[Bugfix][core] replace heartbeat with pid check (#9818)

Signed-off-by: Joe Runde <Joseph.Runde@ibm.com>

tests/mq_llm_engine/test_error_handling.py

@@ -21,7 +21,7 @@ from vllm.usage.usage_lib import UsageContext
 from vllm.utils import FlexibleArgumentParser
 
 MODEL = "google/gemma-1.1-2b-it"
-ENGINE_ARGS = AsyncEngineArgs(model=MODEL)
+ENGINE_ARGS = AsyncEngineArgs(model=MODEL, enforce_eager=True)
 RAISED_ERROR = KeyError
 RAISED_VALUE = "foo"
 
@@ -266,3 +266,28 @@ async def test_mp_cuda_init():
 
     async with build_async_engine_client(args):
         pass
+
+
+@pytest.mark.asyncio
+async def test_engine_process_death(tmp_socket):
+    with RemoteMQLLMEngine(engine_args=ENGINE_ARGS,
+                           ipc_path=tmp_socket) as engine:
+
+        client = await engine.make_client()
+        assert client.is_running
+
+        # kill the engine process
+        engine.proc.kill()
+
+        # Generate call should fail
+        with pytest.raises(MQEngineDeadError):
+            async for _ in client.generate(prompt="Hello my name is",
+                                           sampling_params=SamplingParams(),
+                                           request_id=uuid.uuid4()):
+                pass
+
+        # And the health check should show the engine is dead
+        with pytest.raises(RuntimeError, match="Engine process .* died"):
+            await client.check_health()
+
+        client.close()

tests/mq_llm_engine/utils.py

@@ -68,7 +68,7 @@ class RemoteMQLLMEngine:
 
     async def make_client(self) -> MQLLMEngineClient:
         engine_config = self.engine_args.create_engine_config()
-        client = MQLLMEngineClient(self.ipc_path, engine_config)
+        client = MQLLMEngineClient(self.ipc_path, engine_config, self.proc.pid)
         while True:
             try:
                 await client.setup()

vllm/engine/multiprocessing/client.py

@@ -6,6 +6,7 @@ from typing import (Any, AsyncGenerator, Dict, Iterator, List, Mapping,
                     Optional, Union, cast, overload)
 
 import cloudpickle
+import psutil
 import zmq
 import zmq.asyncio
 from zmq import Frame  # type: ignore[attr-defined]
@@ -77,7 +78,8 @@ class MQLLMEngineClient(EngineClient):
             every N seconds, confirming the engine is healthy
     """
 
-    def __init__(self, ipc_path: str, engine_config: EngineConfig):
+    def __init__(self, ipc_path: str, engine_config: EngineConfig,
+                 engine_pid: int):
         self.context = zmq.asyncio.Context()
         self._errored_with: Optional[BaseException] = None
 
@@ -115,6 +117,7 @@ class MQLLMEngineClient(EngineClient):
         # Loop to check health of the LLMEngine periodically.
         # Started after the MQLLMEngine is ready.
         self.health_loop: Optional[asyncio.Task] = None
+        self._engine_process = psutil.Process(engine_pid)
 
     @staticmethod
     def is_unsupported_config(engine_args: AsyncEngineArgs):
@@ -131,21 +134,22 @@ class MQLLMEngineClient(EngineClient):
             socket.close(linger=0)
 
     async def run_heartbeat_loop(self, timeout: int):
-        """Background loop that continually listens to the RPCServer for
-        heartbeats.
+        """Background loop that continually checks to ensure the engine process
+        is still alive.
         """
         try:
             while True:
-                if await self.heartbeat_socket.poll(timeout=timeout) == 0:
-                    # No heartbeat was received. Set error and exit the loop
+                # Check if the engine process is running:
+                if not self._engine_process.is_running() or (
+                        self._engine_process.status() == psutil.STATUS_ZOMBIE):
+                    # NB: is_running() returns True for zombies
                     self._set_errored(
-                        TimeoutError("No heartbeat received "
-                                     "from MQLLMEngine"))
-                    logger.debug("Shutting down MQLLMEngineClient check "
-                                 "health loop due to timeout")
+                        RuntimeError(
+                            f"Engine process (pid {self._engine_process.pid}) "
+                            "died."))
                     break
 
-                else:
+                if await self.heartbeat_socket.poll(timeout=timeout):
                     # Heartbeat received- check the message
                     await self._check_success(
                         error_message="Heartbeat failed.",
@@ -156,6 +160,11 @@ class MQLLMEngineClient(EngineClient):
         except asyncio.CancelledError:
             logger.debug("Shutting down MQLLMEngineClient check health loop.")
 
+        except psutil.NoSuchProcess:
+            self._set_errored(
+                RuntimeError(
+                    f"Engine process (pid {self._engine_process.pid}) died."))
+
         except Exception as e:
             self._set_errored(e)
 

vllm/engine/multiprocessing/engine.py

@@ -1,7 +1,5 @@
 import pickle
 import signal
-import threading
-import time
 from contextlib import contextmanager
 from typing import Iterator, List, Optional, Union
 
@@ -21,7 +19,7 @@ from vllm.engine.multiprocessing import (ENGINE_DEAD_ERROR, IPC_DATA_EXT,
                                          RPCStartupRequest, RPCStartupResponse,
                                          RPCUProfileRequest)
 # yapf: enable
-from vllm.envs import VLLM_RPC_TIMEOUT, VLLM_USE_V1
+from vllm.envs import VLLM_USE_V1
 from vllm.executor.gpu_executor import GPUExecutor
 from vllm.logger import init_logger
 from vllm.outputs import RequestOutput
@@ -108,20 +106,6 @@ class MQLLMEngine:
         # Error state.
         self._errored_with: Optional[BaseException] = None
 
-        # Heartbeat thread
-        self.heartbeat_thread = threading.Thread(target=self._heartbeat_loop,
-                                                 daemon=True)
-        self._heartbeat_stop_event = threading.Event()
-        # The heartbeat needs to be faster than what the client will wait for
-        # The VLLM_RPC_TIMEOUT duration is in ms, and we need one in seconds
-        self.heartbeat_interval_seconds = VLLM_RPC_TIMEOUT / 5000.0
-
-        self._last_alive_time = time.time()
-        # The heartbeats can tolerate a long period of the engine chugging
-        # away at a generation request.
-        # The VLLM_RPC_TIMEOUT duration is in ms, and we need one in seconds
-        self.last_alive_threshold = VLLM_RPC_TIMEOUT * 3.0 / 1000.0
-
     @property
     def dead_error(self) -> BaseException:
         if self._errored_with is not None:
@@ -157,8 +141,6 @@ class MQLLMEngine:
             try:
                 logger.debug("Starting Startup Loop.")
                 self.run_startup_loop()
-                logger.debug("Starting heartbeat thread")
-                self.heartbeat_thread.start()
                 logger.debug("Starting Engine Loop.")
                 self.run_engine_loop()
             except Exception as e:
@@ -172,7 +154,6 @@ class MQLLMEngine:
     def cleanup(self):
         """Cleanup zeromq state on shutdown."""
         # Closes all sockets and destroys context.
-        self._heartbeat_stop_event.set()
         self.ctx.destroy(linger=0)
         del self.engine
 
@@ -211,11 +192,12 @@ class MQLLMEngine:
         """Core busy loop of the LLMEngine."""
 
         while True:
-            self._alive()
             if not self.engine.has_unfinished_requests():
                 # Poll until there is work to do.
                 while self.input_socket.poll(timeout=POLLING_TIMEOUT_MS) == 0:
-                    self._alive()
+                    # When there's no work, check on engine health and send
+                    # health status back to client
+                    self._health_check()
                     self.engine.do_log_stats()
                     logger.debug("Waiting for new requests in engine loop.")
 
@@ -314,32 +296,16 @@ class MQLLMEngine:
         if self.log_requests:
             logger.info("Aborted request %s.", request.request_id)
 
-    def _heartbeat_loop(self):
-        while not self._heartbeat_stop_event.wait(
-                timeout=self.heartbeat_interval_seconds):
-            # Loops until the stop event is set
-            self._heartbeat()
-
-        logger.debug("Exiting MQLLMEngine heartbeat thread")
-
-    def _heartbeat(self):
+    def _health_check(self):
         # Send unhealthy if engine has already errored
         if self._errored_with is not None:
             self._send_unhealthy(self._errored_with)
-
-        # Check for life of the main loop
-        elif time.time() - self._last_alive_time > self.last_alive_threshold:
-            self._send_unhealthy(RuntimeError("Engine loop has died"))
-
-        else:
-            # Otherwise- check health of the engine
-            # self.engine.check_health() raises on unhealthy
-            try:
-                self.engine.check_health()
-                self._send_healthy()
-            except Exception as e:
-                self._set_errored(e)
-                self._send_unhealthy(e)
+        try:
+            self.engine.check_health()
+            self._send_healthy()
+        except Exception as e:
+            self._set_errored(e)
+            self._send_unhealthy(e)
 
     def _send_outputs(self, outputs: REQUEST_OUTPUTS_T):
         """Send List of RequestOutput to RPCClient."""
@@ -369,9 +335,6 @@ class MQLLMEngine:
         if self._errored_with is None:
             self._errored_with = e
 
-    def _alive(self):
-        self._last_alive_time = time.time()
-
     def start_profile(self) -> None:
         if type(self.engine.model_executor) is GPUExecutor:
             self.engine.model_executor.start_profile()

vllm/entrypoints/openai/api_server.py

@@ -176,13 +176,16 @@ async def build_async_engine_client_from_engine_args(
                                                UsageContext.OPENAI_API_SERVER,
                                                ipc_path))
         engine_process.start()
-        logger.info("Started engine process with PID %d", engine_process.pid)
+        engine_pid = engine_process.pid
+        assert engine_pid is not None, "Engine process failed to start"
+        logger.info("Started engine process with PID %d", engine_pid)
 
         # Build RPCClient, which conforms to EngineClient Protocol.
         # NOTE: Actually, this is not true yet. We still need to support
         # embedding models via RPC (see TODO above)
         engine_config = engine_args.create_engine_config()
-        mp_engine_client = MQLLMEngineClient(ipc_path, engine_config)
+        mp_engine_client = MQLLMEngineClient(ipc_path, engine_config,
+                                             engine_pid)
 
         try:
             while True: