torch.empty((bs,), dtype=torch.int64, device=self.device)
@@ -303,13 +314,13 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
             )
 
     def _debug_check_no_duplicate_pages(self):
-        # span both containers: need_sort (PD disagg) routes frees into release_pages
+        pages = self.get_all_free_pages()
-        pages = torch.cat((self.free_pages, self.release_pages))
         assert len(torch.unique(pages)) == len(pages)
 
     def _release_page_ids(self, *page_ids: torch.Tensor):
         if self.need_sort:
-            self.release_pages = torch.cat((*page_ids, self.release_pages))
+            self.staged_pages.extend(page_ids)
+            self.num_staged_pages += sum(ids.numel() for ids in page_ids)
         else:
             self.free_pages = torch.cat((*page_ids, self.free_pages))
 
@@ -335,7 +346,9 @@ class PagedTokenToKVPoolAllocator(BaseTokenToKVPoolAllocator):
         )
         self.free_group = None
         self.free_page_reps_group = []
-        self.release_pages = torch.empty((0,), dtype=torch.int64, device=self.device)
+        # need_sort only: freed pages wait here, unsorted, until an alloc runs short.
+        self.staged_pages: list[torch.Tensor] = []
+        self.num_staged_pages = 0
 
     def get_cpu_copy(self, indices, mamba_indices=None):
         return self._kvcache.get_cpu_copy(indices, mamba_indices=mamba_indices)

test/registered/unit/layers/test_minicpm_sparse_cache.py

@@ -53,6 +53,9 @@ class RecordingAllocator(BaseTokenToKVPoolAllocator):
     def available_size(self):
         return self.capacity - len(self.live)
 
+    def get_all_free_pages(self):
+        return self.free_pages
+
     def clear(self):
         self.next_slot = 1
         self.live.clear()
@@ -244,7 +247,6 @@ def test_streaming_session_release_frees_compressed_slots():
 def test_mamba_leak_diagnostic_does_not_report_reserved_slots():
     pool, _, _, allocator = make_pool_and_req(capacity=69)
     allocator.free_pages = torch.arange(6, 70, dtype=torch.int64)
-    allocator.release_pages = torch.empty(0, dtype=torch.int64)
     pool.mamba_pool = SimpleNamespace(size=1)
     pool.mamba_allocator = SimpleNamespace(
         size=1,

test/registered/unit/managers/test_kv_page_invariants.py

@@ -22,7 +22,7 @@ def _make_checker(page_size=_PAGE_SIZE, row_width=4096, num_reqs=8, free_pages=N
     alloc = SimpleNamespace(
         page_size=page_size,
         free_pages=free_pages,
-        release_pages=torch.empty(0, dtype=torch.int64),
+        get_all_free_pages=lambda: free_pages,
     )
     tc = SimpleNamespace(slots={})
     _ps, _rtp, _alloc, _tc = page_size, rtp, alloc, tc

test/registered/unit/mem_cache/test_paged_allocator_lazy_release.py

@@ -0,0 +1,70 @@
+"""need_sort allocators stage released pages and merge only under allocation pressure."""
+
+import unittest
+from unittest.mock import patch
+
+import torch
+
+from sglang.srt.mem_cache.allocator.paged import PagedTokenToKVPoolAllocator
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=2, suite="base-a-test-cpu")
+
+PAGE_SIZE = 2
+NUM_PAGES = 16
+
+
+def _make_allocator(*, need_sort: bool) -> PagedTokenToKVPoolAllocator:
+    return PagedTokenToKVPoolAllocator(
+        size=NUM_PAGES * PAGE_SIZE,
+        page_size=PAGE_SIZE,
+        dtype=torch.float16,
+        device="cpu",
+        kvcache=None,
+        need_sort=need_sort,
+    )
+
+
+def _page_ids(indices: torch.Tensor) -> torch.Tensor:
+    return indices[::PAGE_SIZE] // PAGE_SIZE
+
+
+class TestPagedAllocatorLazyRelease(CustomTestCase):
+    def test_release_stages_until_free_pages_run_dry(self):
+        allocator = _make_allocator(need_sort=True)
+        allocated = allocator.alloc(24)
+        free_pages_before = allocator.free_pages
+
+        with patch.object(torch, "cat", wraps=torch.cat) as cat_mock:
+            allocator.free(allocated[:2])
+            allocator.free(allocated[4:6])
+            self.assertIs(allocator.free_pages, free_pages_before)
+            self.assertEqual(allocator.num_staged_pages, 2)
+            self.assertEqual(allocator.available_size(), 12)
+            cat_mock.assert_not_called()
+
+            # free_pages still covers this: no merge.
+            primary = allocator.alloc(8)
+            self.assertTrue(torch.equal(_page_ids(primary), torch.arange(13, 17)))
+            cat_mock.assert_not_called()
+
+            # Pressure: one merge, staged pages come back sorted.
+            reused = allocator.alloc(4)
+            self.assertTrue(torch.equal(_page_ids(reused), torch.tensor([1, 3])))
+            self.assertEqual(cat_mock.call_count, 1)
+
+        self.assertEqual(allocator.num_staged_pages, 0)
+        self.assertEqual(allocator.staged_pages, [])
+
+    def test_clear_drops_staged_pages(self):
+        allocator = _make_allocator(need_sort=True)
+        allocator.free(allocator.alloc(4))
+
+        allocator.clear()
+        self.assertEqual(allocator.available_size(), allocator.size)
+        self.assertEqual(allocator.staged_pages, [])
+
+
+if __name__ == "__main__":
+    unittest.main()

test/registered/unit/mem_cache/test_paged_free_segment.py

@@ -67,11 +67,12 @@ class TestFreeSegment(unittest.TestCase):
         alloc.free_segment(row[:0], start_pos=0)
         self.assertEqual(len(alloc.free_pages), before)
 
-    def test_need_sort_routes_to_release_pages(self):
+    def test_need_sort_defers_released_pages(self):
         alloc = _make_allocator(need_sort=True)
         row = _make_kv_row(alloc, 2 * PAGE_SIZE)
         alloc.free_segment(row, start_pos=0)
-        self.assertEqual(len(alloc.release_pages), 2)
+        self.assertEqual(len(alloc.staged_pages), 1)
+        self.assertEqual(alloc.num_staged_pages, 2)
 
     def test_group_defers_until_group_end(self):
         alloc = _make_allocator()
@@ -108,8 +109,8 @@ class TestFreeSegment(unittest.TestCase):
         with self.assertRaises(AssertionError):
             alloc.free_group_end()
 
-    def test_group_end_debug_assert_covers_release_pages(self):
+    def test_group_end_debug_assert_covers_staged_releases(self):
-        # need_sort routes frees into release_pages; the duplicate check must
+        # need_sort stages frees in chunks; the duplicate check must
         # not go vacuous there (PD disaggregation runs with need_sort=True).
         alloc = _make_allocator(need_sort=True)
         alloc.debug_mode = True