[V1][Misc] stop update prefix cache stats when logs_stats is disabled (vllm-project#16460)

Signed-off-by: vie-serendipity <[email protected]>
Signed-off-by: Yang Wang <[email protected]>

Author: vie-serendipity

tests/v1/core/test_prefix_caching.py

@@ -751,3 +751,25 @@ def test_reset_prefix_cache():
     assert manager.reset_prefix_cache()
     assert not manager.block_pool.cached_block_hash_to_block
     assert all([blk.block_hash is None for blk in manager.block_pool.blocks])
+
+
+def test_prefix_cache_stats_disabled():
+    """Test that prefix_cache_stats is None when log_stats is False."""
+    manager = KVCacheManager(
+        make_kv_cache_config(16, 11),
+        max_model_len=8192,
+        enable_caching=True,
+        log_stats=False,  # Disable logging stats
+    )
+    assert manager.prefix_cache_stats is None
+
+    # Call all functions that check whether log_stats is disabled.
+    req = make_request("0", list(range(16)))
+    computed_blocks, num_computed_tokens = manager.get_computed_blocks(req)
+    assert not computed_blocks
+    assert num_computed_tokens == 0
+    manager.allocate_slots(req, 16, computed_blocks)
+    manager.reset_prefix_cache()
+
+    # Ensure prefix_cache_stats remains None
+    assert manager.prefix_cache_stats is None

vllm/v1/core/kv_cache_manager.py

@@ -39,8 +39,9 @@ def __init__(
 
         self.enable_caching = enable_caching
         self.caching_hash_fn = sha256 if caching_hash_algo == "sha256" else hash
-        # FIXME: make prefix cache stats conditional on log_stats
         self.log_stats = log_stats
+        # FIXME: make prefix cache stats conditional on log_stats
+        self.prefix_cache_stats = PrefixCacheStats() if log_stats else None
         # NOTE(woosuk): To avoid frequent block allocation, we preallocate some
         # blocks for each request. For example, when a request reaches the end
         # of its block table, we preallocate N blocks in advance. This way, we
@@ -79,7 +80,6 @@ def __init__(
         # This is only used to track the RUNNING requests, we do not track the
         # data for reempted ones.
         self.num_cached_block: dict[str, int] = {}
-        self.prefix_cache_stats = PrefixCacheStats()
 
     @property
     def usage(self) -> float:
@@ -90,12 +90,14 @@ def usage(self) -> float:
         """
         return self.block_pool.get_usage()
 
-    def make_prefix_cache_stats(self) -> PrefixCacheStats:
+    def make_prefix_cache_stats(self) -> Optional[PrefixCacheStats]:
         """Get (and reset) the prefix cache stats.
 
         Returns:
-            The current prefix caching stats.
+            The current prefix caching stats, or None if logging is disabled.
         """
+        if not self.log_stats:
+            return None
         stats = self.prefix_cache_stats
         self.prefix_cache_stats = PrefixCacheStats()
         return stats
@@ -125,7 +127,9 @@ def get_computed_blocks(
                                                self.block_size, request)
             self.req_to_block_hashes[request.request_id] = block_hashes
 
-        self.prefix_cache_stats.requests += 1
+        if self.log_stats:
+            assert self.prefix_cache_stats is not None
+            self.prefix_cache_stats.requests += 1
         # When the request requires prompt logprobs, we skip prefix caching.
         if request.sampling_params.prompt_logprobs is not None:
             return [], 0
@@ -145,8 +149,10 @@ def get_computed_blocks(
 
         computed_blocks = (
             self.specialized_manager.find_longest_cache_hit(block_hashes))
-        self.prefix_cache_stats.queries += len(block_hashes)
-        self.prefix_cache_stats.hits += len(computed_blocks)
+        if self.log_stats:
+            assert self.prefix_cache_stats is not None
+            self.prefix_cache_stats.queries += len(block_hashes)
+            self.prefix_cache_stats.hits += len(computed_blocks)
 
         if last_block_hash is not None:
             # Add back the last block hash if it was removed.
@@ -317,17 +323,19 @@ def free(self, request: Request) -> None:
 
     def reset_prefix_cache(self) -> bool:
         """Reset prefix cache. This function may be used in RLHF
-        flows to invalid prefix caching after the weights are updated,
+        flows to invalidate prefix caching after the weights are updated,
         or used for resetting prefix caching status for benchmarking.
 
         Returns:
             bool: True if the prefix cache is successfully reset,
             False otherwise.
         """
-        if self.block_pool.reset_prefix_cache():
+        if not self.block_pool.reset_prefix_cache():
+            return False
+        if self.log_stats:
+            assert self.prefix_cache_stats is not None
             self.prefix_cache_stats.reset = True
-            return True
-        return False
+        return True
 
     def get_num_common_prefix_blocks(
         self,

vllm/v1/core/sched/scheduler.py

@@ -798,11 +798,13 @@ def make_stats(
     ) -> Optional[SchedulerStats]:
         if not self.log_stats:
             return None
+        prefix_cache_stats = self.kv_cache_manager.make_prefix_cache_stats()
+        assert prefix_cache_stats is not None
         return SchedulerStats(
             num_running_reqs=len(self.running),
             num_waiting_reqs=len(self.waiting),
             gpu_cache_usage=self.kv_cache_manager.usage,
-            prefix_cache_stats=self.kv_cache_manager.make_prefix_cache_stats(),
+            prefix_cache_stats=prefix_cache_stats,
             spec_decoding_stats=spec_decoding_stats,
         )