Remove AsyncLLMEngine busy loop, shield background task (#1059)

requirements-dev.txt

@@ -11,3 +11,4 @@ types-setuptools
 # testing
 pytest
 pytest-forked
+pytest-asyncio

tests/async_engine/test_async_llm_engine.py

+import asyncio
+from dataclasses import dataclass
+
+import pytest
+
+from vllm.engine.async_llm_engine import AsyncLLMEngine
+
+
+@dataclass
+class RequestOutput:
+    request_id: int
+    finished: bool = False
+
+
+class MockEngine:
+
+    def __init__(self):
+        self.step_calls = 0
+        self.add_request_calls = 0
+        self.abort_request_calls = 0
+        self.request_id = None
+
+    async def step_async(self):
+        self.step_calls += 1
+        return [RequestOutput(
+            request_id=self.request_id)] if self.request_id else []
+
+    def generate(self, request_id):
+        self.request_id = request_id
+
+    def stop_generating(self):
+        self.request_id = None
+
+    def add_request(self, **kwargs):
+        self.add_request_calls += 1
+        return
+
+    def abort_request(self, request_id):
+        self.abort_request_calls += 1
+        return
+
+
+class MockAsyncLLMEngine(AsyncLLMEngine):
+
+    def _init_engine(self, *args, **kwargs):
+        return MockEngine()
+
+
+@pytest.mark.asyncio
+async def test_new_requests_event():
+    engine = MockAsyncLLMEngine(worker_use_ray=False, engine_use_ray=False)
+    engine.start_background_loop()
+    await asyncio.sleep(0.01)
+    assert engine.engine.step_calls == 0
+
+    await engine.add_request("1", "", None)
+    await asyncio.sleep(0.01)
+    assert engine.engine.add_request_calls == 1
+    assert engine.engine.step_calls == 1
+
+    await engine.add_request("2", "", None)
+    engine.engine.generate("2")
+    await asyncio.sleep(0)
+    assert engine.engine.add_request_calls == 2
+    assert engine.engine.step_calls == 2
+    await asyncio.sleep(0)
+    assert engine.engine.step_calls == 3
+    engine.engine.stop_generating()
+    await asyncio.sleep(0)
+    assert engine.engine.step_calls == 4
+    await asyncio.sleep(0)
+    assert engine.engine.step_calls == 4
+
+    await engine.add_request("3", "", None)
+    await asyncio.sleep(0.01)
+    assert engine.engine.add_request_calls == 3
+    assert engine.engine.step_calls == 5
+    await asyncio.sleep(0.01)
+    assert engine.engine.add_request_calls == 3
+    assert engine.engine.step_calls == 5

tests/async_engine/test_request_tracker.py

@@ -4,10 +4,25 @@ from vllm.engine.async_llm_engine import RequestTracker
 from vllm.outputs import RequestOutput
 
 
+class DummyEvent:
+
+    def __init__(self):
+        self._flag = False
+
+    def set(self):
+        self._flag = True
+
+    def clear(self):
+        self._flag = False
+
+
 def test_request_tracker():
     tracker = RequestTracker()
+    tracker.new_requests_event = DummyEvent()
     stream_1 = tracker.add_request("1")
+    assert tracker.new_requests_event._flag
     new, finished = tracker.get_new_and_finished_requests()
+    assert not tracker.new_requests_event._flag
     assert len(new) == 1
     assert new[0]["request_id"] == "1"
     assert not finished
@@ -15,7 +30,9 @@ def test_request_tracker():
 
     stream_2 = tracker.add_request("2")
     stream_3 = tracker.add_request("3")
+    assert tracker.new_requests_event._flag
     new, finished = tracker.get_new_and_finished_requests()
+    assert not tracker.new_requests_event._flag
     assert len(new) == 2
     assert new[0]["request_id"] == "2"
     assert new[1]["request_id"] == "3"
@@ -26,6 +43,7 @@ def test_request_tracker():
     # request_ids must be unique
     with pytest.raises(KeyError):
         tracker.add_request("1")
+    assert not tracker.new_requests_event._flag
 
     tracker.abort_request("1")
     new, finished = tracker.get_new_and_finished_requests()
@@ -36,6 +54,7 @@ def test_request_tracker():
 
     stream_4 = tracker.add_request("4")
     tracker.abort_request("4")
+    assert tracker.new_requests_event._flag
     new, finished = tracker.get_new_and_finished_requests()
     assert len(finished) == 1
     assert "4" in finished
@@ -43,9 +62,11 @@ def test_request_tracker():
     assert stream_4.finished
 
     stream_5 = tracker.add_request("5")
+    assert tracker.new_requests_event._flag
     tracker.process_request_output(
         RequestOutput("2", "output", [], [], finished=True))
     new, finished = tracker.get_new_and_finished_requests()
+    assert not tracker.new_requests_event._flag
     assert len(finished) == 1
     assert "2" in finished
     assert len(new) == 1

vllm/engine/async_llm_engine.py

 import asyncio
 import time
 from functools import partial
-from typing import Any, Dict, Iterable, List, Optional, Set, Tuple, Type, Union
+from typing import (Any, Dict, Iterable, List, Optional, Set, Tuple, Type,
+                    Union)
 
 from vllm.config import ModelConfig
 from vllm.engine.arg_utils import AsyncEngineArgs
@@ -78,14 +79,24 @@ class RequestTracker:
         self._finished_requests: asyncio.Queue[str] = asyncio.Queue()
         self._new_requests: asyncio.Queue[Tuple[AsyncStream,
                                                 dict]] = asyncio.Queue()
+        self.new_requests_event = None
 
     def __contains__(self, item):
         return item in self._request_streams
 
-    def propagate_exception(self, exc: Exception) -> None:
-        """Propagate an exception to all request streams."""
-        for stream in self._request_streams.values():
-            stream.put(exc)
+    def init_event(self):
+        self.new_requests_event = asyncio.Event()
+
+    def propagate_exception(self,
+                            exc: Exception,
+                            request_id: Optional[str] = None) -> None:
+        """Propagate an exception to request streams
+        (all if request_id is None)."""
+        if request_id is not None:
+            self._request_streams[request_id].put(exc)
+        else:
+            for stream in self._request_streams.values():
+                stream.put(exc)
 
     def process_request_output(self,
                                request_output: RequestOutput,
@@ -112,6 +123,9 @@ class RequestTracker:
             "request_id": request_id,
             **engine_add_request_kwargs
         }))
+
+        self.new_requests_event.set()
+
         return stream
 
     def abort_request(self, request_id: str, *, verbose: bool = False) -> None:
@@ -148,8 +162,13 @@ class RequestTracker:
             self._request_streams[stream.request_id] = stream
             new_requests.append(new_request)
 
+        self.new_requests_event.clear()
+
         return new_requests, finished_requests
 
+    async def wait_for_new_requests(self):
+        await self.new_requests_event.wait()
+
 
 class _AsyncLLMEngine(LLMEngine):
     """Extension of LLMEngine to add async methods."""
@@ -251,9 +270,13 @@ class AsyncLLMEngine:
         self.max_log_len = max_log_len
         self.engine = self._init_engine(*args, **kwargs)
 
-        self.request_tracker: RequestTracker = RequestTracker()
         self.background_loop = None
+        # We need to keep a reference to unshielded
+        # task as well to prevent it from being garbage
+        # collected
+        self._background_loop_unshielded = None
         self.start_engine_loop = start_engine_loop
+        self._request_tracker = RequestTracker()
 
     @property
     def is_running(self) -> bool:
@@ -264,11 +287,14 @@ class AsyncLLMEngine:
         """Start the background loop."""
         if self.is_running:
             raise RuntimeError("Background loop is already running.")
-        self.background_loop = asyncio.get_event_loop().create_task(
-            self.run_engine_loop())
-        self.background_loop.add_done_callback(
+        self._request_tracker.init_event()
+
+        self._background_loop_unshielded = asyncio.get_event_loop(
+        ).create_task(self.run_engine_loop())
+        self._background_loop_unshielded.add_done_callback(
             partial(_raise_exception_on_finish,
-                    request_tracker=self.request_tracker))
+                    request_tracker=self._request_tracker))
+        self.background_loop = asyncio.shield(self._background_loop_unshielded)
 
     def _init_engine(self, *args,
                      **kwargs) -> Union[_AsyncLLMEngine, "ray.ObjectRef"]:
@@ -280,11 +306,13 @@ class AsyncLLMEngine:
             engine_class = ray.remote(num_gpus=1)(self._engine_class).remote
         return engine_class(*args, **kwargs)
 
-    async def engine_step(self):
-        """Kick the engine to process the waiting requests."""
+    async def engine_step(self) -> bool:
+        """Kick the engine to process the waiting requests.
+
+        Returns True if there are in-progress requests."""
 
         new_requests, finished_requests = (
-            self.request_tracker.get_new_and_finished_requests())
+            self._request_tracker.get_new_and_finished_requests())
 
         for new_request in new_requests:
             # Add the request into the vLLM engine's waiting queue.
@@ -304,9 +332,11 @@ class AsyncLLMEngine:
 
         # Put the outputs into the corresponding streams.
         for request_output in request_outputs:
-            self.request_tracker.process_request_output(
+            self._request_tracker.process_request_output(
                 request_output, verbose=self.log_requests)
 
+        return len(request_outputs) > 0
+
     async def _engine_abort(self, request_ids: Iterable[str]):
         if self.engine_use_ray:
             await self.engine.abort_request.remote(request_ids)
@@ -314,8 +344,12 @@ class AsyncLLMEngine:
             self.engine.abort_request(request_ids)
 
     async def run_engine_loop(self):
+        # Initialize the RequestTracker here so it uses the right event loop.
+        has_requests_in_progress = False
         while True:
-            await self.engine_step()
+            if not has_requests_in_progress:
+                await self._request_tracker.wait_for_new_requests()
+            has_requests_in_progress = await self.engine_step()
             await asyncio.sleep(0)
 
     async def add_request(
@@ -350,7 +384,7 @@ class AsyncLLMEngine:
                     "error that caused the background loop to stop "
                     "(AsyncEngineDeadError).")
 
-        stream = self.request_tracker.add_request(
+        stream = self._request_tracker.add_request(
             request_id,
             prompt=prompt,
             sampling_params=sampling_params,
@@ -428,8 +462,8 @@ class AsyncLLMEngine:
         Args:
             request_id: The unique id of the request.
         """
-        self.request_tracker.abort_request(request_id,
-                                           verbose=self.log_requests)
+        self._request_tracker.abort_request(request_id,
+                                            verbose=self.log_requests)
 
     async def get_model_config(self) -> ModelConfig:
         """Get the model configuration of the vLLM engine."""