[Frontend][V1] Online serving performance improvements

These help in particular with TTFT, and ITL variance. Overall throughput doesn't change much.

- Break up output processing (detokenization) to avoid blocking the event loop for too long
- Freeze the heap after startup to reduce GC overhead/pauses
- Optimize a couple of CPU hotspots seen during profiling

Signed-off-by: Nick Hill <nhill@redhat.com>

Author: njhill

vllm/entrypoints/openai/api_server.py

@@ -1,5 +1,6 @@
 import asyncio
 import atexit
+import gc
 import importlib
 import inspect
 import multiprocessing
@@ -104,6 +105,11 @@ async def _force_log():
             task.add_done_callback(_running_tasks.remove)
         else:
             task = None
+
+        # Mark the startup heap as static so that it's ignored by GC.
+        # Reduces pause times of oldest generation collections.
+        gc.collect()
+        gc.freeze()
         try:
             yield
         finally:

vllm/entrypoints/openai/protocol.py

@@ -3,7 +3,7 @@
 import re
 import time
 from argparse import Namespace
-from typing import Any, Dict, List, Literal, Optional, Union
+from typing import Any, ClassVar, Dict, List, Literal, Optional, Set, Union
 
 import torch
 from pydantic import BaseModel, ConfigDict, Field, model_validator
@@ -42,23 +42,31 @@ class OpenAIBaseModel(BaseModel):
     # OpenAI API does allow extra fields
     model_config = ConfigDict(extra="allow")
 
+    # Cache class field names
+    field_names: ClassVar[Optional[Set[str]]] = None
+
     @model_validator(mode="before")
     @classmethod
     def __log_extra_fields__(cls, data):
-        if isinstance(data, dict):
+
+        field_names = cls.field_names
+        if field_names is None:
+            if not isinstance(data, dict):
+                return data
             # Get all class field names and their potential aliases
             field_names = set()
             for field_name, field in cls.model_fields.items():
                 field_names.add(field_name)
-                if hasattr(field, 'alias') and field.alias:
-                    field_names.add(field.alias)
-
-            # Compare against both field names and aliases
-            extra_fields = data.keys() - field_names
-            if extra_fields:
-                logger.warning(
-                    "The following fields were present in the request "
-                    "but ignored: %s", extra_fields)
+                if alias := getattr(field, 'alias', None):
+                    field_names.add(alias)
+            cls.field_names = field_names
+
+        # Compare against both field names and aliases
+        if any(k not in field_names for k in data):
+            logger.warning(
+                "The following fields were present in the request "
+                "but ignored: %s",
+                data.keys() - field_names)
         return data
 
 

vllm/v1/engine/async_llm.py

@@ -1,7 +1,10 @@
 import asyncio
+import math
 import os
 from typing import AsyncGenerator, List, Mapping, Optional, Type, Union
 
+import numpy as np
+
 from vllm.config import ModelConfig, VllmConfig
 from vllm.engine.arg_utils import AsyncEngineArgs
 from vllm.engine.protocol import EngineClient
@@ -26,6 +29,11 @@
 
 logger = init_logger(__name__)
 
+# For now determined empirically.
+# Larger => higher ITL variance
+# Smaller => higher TTFT, throughput impacted
+OUTPUT_PROCESSING_CHUNK_SIZE = 128
+
 
 class AsyncLLM(EngineClient):
 
@@ -205,17 +213,15 @@ async def generate(
 
             # The output_handler task pushes items into the queue.
             # This task pulls from the queue and yields to caller.
-            while True:
+            finished = False
+            while not finished:
                 # Note: drain queue without await if possible (avoids
                 # task switching under load which helps performance).
-                out = q.get_nowait() if q.qsize() > 0 else await q.get()
+                out = q.get_nowait() if not q.empty() else await q.get()
 
                 # Note: both OutputProcessor and EngineCore handle their
                 # own request cleanup based on finished.
-                if out.finished:
-                    yield out
-                    break
-
+                finished = out.finished
                 yield out
 
         # If the request is disconnected by the client, the
@@ -233,22 +239,41 @@ async def _run_output_handler(self):
                 # 1) Pull EngineCoreOutputs from the EngineCore.
                 outputs = await self.engine_core.get_output_async()
 
-                # 2) Process EngineCoreOutputs.
-                processed_outputs = self.output_processor.process_outputs(
-                    outputs.outputs)
-                # NOTE: RequestOutputs are pushed to their queues.
-                assert len(processed_outputs.request_outputs) == 0
-
-                # 3) Abort any reqs that finished due to stop strings.
-                await self.engine_core.abort_requests_async(
-                    processed_outputs.reqs_to_abort)
+                # Split outputs into chunks of at most
+                # OUTPUT_PROCESSING_CHUNK_SIZE, so that we don't block the
+                # event loop for too long.
+                num_outputs = len(outputs.outputs)
+                if num_outputs <= OUTPUT_PROCESSING_CHUNK_SIZE:
+                    slices = (outputs.outputs,)
+                else:
+                    slices = np.array_split(outputs.outputs,
+                        math.ceil(num_outputs / OUTPUT_PROCESSING_CHUNK_SIZE))
+
+                iteration_stats = None
+                for i, outputs_slice in enumerate(slices):
+
+                    # 2) Process EngineCoreOutputs.
+                    processed_outputs = self.output_processor.process_outputs(
+                        outputs_slice, iteration_stats)
+                    # NOTE: RequestOutputs are pushed to their queues.
+                    assert not processed_outputs.request_outputs
+                    iteration_stats = processed_outputs.iteration_stats
+
+                    # Allow other asyncio tasks to run between chunks
+                    if i + 1 < len(slices):
+                        await asyncio.sleep(0)
+
+                    # 3) Abort any reqs that finished due to stop strings.
+                    await self.engine_core.abort_requests_async(
+                        processed_outputs.reqs_to_abort)
 
                 # 4) Logging.
                 # TODO(rob): make into a coroutine and launch it in
                 # background thread once we add Prometheus.
+                assert iteration_stats is not None
                 self._log_stats(
                     scheduler_stats=outputs.scheduler_stats,
-                    iteration_stats=processed_outputs.iteration_stats,
+                    iteration_stats=iteration_stats,
                 )
 
         except Exception as e:

vllm/v1/engine/output_processor.py

@@ -101,6 +101,7 @@ def add_request(
     def process_outputs(
         self,
         engine_core_outputs: List[EngineCoreOutput],
+        iteration_stats: Optional[IterationStats] = None,
     ) -> OutputProcessorOutput:
         """
         Process the EngineCoreOutputs:
@@ -133,7 +134,8 @@ def process_outputs(
 
         request_outputs: List[RequestOutput] = []
         reqs_to_abort: List[str] = []
-        iteration_stats = IterationStats(self.log_stats)
+        if not iteration_stats:
+            iteration_stats = IterationStats(self.log_stats)
         for engine_core_output in engine_core_outputs:
             req_id = engine_core_output.request_id
             req_state = self.request_states.get(req_id)
@@ -175,8 +177,8 @@ def process_outputs(
             iteration_stats=iteration_stats,
         )
 
+    @staticmethod
     def _make_request_output(
-        self,
         request_state: RequestState,
         detokenizer_output: Optional[DetokenizerOutput],
     ) -> Optional[RequestOutput]:

vllm/v1/request.py

@@ -64,6 +64,12 @@ def __init__(
         # recomputing.
         self._kv_block_hashes: List[BlockHashType] = []
 
+        # Read-only views
+        # Prevent directly appending to the these lists since
+        # they should also be updated simultaneously.
+        self.output_token_ids = ConstantList(self._output_token_ids)
+        self.all_token_ids = ConstantList(self._all_token_ids)
+
     @classmethod
     def from_engine_core_request(cls, request: EngineCoreRequest) -> "Request":
         return cls(
@@ -79,18 +85,6 @@ def from_engine_core_request(cls, request: EngineCoreRequest) -> "Request":
             lora_request=request.lora_request,
         )
 
-    @property
-    def output_token_ids(self) -> ConstantList[int]:
-        # Prevent directly appending to the output_token_ids since
-        # all_token_ids should also be updated simultaneously.
-        return ConstantList(self._output_token_ids)
-
-    @property
-    def all_token_ids(self) -> ConstantList[int]:
-        # Prevent directly appending to the all_token_ids since
-        # output_token_ids should also be updated simultaneously
-        return ConstantList(self._all_token_ids)
-
     def append_output_token_ids(
         self,
         token_ids: Union[int, List[int]],