[Bugfix] Use heartbeats instead of health checks (#8583)

tests/mq_llm_engine/test_error_handling.py

@@ -153,27 +153,20 @@ async def test_failed_abort(tmp_socket):
         await client.check_health()
 
         # Trigger an abort on the client side.
-        async def bad_abort_after_2s():
-            await asyncio.sleep(2.0)
-            await client.abort(request_id="foo")
+        # This request ID does not exist, and will cause the engine to error
+        await client.abort(request_id="foo")
 
-        # Trigger an abort in 2s from now.
-        abort_task = asyncio.create_task(bad_abort_after_2s())
-
-        # Exception in abort() will happen during this generation.
-        # This will kill the engine and should return ENGINE_DEAD_ERROR
+        # Future generation requests will now fail
         # with reference to the original KeyError("foo")
         with pytest.raises(MQEngineDeadError) as execinfo:
             async for _ in client.generate(
                     inputs="Hello my name is",
-                    sampling_params=SamplingParams(max_tokens=2000),
+                    sampling_params=SamplingParams(max_tokens=10),
                     request_id=uuid.uuid4()):
                 pass
         assert "KeyError" in repr(execinfo.value)
         assert client.errored
 
-        await abort_task
-
         # This should raise the original error.
         with pytest.raises(RAISED_ERROR):
             await client.check_health()

vllm/engine/multiprocessing/__init__.py

@@ -43,10 +43,6 @@ class RPCAbortRequest:
     request_id: str
 
 
-class RPCHealthRequest:
-    pass
-
-
 class RPCStartupRequest(Enum):
     IS_SERVER_READY = 1
 
@@ -56,8 +52,7 @@ class RPCStartupResponse:
     tracing_enabled: bool
 
 
-RPC_REQUEST_T = Union[RPCProcessRequest, RPCAbortRequest, RPCHealthRequest,
-                      RPCStartupRequest]
+RPC_REQUEST_T = Union[RPCProcessRequest, RPCAbortRequest, RPCStartupRequest]
 
 REQUEST_OUTPUTS_T = Union[List[RequestOutput], RPCError]
 

vllm/engine/multiprocessing/client.py

@@ -20,9 +20,8 @@ from vllm.engine.multiprocessing import (ENGINE_DEAD_ERROR, IPC_DATA_EXT,
                                          IPC_HEALTH_EXT, IPC_INPUT_EXT,
                                          IPC_OUTPUT_EXT, RPC_REQUEST_T,
                                          VLLM_RPC_SUCCESS_STR, RPCAbortRequest,
-                                         RPCError, RPCHealthRequest,
-                                         RPCProcessRequest, RPCStartupRequest,
-                                         RPCStartupResponse)
+                                         RPCError, RPCProcessRequest,
+                                         RPCStartupRequest, RPCStartupResponse)
 # yapf: enable
 from vllm.envs import VLLM_RPC_TIMEOUT
 from vllm.inputs import PromptInputs
@@ -95,9 +94,9 @@ class MQLLMEngineClient:
         self.output_socket: Socket = self.context.socket(zmq.constants.PULL)
         self.output_socket.connect(f"{ipc_path}{IPC_OUTPUT_EXT}")
 
-        # IPC path for ack of check_health requests.
-        self.health_socket: Socket = self.context.socket(zmq.constants.PULL)
-        self.health_socket.connect(f"{ipc_path}{IPC_HEALTH_EXT}")
+        # IPC path for acking heartbeats.
+        self.heartbeat_socket: Socket = self.context.socket(zmq.constants.PULL)
+        self.heartbeat_socket.connect(f"{ipc_path}{IPC_HEALTH_EXT}")
 
         # IPC path for the data socket.
         self.data_ipc_path = f"{ipc_path}{IPC_DATA_EXT}"
@@ -124,34 +123,28 @@ class MQLLMEngineClient:
         finally:
             socket.close(linger=0)
 
-    async def run_check_health_loop(self, timeout: int):
-        """Background loop that continually probes the RPCServer for health.
-        
-        The loop sends CHECK_HEALTH requests to the INPUT_SOCKET, which
-        the MQLLMEngine server is blocking on.
-
-        The Server replies on the HEALTH_SOCKET (rather than on the 
-        OUTPUT_SOCKET such that the messages are not intermingled with
-        output streaming).
+    async def run_heartbeat_loop(self, timeout: int):
+        """Background loop that continually listens to the RPCServer for
+        heartbeats.
         """
-
         try:
             while True:
-                if await self.health_socket.poll(timeout=timeout) == 0:
-                    # Wakeup every N seconds and do a health probe.
-                    await self._send_one_way_rpc_request(
-                        RPCHealthRequest(), self.input_socket)
-
-                    # Wait for ack from the health socket.
-                    await self._await_ack(error_message="Health check failed.",
-                                          socket=self.health_socket)
+                if await self.heartbeat_socket.poll(timeout=timeout) == 0:
+                    # No heartbeat was received. Set error and exit the loop
+                    self._set_errored(
+                        TimeoutError("No heartbeat received "
+                                     "from MQLLMEngine"))
+                    logger.debug("Shutting down MQLLMEngineClient check "
+                                 "health loop due to timeout")
+                    break
+
                 else:
-                    # Server sent a health status message unprompted.
+                    # Heartbeat received- check the message
                     await self._check_success(
-                        error_message="Health check failed.",
-                        socket=self.health_socket)
+                        error_message="Heartbeat failed.",
+                        socket=self.heartbeat_socket)
 
-                logger.debug("Health probe successful.")
+                logger.debug("Heartbeat successful.")
 
         except asyncio.CancelledError:
             logger.debug("Shutting down MQLLMEngineClient check health loop.")
@@ -234,7 +227,7 @@ class MQLLMEngineClient:
 
             # Start health_loop.
             self.health_loop = asyncio.create_task(
-                self.run_check_health_loop(timeout=VLLM_RPC_TIMEOUT))
+                self.run_heartbeat_loop(timeout=VLLM_RPC_TIMEOUT))
 
     def close(self):
         """Destroy the ZeroMQ Context."""

vllm/engine/multiprocessing/engine.py

 import pickle
 import signal
+import threading
+import time
 from contextlib import contextmanager
 from typing import Iterator, List, Optional, Union
 
@@ -15,10 +17,10 @@ from vllm.engine.multiprocessing import (ENGINE_DEAD_ERROR, IPC_DATA_EXT,
                                          IPC_HEALTH_EXT, IPC_INPUT_EXT,
                                          IPC_OUTPUT_EXT, REQUEST_OUTPUTS_T,
                                          VLLM_RPC_SUCCESS_STR, RPCAbortRequest,
-                                         RPCError, RPCHealthRequest,
-                                         RPCProcessRequest, RPCStartupRequest,
-                                         RPCStartupResponse)
+                                         RPCError, RPCProcessRequest,
+                                         RPCStartupRequest, RPCStartupResponse)
 # yapf: enable
+from vllm.envs import VLLM_RPC_TIMEOUT
 from vllm.logger import init_logger
 from vllm.outputs import RequestOutput
 from vllm.usage.usage_lib import UsageContext
@@ -91,9 +93,9 @@ class MQLLMEngine:
         self.output_socket = self.ctx.socket(zmq.constants.PUSH)
         self.output_socket.bind(f"{ipc_path}{IPC_OUTPUT_EXT}")
 
-        # Send health status back to client.
-        self.health_socket = self.ctx.socket(zmq.constants.PUSH)
-        self.health_socket.bind(f"{ipc_path}{IPC_HEALTH_EXT}")
+        # Send heartbeats back to client.
+        self.heartbeat_socket = self.ctx.socket(zmq.constants.PUSH)
+        self.heartbeat_socket.bind(f"{ipc_path}{IPC_HEALTH_EXT}")
 
         # IPC path for the data socket.
         self.data_ipc_path = f"{ipc_path}{IPC_DATA_EXT}"
@@ -101,6 +103,20 @@ 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:
@@ -131,6 +147,8 @@ 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:
@@ -144,6 +162,7 @@ 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
 
@@ -182,9 +201,11 @@ 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()
                     self.engine.do_log_stats()
                     logger.debug("Waiting for new requests in engine loop.")
 
@@ -200,7 +221,6 @@ class MQLLMEngine:
 
     def engine_step(self) -> List[RequestOutput]:
         """Engine step wrapper with error handling."""
-
         try:
             return self.engine.step()
         except SystemExit:
@@ -229,10 +249,9 @@ class MQLLMEngine:
                     self._handle_process_request(request)
                 elif isinstance(request, RPCAbortRequest):
                     self._handle_abort_request(request)
-                elif isinstance(request, RPCHealthRequest):
-                    self._handle_health_request()
                 else:
-                    raise ValueError("Unknown RPCRequest Type: {request}")
+                    raise ValueError("Unknown RPCRequest Type: "
+                                     f"{type(request)}")
 
         except Exception as e:
             self._set_errored(e)
@@ -279,13 +298,32 @@ class MQLLMEngine:
         if self.log_requests:
             logger.info("Aborted request %s.", request.request_id)
 
-    def _handle_health_request(self):
+    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):
+        # Send unhealthy if engine has already errored
         if self._errored_with is not None:
             self._send_unhealthy(self._errored_with)
 
-        # Raises error if unhealthy.
-        self.engine.check_health()
-        self._send_healthy()
+        # 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)
 
     def _send_outputs(self, outputs: REQUEST_OUTPUTS_T):
         """Send List of RequestOutput to RPCClient."""
@@ -295,12 +333,14 @@ class MQLLMEngine:
 
     def _send_healthy(self):
         """Send HEALTHY message to RPCClient."""
-        self.health_socket.send_multipart(HEALTHY_RESPONSE, copy=False)
+        if not self.heartbeat_socket.closed:
+            self.heartbeat_socket.send_multipart(HEALTHY_RESPONSE, copy=False)
 
     def _send_unhealthy(self, error: BaseException):
         """Send UNHEALTHY message to RPCClient."""
-        error_bytes = pickle.dumps(error)
-        self.health_socket.send_multipart((error_bytes, ), copy=False)
+        if not self.heartbeat_socket.closed:
+            error_bytes = pickle.dumps(error)
+            self.heartbeat_socket.send_multipart((error_bytes, ), copy=False)
 
     def _async_socket_engine_callback(self,
                                       request_outputs: REQUEST_OUTPUTS_T):
@@ -313,6 +353,9 @@ class MQLLMEngine:
         if self._errored_with is None:
             self._errored_with = e
 
+    def _alive(self):
+        self._last_alive_time = time.time()
+
 
 def run_mp_engine(engine_args: AsyncEngineArgs, usage_context: UsageContext,
                   ipc_path: str):