[Core] Don't count preempted tokens in prefix cache hit rate (#25787)

Signed-off-by: Zhuohan Li <zhuohan123@gmail.com>

vllm/v1/core/kv_cache_manager.py

@@ -27,8 +27,8 @@ class KVCacheBlocks:
     `blocks[i][j]` refers to the i-th kv_cache_group
     and the j-th block of tokens.We don't use block of
     tokens as the outer dimension because it assumes all
-    kv_cache_groups have the same number of blocks, which is true for now but 
-    will be broken if we want to give different block_size to different 
+    kv_cache_groups have the same number of blocks, which is true for now but
+    will be broken if we want to give different block_size to different
     kv_cache_groups in the future.
     """
 
@@ -184,9 +184,17 @@ class KVCacheManager:
 
         if self.log_stats:
             assert self.prefix_cache_stats is not None
-            self.prefix_cache_stats.requests += 1
-            self.prefix_cache_stats.queries += request.num_tokens
-            self.prefix_cache_stats.hits += num_new_computed_tokens
+            if request.num_preemptions > 0:
+                # Previously preempted request
+                self.prefix_cache_stats.preempted_requests += 1
+                self.prefix_cache_stats.preempted_queries += request.num_tokens
+                self.prefix_cache_stats.preempted_hits += (
+                    num_new_computed_tokens)
+            else:
+                # New request
+                self.prefix_cache_stats.requests += 1
+                self.prefix_cache_stats.queries += request.num_tokens
+                self.prefix_cache_stats.hits += num_new_computed_tokens
 
         return KVCacheBlocks(computed_blocks), num_new_computed_tokens
 
@@ -209,10 +217,10 @@ class KVCacheManager:
                 already been computed locally (i.e. new_computed_blocks).
             num_new_computed_tokens: The number of new computed tokens just
                 hitting the prefix caching, excluding external tokens.
-            new_computed_blocks: The cached blocks for the above new computed 
+            new_computed_blocks: The cached blocks for the above new computed
                 tokens.
             num_lookahead_tokens: The number of speculative tokens to allocate.
-                This is used by spec decode proposers with kv-cache such 
+                This is used by spec decode proposers with kv-cache such
                 as eagle.
             delay_cache_blocks: Whether to skip caching the blocks. This is
                 used by P/D when allocating blocks used in a KV transfer
@@ -365,7 +373,7 @@ class KVCacheManager:
                 requests in the current step.
 
         Returns:
-            list[int]: The number of common prefix blocks for each kv cache 
+            list[int]: The number of common prefix blocks for each kv cache
             group.
         """
         assert request.status == RequestStatus.RUNNING

vllm/v1/core/sched/scheduler.py

@@ -251,46 +251,48 @@ class Scheduler(SchedulerInterface):
                 req_index += 1
                 continue
 
+            # Schedule newly needed KV blocks for the request.
             while True:
                 new_blocks = self.kv_cache_manager.allocate_slots(
                     request,
                     num_new_tokens,
                     num_lookahead_tokens=self.num_lookahead_tokens)
-                if new_blocks is None:
-                    # The request cannot be scheduled.
-                    # Preempt the lowest-priority request.
-                    if self.policy == SchedulingPolicy.PRIORITY:
-                        preempted_req = max(
-                            self.running,
-                            key=lambda r: (r.priority, r.arrival_time),
-                        )
-                        self.running.remove(preempted_req)
-                        if preempted_req in scheduled_running_reqs:
-                            scheduled_running_reqs.remove(preempted_req)
-                    else:
-                        preempted_req = self.running.pop()
-
-                    self.kv_cache_manager.free(preempted_req)
-                    self.encoder_cache_manager.free(preempted_req)
-                    preempted_req.status = RequestStatus.PREEMPTED
-                    preempted_req.num_computed_tokens = 0
-                    if self.log_stats:
-                        preempted_req.record_event(
-                            EngineCoreEventType.PREEMPTED, scheduled_timestamp)
-
-                    self.waiting.prepend_request(preempted_req)
-                    preempted_reqs.append(preempted_req)
-                    if preempted_req == request:
-                        # No more request to preempt.
-                        can_schedule = False
-                        break
-                else:
+
+                if new_blocks is not None:
                     # The request can be scheduled.
-                    can_schedule = True
                     break
-            if not can_schedule:
+
+                # The request cannot be scheduled.
+                # Preempt the lowest-priority request.
+                if self.policy == SchedulingPolicy.PRIORITY:
+                    preempted_req = max(
+                        self.running,
+                        key=lambda r: (r.priority, r.arrival_time),
+                    )
+                    self.running.remove(preempted_req)
+                    if preempted_req in scheduled_running_reqs:
+                        scheduled_running_reqs.remove(preempted_req)
+                else:
+                    preempted_req = self.running.pop()
+
+                self.kv_cache_manager.free(preempted_req)
+                self.encoder_cache_manager.free(preempted_req)
+                preempted_req.status = RequestStatus.PREEMPTED
+                preempted_req.num_computed_tokens = 0
+                preempted_req.num_preemptions += 1
+                if self.log_stats:
+                    preempted_req.record_event(EngineCoreEventType.PREEMPTED,
+                                               scheduled_timestamp)
+
+                self.waiting.prepend_request(preempted_req)
+                preempted_reqs.append(preempted_req)
+                if preempted_req == request:
+                    # No more request to preempt. Cannot schedule this request.
+                    break
+
+            if new_blocks is None:
+                # Cannot schedule this request.
                 break
-            assert new_blocks is not None
 
             # Schedule the request.
             scheduled_running_reqs.append(request)

vllm/v1/metrics/stats.py

@@ -17,13 +17,19 @@ class PrefixCacheStats:
     """Stores prefix cache hit statistics."""
     # Whether reset_prefix_cache was invoked.
     reset: bool = False
-    # The number of requests in this update.
+    # The number of new requests in this update.
     requests: int = 0
     # The number of queries in these requests. Note that "queries" here
     # means the number of tokens that were queried from the cache.
     queries: int = 0
     # The number of hits in these requests.
     hits: int = 0
+    # The number of previously preempted requests in this update.
+    preempted_requests: int = 0
+    # The `queries` number for preempted requests.
+    preempted_queries: int = 0
+    # The `hits` number for preempted requests.
+    preempted_hits: int = 0
 
 
 @dataclass

vllm/v1/request.py

@@ -115,6 +115,9 @@ class Request:
         # indicates that the output is corrupted
         self.num_nans_in_logits = 0
 
+        # The number of requests being preempted by the scheduler
+        self.num_preemptions = 0
+
         self.block_hashes: list[BlockHash] = []
         self.get_hash_new_full_blocks: Optional[Callable[
             [], list[BlockHash]]] = None