Make `AsyncLLMEngine` more robust & fix batched abort (#969)

Signed-off-by: Antoni Baum <antoni.baum@protonmail.com>
Co-authored-by: Avnish Narayan <38871737+avnishn@users.noreply.github.com>

tests/async_engine/api_server_async_engine.py

+"""vllm.entrypoints.api_server with some extra logging for testing."""
+import argparse
+from typing import Any, Dict
+
+import uvicorn
+from fastapi.responses import JSONResponse, Response
+
+import vllm.entrypoints.api_server
+from vllm.engine.arg_utils import AsyncEngineArgs
+from vllm.engine.async_llm_engine import AsyncLLMEngine
+
+app = vllm.entrypoints.api_server.app
+
+
+class AsyncLLMEngineWithStats(AsyncLLMEngine):
+
+    def __init__(self, *args, **kwargs):
+        super().__init__(*args, **kwargs)
+        self._num_aborts = 0
+
+    async def abort(self, request_id: str) -> None:
+        await super().abort(request_id)
+        self._num_aborts += 1
+
+    def testing_stats(self) -> Dict[str, Any]:
+        return {"num_aborted_requests": self._num_aborts}
+
+
+@app.get("/stats")
+def stats() -> Response:
+    """Get the statistics of the engine."""
+    return JSONResponse(engine.testing_stats())
+
+
+if __name__ == "__main__":
+    parser = argparse.ArgumentParser()
+    parser.add_argument("--host", type=str, default="localhost")
+    parser.add_argument("--port", type=int, default=8000)
+    parser = AsyncEngineArgs.add_cli_args(parser)
+    args = parser.parse_args()
+
+    engine_args = AsyncEngineArgs.from_cli_args(args)
+    engine = AsyncLLMEngineWithStats.from_engine_args(engine_args,
+                                                      start_engine_loop=False)
+    vllm.entrypoints.api_server.engine = engine
+    uvicorn.run(
+        app,
+        host=args.host,
+        port=args.port,
+        log_level="debug",
+        timeout_keep_alive=vllm.entrypoints.api_server.TIMEOUT_KEEP_ALIVE)

tests/async_engine/test_api_server.py

+import subprocess
+import sys
+import time
+from multiprocessing import Pool
+from pathlib import Path
+
+import pytest
+import requests
+
+
+def _query_server(prompt: str) -> dict:
+    response = requests.post("http://localhost:8000/generate",
+                             json={
+                                 "prompt": prompt,
+                                 "max_tokens": 100,
+                                 "temperature": 0,
+                                 "ignore_eos": True
+                             })
+    response.raise_for_status()
+    return response.json()
+
+
+@pytest.fixture
+def api_server():
+    script_path = Path(__file__).parent.joinpath(
+        "api_server_async_engine.py").absolute()
+    uvicorn_process = subprocess.Popen([
+        sys.executable, "-u",
+        str(script_path), "--model", "facebook/opt-125m"
+    ])
+    yield
+    uvicorn_process.terminate()
+
+
+def test_api_server(api_server):
+    """
+    Run the API server and test it.
+
+    We run both the server and requests in separate processes.
+
+    We test that the server can handle incoming requests, including
+    multiple requests at the same time, and that it can handle requests
+    being cancelled without crashing.
+    """
+    with Pool(32) as pool:
+        # Wait until the server is ready
+        prompts = ["Hello world"] * 1
+        result = None
+        while not result:
+            try:
+                for result in pool.map(_query_server, prompts):
+                    break
+            except:
+                time.sleep(1)
+
+        # Actual tests start here
+        # Try with 1 prompt
+        for result in pool.map(_query_server, prompts):
+            assert result
+
+        num_aborted_requests = requests.get(
+            "http://localhost:8000/stats").json()["num_aborted_requests"]
+        assert num_aborted_requests == 0
+
+        # Try with 100 prompts
+        prompts = ["Hello world"] * 100
+        for result in pool.map(_query_server, prompts):
+            assert result
+
+        # Cancel requests
+        pool.map_async(_query_server, prompts)
+        time.sleep(0.01)
+        pool.terminate()
+        pool.join()
+
+        # check cancellation stats
+        num_aborted_requests = requests.get(
+            "http://localhost:8000/stats").json()["num_aborted_requests"]
+        assert num_aborted_requests > 0
+
+    # check that server still runs after cancellations
+    with Pool(32) as pool:
+        # Try with 100 prompts
+        prompts = ["Hello world"] * 100
+        for result in pool.map(_query_server, prompts):
+            assert result

tests/async_engine/test_request_tracker.py

+import pytest
+
+from vllm.engine.async_llm_engine import RequestTracker
+from vllm.outputs import RequestOutput
+
+
+def test_request_tracker():
+    tracker = RequestTracker()
+    stream_1 = tracker.add_request("1")
+    new, finished = tracker.get_new_and_finished_requests()
+    assert len(new) == 1
+    assert new[0]["request_id"] == "1"
+    assert not finished
+    assert not stream_1.finished
+
+    stream_2 = tracker.add_request("2")
+    stream_3 = tracker.add_request("3")
+    new, finished = tracker.get_new_and_finished_requests()
+    assert len(new) == 2
+    assert new[0]["request_id"] == "2"
+    assert new[1]["request_id"] == "3"
+    assert not finished
+    assert not stream_2.finished
+    assert not stream_3.finished
+
+    # request_ids must be unique
+    with pytest.raises(KeyError):
+        tracker.add_request("1")
+
+    tracker.abort_request("1")
+    new, finished = tracker.get_new_and_finished_requests()
+    assert len(finished) == 1
+    assert "1" in finished
+    assert not new
+    assert stream_1.finished
+
+    stream_4 = tracker.add_request("4")
+    tracker.abort_request("4")
+    new, finished = tracker.get_new_and_finished_requests()
+    assert len(finished) == 1
+    assert "4" in finished
+    assert not new
+    assert stream_4.finished
+
+    stream_5 = tracker.add_request("5")
+    tracker.process_request_output(
+        RequestOutput("2", "output", [], [], finished=True))
+    new, finished = tracker.get_new_and_finished_requests()
+    assert len(finished) == 1
+    assert "2" in finished
+    assert len(new) == 1
+    assert new[0]["request_id"] == "5"
+    assert stream_2.finished
+    assert not stream_5.finished

vllm/core/scheduler.py

@@ -92,7 +92,10 @@ class Scheduler:
             request_id = (request_id, )
         request_ids = set(request_id)
         for state_queue in [self.waiting, self.running, self.swapped]:
-            for seq_group in state_queue:
+            # We need to reverse the list as we are removing elements
+            # from it as we iterate over it. If we don't do it,
+            # indices will get messed up and we will skip over elements.
+            for seq_group in reversed(state_queue):
                 if seq_group.request_id in request_ids:
                     # Remove the sequence group from the state queue.
                     state_queue.remove(seq_group)

vllm/engine/async_llm_engine.py

 import asyncio
 import time
 from functools import partial
-from typing import Any, Dict, Iterable, List, Optional, 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
@@ -14,6 +14,28 @@ from vllm.sampling_params import SamplingParams
 logger = init_logger(__name__)
 
 
+class AsyncEngineDeadError(RuntimeError):
+    pass
+
+
+def _raise_exception_on_finish(task: asyncio.Task,
+                               request_tracker: "RequestTracker") -> None:
+    msg = ("Task finished unexpectedly. This should never happen! "
+           "Please open an issue on Github.")
+    try:
+        try:
+            task.result()
+        except asyncio.CancelledError:
+            return
+        except Exception as exc:
+            raise AsyncEngineDeadError(
+                msg + " See stack trace above for the actual cause.") from exc
+        raise AsyncEngineDeadError(msg)
+    except Exception as exc:
+        request_tracker.propagate_exception(exc)
+        raise exc
+
+
 class AsyncStream:
     """A stream of RequestOutputs for a request that can be
     iterated over asynchronously."""
@@ -43,15 +65,90 @@ class AsyncStream:
         result = await self._queue.get()
         if result is StopIteration:
             raise StopAsyncIteration
+        elif isinstance(result, Exception):
+            raise result
         return result
 
 
-def _raise_exception_on_finish(task: asyncio.Task) -> None:
-    try:
-        task.result()
-    except Exception as e:
-        raise RuntimeError("Task finished unexpectedly.") from e
-    raise RuntimeError("Task finished unexpectedly.")
+class RequestTracker:
+    """Synchronous abstraction for tracking requests."""
+
+    def __init__(self) -> None:
+        self._request_streams: Dict[str, AsyncStream] = {}
+        self._finished_requests: asyncio.Queue[str] = asyncio.Queue()
+        self._new_requests: asyncio.Queue[Tuple[AsyncStream,
+                                                dict]] = asyncio.Queue()
+
+    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 process_request_output(self,
+                               request_output: RequestOutput,
+                               *,
+                               verbose: bool = False) -> None:
+        """Process a request output from the engine."""
+        request_id = request_output.request_id
+
+        self._request_streams[request_id].put(request_output)
+        if request_output.finished:
+            if verbose:
+                logger.info(f"Finished request {request_id}.")
+            self.abort_request(request_id)
+
+    def add_request(self, request_id: str,
+                    **engine_add_request_kwargs) -> AsyncStream:
+        """Add a request to be sent to the engine on the next background
+        loop iteration."""
+        if request_id in self._request_streams:
+            raise KeyError(f"Request {request_id} already exists.")
+
+        stream = AsyncStream(request_id)
+        self._new_requests.put_nowait((stream, {
+            "request_id": request_id,
+            **engine_add_request_kwargs
+        }))
+        return stream
+
+    def abort_request(self, request_id: str, *, verbose: bool = False) -> None:
+        """Abort a request during next background loop iteration."""
+        if verbose:
+            logger.info(f"Aborted request {request_id}.")
+
+        self._finished_requests.put_nowait(request_id)
+
+        if request_id not in self._request_streams or self._request_streams[
+                request_id].finished:
+            # The request has already finished or been aborted.
+            return
+
+        self._request_streams[request_id].finish()
+
+    def get_new_and_finished_requests(self) -> Tuple[List[dict], Set[str]]:
+        """Get the new requests and finished requests to be
+        sent to the engine."""
+        new_requests: List[dict] = []
+        finished_requests: Set[str] = set()
+
+        while not self._finished_requests.empty():
+            request_id = self._finished_requests.get_nowait()
+            finished_requests.add(request_id)
+            self._request_streams.pop(request_id, None)
+
+        while not self._new_requests.empty():
+            stream, new_request = self._new_requests.get_nowait()
+            if stream.request_id in finished_requests:
+                # The request has already been aborted.
+                stream.finish()
+                continue
+            self._request_streams[stream.request_id] = stream
+            new_requests.append(new_request)
+
+        return new_requests, finished_requests
 
 
 class _AsyncLLMEngine(LLMEngine):
@@ -150,16 +247,15 @@ class AsyncLLMEngine:
         self.log_requests = log_requests
         self.engine = self._init_engine(*args, **kwargs)
 
-        # Request id -> stream.
-        self.request_streams: Dict[str, AsyncStream] = {}
-        self.finished_requests: asyncio.Queue[str] = asyncio.Queue()
+        self.request_tracker: RequestTracker = RequestTracker()
         self.background_loop = None
         if start_engine_loop:
             self.start_background_loop()
 
     @property
     def is_running(self) -> bool:
-        return self.background_loop is not None
+        return (self.background_loop is not None
+                and not self.background_loop.done())
 
     def start_background_loop(self) -> None:
         """Start the background loop."""
@@ -167,7 +263,9 @@ class AsyncLLMEngine:
             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(_raise_exception_on_finish)
+        self.background_loop.add_done_callback(
+            partial(_raise_exception_on_finish,
+                    request_tracker=self.request_tracker))
 
     def _init_engine(self, *args,
                      **kwargs) -> Union[_AsyncLLMEngine, "ray.ObjectRef"]:
@@ -181,6 +279,21 @@ class AsyncLLMEngine:
 
     async def engine_step(self):
         """Kick the engine to process the waiting requests."""
+
+        new_requests, 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.
+            # TODO: Maybe add add_request_batch to reduce Ray overhead
+            if self.engine_use_ray:
+                await self.engine.add_request.remote(**new_request)
+            else:
+                self.engine.add_request(**new_request)
+
+        if finished_requests:
+            await self._engine_abort(finished_requests)
+
         if self.engine_use_ray:
             request_outputs = await self.engine.step.remote()
         else:
@@ -188,20 +301,8 @@ class AsyncLLMEngine:
 
         # Put the outputs into the corresponding streams.
         for request_output in request_outputs:
-            request_id = request_output.request_id
-            self.request_streams[request_id].put(request_output)
-            if request_output.finished:
-                if self.log_requests:
-                    logger.info(f"Finished request {request_id}.")
-                self.request_streams[request_id].finish()
-                self.finished_requests.put_nowait(request_id)
-
-        finished_request = set()
-        while not self.finished_requests.empty():
-            finished_request.add(self.finished_requests.get_nowait())
-        await self._engine_abort(finished_request)
-        for request_id in finished_request:
-            del self.request_streams[request_id]
+            self.request_tracker.process_request_output(
+                request_output, verbose=self.log_requests)
 
     async def _engine_abort(self, request_ids: Iterable[str]):
         if self.engine_use_ray:
@@ -228,23 +329,17 @@ class AsyncLLMEngine:
                         f"sampling params: {sampling_params}, "
                         f"prompt token ids: {prompt_token_ids}.")
 
-        if request_id in self.request_streams:
-            raise KeyError(f"Request {request_id} already exists.")
-        stream = AsyncStream(request_id)
-        self.request_streams[request_id] = stream
+        if not self.is_running:
+            raise AsyncEngineDeadError(
+                "Background loop is not running. If it was running, "
+                "inspect the output to find the stacktrace of the "
+                "error that caused the background loop to stop "
+                "(AsyncEngineDeadError).")
 
-        # Add the request into the vLLM engine's waiting queue.
-        if self.engine_use_ray:
-            await self.engine.add_request.remote(
+        stream = self.request_tracker.add_request(
             request_id,
-                prompt,
-                sampling_params,
-                prompt_token_ids=prompt_token_ids,
-                arrival_time=arrival_time)
-        else:
-            self.engine.add_request(request_id,
-                                    prompt,
-                                    sampling_params,
+            prompt=prompt,
+            sampling_params=sampling_params,
             prompt_token_ids=prompt_token_ids,
             arrival_time=arrival_time)
 
@@ -300,6 +395,13 @@ class AsyncLLMEngine:
         Args:
             request_id: The unique id of the request.
         """
+        if not self.is_running:
+            raise AsyncEngineDeadError(
+                "Background loop is not running. If it was running, "
+                "inspect the output to find the stacktrace of the "
+                "error that caused the background loop to stop "
+                "(AsyncEngineDeadError).")
+
         return self._abort(request_id)
 
     def _abort(self, request_id: str) -> None:
@@ -311,16 +413,8 @@ class AsyncLLMEngine:
         Args:
             request_id: The unique id of the request.
         """
-        if request_id not in self.request_streams or self.request_streams[
-                request_id].finished:
-            # The request has already finished or been aborted.
-            return
-
-        if self.log_requests:
-            logger.info(f"Aborted request {request_id}.")
-
-        self.request_streams[request_id].finish()
-        self.finished_requests.put_nowait(request_id)
+        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."""

vllm/entrypoints/api_server.py

@@ -14,6 +14,7 @@ from vllm.utils import random_uuid
 TIMEOUT_KEEP_ALIVE = 5  # seconds.
 TIMEOUT_TO_PREVENT_DEADLOCK = 1  # seconds.
 app = FastAPI()
+engine = None
 
 
 @app.post("/generate")

vllm/entrypoints/openai/api_server.py

@@ -44,6 +44,7 @@ TIMEOUT_KEEP_ALIVE = 5  # seconds
 logger = init_logger(__name__)
 served_model = None
 app = fastapi.FastAPI()
+engine = None
 
 
 def create_error_response(status_code: HTTPStatus,