[V1][DP] More robust DP/EP dummy request coordination (#16277)

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

Author: njhill

tests/v1/test_async_llm_dp.py

@@ -101,9 +101,9 @@ async def test_load(output_kind: RequestOutputKind):
         # the engines only synchronize stopping every N steps so
         # allow a small amount of time here.
         for _ in range(10):
-            if core_client.num_engines_running == 0:
+            if not core_client.engines_running:
                 break
             await asyncio.sleep(0.5)
 
-        assert core_client.num_engines_running == 0
+        assert not core_client.engines_running
         assert not core_client.reqs_in_flight

vllm/v1/engine/__init__.py

@@ -61,6 +61,11 @@ class EngineCoreRequest(
     arrival_time: float
     lora_request: Optional[LoRARequest]
 
+    # Used in DP case to indicate which wave of requests this is expected to
+    # belong to, to cover a race condition where the request is sent before
+    # a wave finished notification is received.
+    current_wave: int = 0
+
 
 class EngineCoreEventType(enum.IntEnum):
     """The type of engine core request event."""
@@ -139,8 +144,12 @@ class EngineCoreOutputs(
     utility_output: Optional[UtilityOutput] = None
     finished_requests: Optional[set[str]] = None
 
-    # In DP case, used to signal that the engine is paused.
-    engine_paused: bool = False
+    # In DP case, used to signal that the current wave of requests
+    # has finished and the engines are paused.
+    wave_complete: Optional[int] = None
+    # In DP case, used to signal that a request was received for an
+    # "old" wave, so the next wave needs to be started in other engines.
+    start_wave: Optional[int] = None
 
     def __post_init__(self):
         if self.timestamp == 0.0:
@@ -154,7 +163,7 @@ class EngineCoreRequestType(enum.Enum):
     """
     ADD = b'\x00'
     ABORT = b'\x01'
-    START_DP = b'\x02'
+    START_DP_WAVE = b'\x02'
     UTILITY = b'\x03'
     # Sentinel used within EngineCoreProc.
     EXECUTOR_FAILED = b'\x04'

vllm/v1/engine/core.py

@@ -325,7 +325,7 @@ def __init__(
 
         self.step_fn = (self.step if self.batch_queue is None else
                         self.step_with_batch_queue)
-        self.global_unfinished_reqs = False
+        self.engines_running = False
 
         # Background Threads and Queues for IO. These enable us to
         # overlap ZMQ socket IO with GPU since they release the GIL,
@@ -410,19 +410,15 @@ def _process_input_queue(self):
         """Exits when an engine step needs to be performed."""
 
         waited = False
-        while not self.global_unfinished_reqs and not (
-                self.scheduler.has_requests()):
+        while not self.engines_running and not (self.scheduler.has_requests()):
             if logger.isEnabledFor(DEBUG) and self.input_queue.empty():
                 logger.debug("EngineCore waiting for work.")
                 waited = True
             req = self.input_queue.get()
             self._handle_client_request(*req)
 
         if waited:
-            logger.debug(
-                "EngineCore loop active - local unfinished: %s, finished: %s.",
-                self.scheduler.has_unfinished_requests(),
-                self.scheduler.has_finished_requests())
+            logger.debug("EngineCore loop active.")
 
         # Handle any more client requests.
         while not self.input_queue.empty():
@@ -446,10 +442,6 @@ def _handle_client_request(self, request_type: EngineCoreRequestType,
             self.add_request(request)
         elif request_type == EngineCoreRequestType.ABORT:
             self.abort_requests(request)
-        elif request_type == EngineCoreRequestType.START_DP:
-            if not self.global_unfinished_reqs:
-                logger.debug("EngineCore starting idle loop.")
-                self.global_unfinished_reqs = True
         elif request_type == EngineCoreRequestType.UTILITY:
             call_id, method_name, args = request
             output = UtilityOutput(call_id)
@@ -548,9 +540,6 @@ def process_output_socket(self, output_path: str, engine_index: int):
                 socket.send_multipart(buffers, copy=False)
 
 
-ENGINE_PAUSED_OUTPUTS = EngineCoreOutputs(engine_paused=True)
-
-
 class DPEngineCoreProc(EngineCoreProc):
     """ZMQ-wrapper for running EngineCore in background process
     in a data parallel context."""
@@ -587,7 +576,9 @@ def __init__(
                 for i in range(local_dp_rank * tp_size, (local_dp_rank + 1) *
                                tp_size))
 
+        self.local_dp_rank = local_dp_rank
         self.dp_group = vllm_config.parallel_config.stateless_init_dp_group()
+        self.current_wave = 0
 
         # Initialize the engine after setting up environment.
         super().__init__(input_path, output_path, vllm_config, executor_class,
@@ -602,6 +593,31 @@ def shutdown(self):
         if dp_group := getattr(self, "dp_group", None):
             stateless_destroy_torch_distributed_process_group(dp_group)
 
+    def add_request(self, request: EngineCoreRequest):
+        if request.current_wave != self.current_wave:
+            if request.current_wave > self.current_wave:
+                self.current_wave = request.current_wave
+            elif not self.engines_running:
+                # Request received for an already-completed wave, notify
+                # front-end that we need to start the next one.
+                self.output_queue.put_nowait(
+                    EngineCoreOutputs(start_wave=self.current_wave))
+
+        super().add_request(request)
+
+    def _handle_client_request(self, request_type: EngineCoreRequestType,
+                               request: Any) -> None:
+        if request_type == EngineCoreRequestType.START_DP_WAVE:
+            new_wave: int = request
+            if new_wave >= self.current_wave:
+                self.current_wave = new_wave
+                if not self.engines_running:
+                    logger.debug("EngineCore starting idle loop for wave %d.",
+                                 new_wave)
+                    self.engines_running = True
+        else:
+            super()._handle_client_request(request_type, request)
+
     def run_busy_loop(self):
         """Core busy loop of the EngineCore for data parallel case."""
 
@@ -628,7 +644,7 @@ def run_busy_loop(self):
                     # up-to-date state is returned in the engine outputs.
                     self._process_engine_step()
 
-                if not self.global_unfinished_reqs:
+                if not self.engines_running:
                     # All engines are idle.
                     continue
 
@@ -637,18 +653,23 @@ def run_busy_loop(self):
                 self.execute_dummy_batch()
 
             # 3) All-reduce operation to determine global unfinished reqs.
-            self.global_unfinished_reqs = self._has_global_unfinished_reqs(
+            self.engines_running = self._has_global_unfinished_reqs(
                 local_unfinished_reqs)
 
-            if not self.global_unfinished_reqs:
-                # Notify client that we are pausing the loop.
-                self.output_queue.put_nowait(ENGINE_PAUSED_OUTPUTS)
+            if not self.engines_running:
+                if self.local_dp_rank == 0:
+                    # Notify client that we are pausing the loop.
+                    logger.debug("Wave %d finished, pausing engine loop.",
+                                 self.current_wave)
+                    self.output_queue.put_nowait(
+                        EngineCoreOutputs(wave_complete=self.current_wave))
+                self.current_wave += 1
 
     def _has_global_unfinished_reqs(self, local_unfinished: bool) -> bool:
 
-        # Optimization - only perform finish-sync all-reduce every 16 steps.
+        # Optimization - only perform finish-sync all-reduce every 24 steps.
         self.counter += 1
-        if self.counter != 16:
+        if self.counter != 24:
             return True
         self.counter = 0
 

vllm/v1/engine/core_client.py

@@ -792,15 +792,12 @@ class DPAsyncMPClient(AsyncMPClient):
     def __init__(self, vllm_config: VllmConfig, executor_class: type[Executor],
                  log_stats: bool):
 
-        self.num_engines_running = 0
+        self.current_wave = 0
+        self.engines_running = False
         self.reqs_in_flight: dict[str, CoreEngine] = {}
 
         super().__init__(vllm_config, executor_class, log_stats)
 
-        # Control message used for triggering dp idle mode loop.
-        self.start_dp_msg = (EngineCoreRequestType.START_DP.value,
-                             *self.encoder.encode(None))
-
         assert len(self.core_engines) > 1
 
     def _init_core_engines(
@@ -829,23 +826,23 @@ async def add_request_async(self, request: EngineCoreRequest) -> None:
         # NOTE: text prompt is not needed in the core engine as it has been
         # tokenized.
         request.prompt = None
-
-        msg = (EngineCoreRequestType.ADD.value, *self.encoder.encode(request))
+        request.current_wave = self.current_wave
 
         chosen_engine = self.get_core_engine_for_request()
         self.reqs_in_flight[request.request_id] = chosen_engine
         chosen_engine.num_reqs_in_flight += 1
-        if self.num_engines_running >= len(self.core_engines):
-            await self._send_input_message(msg, chosen_engine)
-        else:
+
+        to_await = self._send_input(EngineCoreRequestType.ADD, request,
+                                    chosen_engine)
+        if not self.engines_running:
             # Send request to chosen engine and dp start loop
             # control message to all other engines.
-            self.num_engines_running += len(self.core_engines)
-            await asyncio.gather(*[
-                self._send_input_message(
-                    msg if engine is chosen_engine else self.start_dp_msg,
-                    engine) for engine in self.core_engines
-            ])
+            self.engines_running = True
+            to_await = asyncio.gather(
+                to_await,  # type: ignore[assignment]
+                *self._start_wave_coros(exclude_index=chosen_engine.index))
+
+        await to_await
 
         self._ensure_output_queue_task()
 
@@ -860,21 +857,31 @@ async def process_engine_outputs(self: "DPAsyncMPClient",
                 if engine := self.reqs_in_flight.pop(req_id, None):
                     engine.num_reqs_in_flight -= 1
 
-        if outputs.engine_paused:
-            assert self.num_engines_running >= 1
-            self.num_engines_running -= 1
-            if not self.num_engines_running and self.reqs_in_flight:
-                # If there are requests in flight here, they must have
-                # been sent after the engines paused. We must make
-                # sure to start the other engines:
-                self.num_engines_running = len(self.core_engines)
-                coros = [
-                    self._send_input_message(self.start_dp_msg, engine)
-                    for engine in self.core_engines
-                    if not engine.num_reqs_in_flight
-                ]
-                if coros:
-                    await asyncio.gather(*coros)
+        if outputs.wave_complete is not None:
+            # Current wave is complete, move to next wave number
+            # and mark engines as paused.
+            if self.current_wave <= outputs.wave_complete:
+                self.current_wave = outputs.wave_complete + 1
+                self.engines_running = False
+
+        elif outputs.start_wave is not None and (
+                outputs.start_wave > self.current_wave or
+            (outputs.start_wave == self.current_wave
+             and not self.engines_running)):
+            # Engine received request for a non-current wave so we must ensure
+            # that other engines progress to the next wave.
+            self.current_wave = outputs.start_wave
+            self.engines_running = True
+            await asyncio.gather(*self._start_wave_coros(
+                exclude_index=outputs.engine_index))
+
+    def _start_wave_coros(self, exclude_index: int) -> list[Awaitable[None]]:
+        logger.debug("Sending start DP wave %d.", self.current_wave)
+        return [
+            self._send_input(EngineCoreRequestType.START_DP_WAVE,
+                             self.current_wave, engine)
+            for engine in self.core_engines if engine.index != exclude_index
+        ]
 
     async def abort_requests_async(self, request_ids: list[str]) -> None:
         if not request_ids: