[BugFix] Harden distributed DP startup (#21538)

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

vllm/utils/__init__.py

@@ -2794,6 +2794,9 @@ def make_zmq_socket(
     if linger is not None:
         socket.setsockopt(zmq.LINGER, linger)
 
+    if socket_type == zmq.XPUB:
+        socket.setsockopt(zmq.XPUB_VERBOSE, True)
+
     # Determine if the path is a TCP socket with an IPv6 address.
     # Enable IPv6 on the zmq socket if so.
     scheme, host, _ = split_zmq_path(path)

vllm/v1/engine/coordinator.py

@@ -172,6 +172,18 @@ class DPCoordinatorProc:
                 bind=True,
         ) as publish_back:
 
+            # Wait until all engines subscribe.
+            for _ in self.engines:
+                if publish_back.recv() != b'\x01':
+                    logger.error(
+                        "DP Coordinator received unexpected message while "
+                        "waiting for engines to subscribe")
+                    return
+            # Send ready message to engines.
+            publish_back.send(b"READY")
+
+            logger.info("All engine subscriptions received by DP coordinator")
+
             poller = zmq.Poller()
             poller.register(publish_front, zmq.POLLIN)
             poller.register(output_back, zmq.POLLIN)

vllm/v1/engine/core.py

@@ -461,8 +461,11 @@ class EngineCoreProc(EngineCore):
             self.has_coordinator = addresses.coordinator_output is not None
             self.frontend_stats_publish_address = (
                 addresses.frontend_stats_publish_address)
+            logger.debug("Has DP Coordinator: %s, stats publish address: %s",
+                         self.has_coordinator,
+                         self.frontend_stats_publish_address)
             # Only publish request queue stats to coordinator for "internal"
-            # LB mode.
+            # and "hybrid" LB modes .
             self.publish_dp_lb_stats = (
                 self.has_coordinator
                 and not vllm_config.parallel_config.data_parallel_external_lb)
@@ -472,25 +475,38 @@ class EngineCoreProc(EngineCore):
             super().__init__(vllm_config, executor_class, log_stats,
                              executor_fail_callback)
 
+            # Background Threads and Queues for IO. These enable us to
+            # overlap ZMQ socket IO with GPU since they release the GIL,
+            # and to overlap some serialization/deserialization with the
+            # model forward pass.
+            # Threads handle Socket <-> Queues and core_busy_loop uses Queue.
+            ready_event = threading.Event()
+            input_thread = threading.Thread(target=self.process_input_sockets,
+                                            args=(addresses.inputs,
+                                                  addresses.coordinator_input,
+                                                  identity, ready_event),
+                                            daemon=True)
+            input_thread.start()
+
+            self.output_thread = threading.Thread(
+                target=self.process_output_sockets,
+                args=(addresses.outputs, addresses.coordinator_output,
+                      self.engine_index),
+                daemon=True)
+            self.output_thread.start()
+
+            # Don't complete handshake until DP coordinator ready message is
+            # received.
+            while not ready_event.wait(timeout=10):
+                if not input_thread.is_alive():
+                    raise RuntimeError(
+                        "Input socket thread died during startup")
+                assert addresses.coordinator_input is not None
+                logger.info("Waiting for READY message from DP Coordinator...")
+
         self.step_fn = (self.step if self.batch_queue is None else
                         self.step_with_batch_queue)
 
-        # Background Threads and Queues for IO. These enable us to
-        # overlap ZMQ socket IO with GPU since they release the GIL,
-        # and to overlap some serialization/deserialization with the
-        # model forward pass.
-        # Threads handle Socket <-> Queues and core_busy_loop uses Queue.
-        threading.Thread(target=self.process_input_sockets,
-                         args=(addresses.inputs, addresses.coordinator_input,
-                               identity),
-                         daemon=True).start()
-        self.output_thread = threading.Thread(
-            target=self.process_output_sockets,
-            args=(addresses.outputs, addresses.coordinator_output,
-                  self.engine_index),
-            daemon=True)
-        self.output_thread.start()
-
     @contextmanager
     def _perform_handshakes(
         self,
@@ -505,10 +521,10 @@ class EngineCoreProc(EngineCore):
 
         For DP=1 or offline mode, this is with the colocated front-end process.
 
-        For DP>1 with internal loadbalancing this is with the shared front-end
+        For DP>1 with internal load-balancing this is with the shared front-end
         process which may reside on a different node.
 
-        For DP>1 with external or hybrid loadbalancing, two handshakes are
+        For DP>1 with external or hybrid load-balancing, two handshakes are
         performed:
             - With the rank 0 front-end process which retrieves the
               DP Coordinator ZMQ addresses and DP process group address.
@@ -772,7 +788,7 @@ class EngineCoreProc(EngineCore):
 
     def process_input_sockets(self, input_addresses: list[str],
                               coord_input_address: Optional[str],
-                              identity: bytes):
+                              identity: bytes, ready_event: threading.Event):
         """Input socket IO thread."""
 
         # Msgpack serialization decoding.
@@ -809,9 +825,14 @@ class EngineCoreProc(EngineCore):
                 # back to us.
                 input_socket.send(b'')
                 poller.register(input_socket, zmq.POLLIN)
+
             if coord_socket is not None:
+                # Wait for ready message from coordinator.
+                assert coord_socket.recv() == b"READY"
                 poller.register(coord_socket, zmq.POLLIN)
 
+            ready_event.set()
+            del ready_event
             while True:
                 for input_socket, _ in poller.poll():
                     # (RequestType, RequestData)