[HiCache] Batch PP write and load completion sync (#33473)

python/sglang/srt/mem_cache/unified_radix_cache.py

@@ -2568,8 +2568,18 @@ class UnifiedRadixCache(BasePrefixCache):
         self._drain_async_work()
 
         if self.pp_size != 1:
-            self.writing_check()
+            finish_counts = torch.zeros(2, dtype=torch.int, device="cpu")
-            self.loading_check()
+            if self.pp_rank == 0 and self.cache_controller is not None:
+                finish_counts[0] = self._count_ready_acks(
+                    self.cache_controller.ack_write_queue
+                )
+                finish_counts[1] = self._count_ready_acks(
+                    self.cache_controller.ack_load_queue
+                )
+            self._all_reduce(finish_counts, torch.distributed.ReduceOp.MIN)
+            write_finish_count, load_finish_count = map(int, finish_counts.tolist())
+            self.writing_check(finish_count=write_finish_count)
+            self.loading_check(finish_count=load_finish_count)
             if self.enable_storage:
                 self.drain_storage_control_queues()
         else:

test/registered/unit/mem_cache/test_hiradix_pp_sync_drain.py

@@ -1,6 +1,8 @@
-"""Unit test for HiRadixCache._drain_async_work PP-sync backpressure."""
+"""Unit tests for HiCache PP synchronization."""
 
 import unittest
+from types import SimpleNamespace
+from unittest.mock import MagicMock
 
 from sglang.srt.mem_cache.hiradix_cache import HiRadixCache
 from sglang.srt.mem_cache.unified_radix_cache import UnifiedRadixCache
@@ -46,5 +48,57 @@ class TestPPSyncDrain(unittest.TestCase):
             self.assertEqual(holder.work_list, [])
 
 
+class TestUnifiedPPSyncBatching(unittest.TestCase):
+    def _make_cache(self, pp_rank, write_ready, load_ready):
+        cache = object.__new__(UnifiedRadixCache)
+        cache.tree_core = SimpleNamespace(enable_storage=False)
+        cache.pp_rank = pp_rank
+        cache.pp_size = 2
+        cache.enable_storage_metrics = False
+        cache.storage_metrics_collector = None
+        cache._drain_async_work = MagicMock()
+        cache._all_reduce = MagicMock()
+        cache.writing_check = MagicMock()
+        cache.loading_check = MagicMock()
+        cache.drain_storage_control_queues = MagicMock()
+        cache.cache_controller = SimpleNamespace(
+            ack_write_queue=[
+                SimpleNamespace(
+                    finish_event=SimpleNamespace(query=MagicMock(return_value=ready))
+                )
+                for ready in write_ready
+            ],
+            ack_load_queue=[
+                SimpleNamespace(
+                    finish_event=SimpleNamespace(query=MagicMock(return_value=ready))
+                )
+                for ready in load_ready
+            ],
+        )
+        return cache
+
+    def test_pp_batches_write_and_load_counts_once(self):
+        leader = self._make_cache(0, [True, False], [True, True])
+        leader.check_hicache_events()
+
+        leader._all_reduce.assert_called_once()
+        self.assertEqual(leader._all_reduce.call_args.args[0].tolist(), [1, 2])
+        leader.writing_check.assert_called_once_with(finish_count=1)
+        leader.loading_check.assert_called_once_with(finish_count=2)
+
+        follower = self._make_cache(1, [True], [True])
+        follower._all_reduce.side_effect = lambda counts, _: counts.fill_(1)
+        follower.check_hicache_events()
+
+        for queue in (
+            follower.cache_controller.ack_write_queue,
+            follower.cache_controller.ack_load_queue,
+        ):
+            queue[0].finish_event.query.assert_not_called()
+        follower._all_reduce.assert_called_once()
+        follower.writing_check.assert_called_once_with(finish_count=1)
+        follower.loading_check.assert_called_once_with(finish_count=1)
+
+
 if __name__ == "__main__":
     unittest.main()