[BugFix] Async scheduling and PP compatibility with DP (#23770)

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

tests/v1/engine/test_engine_core.py

@@ -306,17 +306,17 @@ def test_engine_core_concurrent_batches(monkeypatch: pytest.MonkeyPatch):
 
         # Schedule Batch 1: (10, req0)
         assert engine_core.step_with_batch_queue()[0] is None
-        assert engine_core.batch_queue.qsize() == 1
-        scheduler_output = engine_core.batch_queue.queue[-1][1]
+        assert len(engine_core.batch_queue) == 1
+        scheduler_output = engine_core.batch_queue[-1][1]
         assert scheduler_output.num_scheduled_tokens["0"] == 10
         # num_computed_tokens should have been updated immediately.
         assert engine_core.scheduler.requests[
             req0.request_id].num_computed_tokens == 10
 
         # Schedule Batch 2: (2, req0), (8, req1)
-        assert engine_core.step_with_batch_queue()[0] is None
-        assert engine_core.batch_queue.qsize() == 2
-        scheduler_output = engine_core.batch_queue.queue[-1][1]
+        assert engine_core.step_with_batch_queue()[0] == {}
+        assert len(engine_core.batch_queue) == 1
+        scheduler_output = engine_core.batch_queue[-1][1]
         assert scheduler_output.num_scheduled_tokens["0"] == 2
         assert scheduler_output.num_scheduled_tokens["1"] == 8
         # num_computed_tokens should have been updated immediately.
@@ -325,42 +325,32 @@ def test_engine_core_concurrent_batches(monkeypatch: pytest.MonkeyPatch):
 
         assert engine_core.scheduler.get_num_unfinished_requests() == 2
 
-        # Batch queue is full. Finish Batch 1.
-        engine_core.step_with_batch_queue()
-
-        # Schedule Batch 3: (4, req1). Note that req0 cannot be scheduled
+        # Finish Batch 1 and schedule Batch 3: (4, req1).
+        # Note that req0 cannot be scheduled
         # because it is in the decoding stage now.
         engine_core.step_with_batch_queue()
-        assert engine_core.batch_queue.qsize() == 2
-        scheduler_output = engine_core.batch_queue.queue[-1][1]
+        assert len(engine_core.batch_queue) == 1
+        scheduler_output = engine_core.batch_queue[-1][1]
         assert scheduler_output.num_scheduled_tokens["1"] == 4
 
-        # Batch queue is full. Finish Batch 2. Get first token of req0.
+        # Finish Batch 2. Get first token of req0.
+        # Schedule Batch 4: (1, req0).
         output = engine_core.step_with_batch_queue()[0].get(0)
         assert output is not None
         assert len(output.outputs) == 1
         assert engine_core.scheduler.requests[req0.request_id].num_tokens == 13
-
-        # Schedule Batch 4: (1, req0).
-        engine_core.step_with_batch_queue()
-        assert engine_core.batch_queue.qsize() == 2
-        scheduler_output = engine_core.batch_queue.queue[-1][1]
+        scheduler_output = engine_core.batch_queue[-1][1]
         assert scheduler_output.num_scheduled_tokens["0"] == 1
 
-        # Batch queue is full. Finish Batch 3. Get first token of req1.
+        # Finish Batch 3. Get first token of req1. Schedule Batch 5: (1, req1).
         output = engine_core.step_with_batch_queue()[0].get(0)
         assert output is not None
         assert len(output.outputs) == 1
         assert engine_core.scheduler.requests[req1.request_id].num_tokens == 13
-
-        # Schedule Batch 5: (1, req1).
-        engine_core.step_with_batch_queue()
-        assert engine_core.batch_queue.qsize() == 2
-        scheduler_output = engine_core.batch_queue.queue[-1][1]
+        scheduler_output = engine_core.batch_queue[-1][1]
         assert scheduler_output.num_scheduled_tokens["1"] == 1
 
         # Loop until req0 is finished.
-        step = 0
         req_id = 0
         expected_num_tokens = [
             engine_core.scheduler.requests["0"].num_tokens + 1,
@@ -368,8 +358,7 @@ def test_engine_core_concurrent_batches(monkeypatch: pytest.MonkeyPatch):
         ]
         while engine_core.scheduler.get_num_unfinished_requests() == 2:
             output = engine_core.step_with_batch_queue()[0]
-            if step % 2 == 0:
-                # Even steps consumes an output.
+            # Every step consumes an output.
             assert output is not None
             assert len(output[0].outputs) == 1
             if req_id in engine_core.scheduler.requests:
@@ -377,10 +366,6 @@ def test_engine_core_concurrent_batches(monkeypatch: pytest.MonkeyPatch):
                     req_id].num_tokens == expected_num_tokens[req_id]
             expected_num_tokens[req_id] += 1
             req_id = (req_id + 1) % 2
-            else:
-                # Odd steps schedules a new batch.
-                assert output is None
-            step += 1
 
 
 @multi_gpu_test(num_gpus=2)

tests/v1/test_async_llm_dp.py

@@ -75,9 +75,10 @@ async def generate(
     ],
 )
 @pytest.mark.parametrize("data_parallel_backend", ["mp", "ray"])
+@pytest.mark.parametrize("async_scheduling", [True, False])
 @pytest.mark.asyncio
-async def test_load(output_kind: RequestOutputKind,
-                    data_parallel_backend: str):
+async def test_load(output_kind: RequestOutputKind, data_parallel_backend: str,
+                    async_scheduling: bool):
 
     stats_loggers = {}
 
@@ -105,6 +106,7 @@ async def test_load(output_kind: RequestOutputKind,
         prompt = "This is a test of data parallel"
 
         engine_args.data_parallel_backend = data_parallel_backend
+        engine_args.async_scheduling = async_scheduling
         engine = AsyncLLM.from_engine_args(engine_args,
                                            stat_loggers=[SimpleStatsLogger])
         after.callback(engine.shutdown)

vllm/executor/ray_utils.py

@@ -10,6 +10,7 @@ import msgspec
 
 import vllm.platforms
 from vllm.config import ParallelConfig
+from vllm.distributed import get_pp_group
 from vllm.executor.msgspec_utils import decode_hook, encode_hook
 from vllm.logger import init_logger
 from vllm.platforms import current_platform
@@ -136,6 +137,11 @@ try:
                 scheduler_output, intermediate_tensors)
             if isinstance(output, IntermediateTensors):
                 output = scheduler_output, output
+            elif not get_pp_group().is_last_rank:
+                # Case where there are no scheduled requests
+                # but may still be finished requests.
+                assert not output or not output.req_ids
+                output = scheduler_output, None
             return output
 
         def override_env_vars(self, vars: Dict[str, str]):

vllm/v1/engine/core.py

@@ -138,12 +138,12 @@ class EngineCore:
         # schedule and execute batches, and is required by pipeline parallelism
         # to eliminate pipeline bubbles.
         self.batch_queue_size = self.model_executor.max_concurrent_batches
-        self.batch_queue: Optional[queue.Queue[tuple[Future[ModelRunnerOutput],
+        self.batch_queue: Optional[deque[tuple[Future[ModelRunnerOutput],
                                                SchedulerOutput]]] = None
         if self.batch_queue_size > 1:
             logger.info("Batch queue is enabled with size %d",
                         self.batch_queue_size)
-            self.batch_queue = queue.Queue(self.batch_queue_size)
+            self.batch_queue = deque(maxlen=self.batch_queue_size)
 
         self.request_block_hasher: Optional[Callable[[Request],
                                                      list[BlockHash]]] = None
@@ -319,41 +319,43 @@ class EngineCore:
         batch in the job queue is finished.
         3. Update the scheduler from the output.
         """
-        assert self.batch_queue is not None
+        batch_queue = self.batch_queue
+        assert batch_queue is not None
 
-        engine_core_outputs = None
-        scheduler_output = None
         # Try to schedule a new batch if the batch queue is not full, but
         # the scheduler may return an empty batch if all requests are scheduled.
         # Note that this is not blocking.
-        if not self.batch_queue.full():
+        assert len(batch_queue) < self.batch_queue_size
+
+        model_executed = False
+        if self.scheduler.has_requests():
             scheduler_output = self.scheduler.schedule()
-            if scheduler_output.total_num_scheduled_tokens > 0:
             future = self.model_executor.execute_model(scheduler_output)
-                self.batch_queue.put_nowait(
-                    (future, scheduler_output))  # type: ignore
-
-        scheduled_batch = (scheduler_output is not None
-                           and scheduler_output.total_num_scheduled_tokens > 0)
-
-        # If no more requests can be scheduled and the job queue is not empty,
-        # block until the first batch in the job queue is finished.
-        # TODO(comaniac): Ideally we should peek the first batch in the
-        # job queue to check if it's finished before scheduling a new batch,
-        # but peeking the first element in a queue is not thread-safe,
-        # so we need more work.
-        if not scheduled_batch and not self.batch_queue.empty():
-            future, scheduler_output = self.batch_queue.get_nowait()
-
-            # Blocking until the first result is available.
+            batch_queue.appendleft(
+                (future, scheduler_output))  # type: ignore[arg-type]
+
+            model_executed = scheduler_output.total_num_scheduled_tokens > 0
+            if model_executed and len(batch_queue) < self.batch_queue_size \
+                and not batch_queue[-1][0].done():
+                # Don't block on next worker response unless the queue is full
+                # or there are no more requests to schedule.
+                return None, True
+
+        elif not batch_queue:
+            # Queue is empty. We should not reach here since this method should
+            # only be called when the scheduler contains requests or the queue
+            # is non-empty.
+            return None, False
+
+        # Block until the next result is available.
+        future, scheduler_output = batch_queue.pop()
         model_output = self.execute_model_with_error_logging(
             lambda _: future.result(), scheduler_output)
 
-            self.batch_queue.task_done()
-            engine_core_outputs = (self.scheduler.update_from_output(
-                scheduler_output, model_output))
+        engine_core_outputs = self.scheduler.update_from_output(
+            scheduler_output, model_output)
 
-        return engine_core_outputs, scheduled_batch
+        return engine_core_outputs, model_executed
 
     def shutdown(self):
         self.structured_output_manager.clear_backend()
@@ -388,7 +390,7 @@ class EngineCore:
         return self.model_executor.is_sleeping
 
     def execute_dummy_batch(self):
-        self.model_executor.collective_rpc("execute_dummy_batch")
+        self.model_executor.execute_dummy_batch()
 
     def add_lora(self, lora_request: LoRARequest) -> bool:
         return self.model_executor.add_lora(lora_request)
@@ -733,7 +735,8 @@ class EngineCoreProc(EngineCore):
         """Exits when an engine step needs to be performed."""
 
         waited = False
-        while not self.engines_running and not self.scheduler.has_requests():
+        while not self.engines_running and not self.scheduler.has_requests() \
+                and not self.batch_queue:
             if logger.isEnabledFor(DEBUG) and self.input_queue.empty():
                 logger.debug("EngineCore waiting for work.")
                 waited = True

vllm/v1/executor/abstract.py

@@ -81,12 +81,10 @@ class Executor(ExecutorBase):
         pass
 
     def determine_available_memory(self) -> list[int]:  # in bytes
-        output = self.collective_rpc("determine_available_memory")
-        return output
+        return self.collective_rpc("determine_available_memory")
 
     def get_kv_cache_specs(self) -> list[dict[str, KVCacheSpec]]:
-        output = self.collective_rpc("get_kv_cache_spec")
-        return output
+        return self.collective_rpc("get_kv_cache_spec")
 
     def execute_model(
         self,
@@ -96,6 +94,9 @@ class Executor(ExecutorBase):
                                      args=(scheduler_output, ))
         return output[0]
 
+    def execute_dummy_batch(self) -> None:
+        self.collective_rpc("execute_dummy_batch")
+
     def take_draft_token_ids(self) -> Optional[DraftTokenIds]:
         output = self.collective_rpc("take_draft_token_ids")
         return output[0]

vllm/v1/executor/multiproc_executor.py

@@ -191,6 +191,10 @@ class MultiprocExecutor(Executor):
                 outputs, self.output_rank)
         return self.kv_output_aggregator.aggregate(outputs, self.output_rank)
 
+    def execute_dummy_batch(self) -> None:
+        self.collective_rpc("execute_dummy_batch",
+                            unique_reply_rank=self.output_rank)
+
     def take_draft_token_ids(self) -> Optional[DraftTokenIds]:
         # OPTIMIZATION: Get output only from a single worker (output_rank)
         outputs = self.collective_rpc("take_draft_token_ids",
@@ -242,12 +246,17 @@ class MultiprocExecutor(Executor):
                 dequeue_timeout = None if deadline is None else (
                     deadline - time.monotonic())
 
-                if non_block:
+                if self.io_thread_pool is not None:
+                    # We must consume worker_response_mq from a single thread.
                     result = self.io_thread_pool.submit(  # type: ignore
                         get_response, w, dequeue_timeout, self.shutdown_event)
-                else:
+                    if not non_block:
+                        result = result.result()
+                elif not non_block:
                     result = get_response(w, dequeue_timeout)
-
+                else:
+                    raise RuntimeError("non_block can only be used when"
+                                       " max_concurrent_batches > 1")
                 responses.append(result)
 
             return responses

vllm/v1/worker/gpu_worker.py

@@ -354,18 +354,22 @@ class Worker(WorkerBase):
         scheduler_output: "SchedulerOutput",
     ) -> Optional[ModelRunnerOutput]:
         intermediate_tensors = None
-        if not get_pp_group().is_first_rank:
+        forward_pass = scheduler_output.total_num_scheduled_tokens > 0
+        if forward_pass and not get_pp_group().is_first_rank:
             intermediate_tensors = IntermediateTensors(
                 get_pp_group().recv_tensor_dict(
                     all_gather_group=get_tp_group()))
 
         output = self.model_runner.execute_model(scheduler_output,
                                                  intermediate_tensors)
+        if isinstance(output, ModelRunnerOutput):
+            return output
 
-        parallel_config = self.vllm_config.parallel_config
-        if parallel_config.distributed_executor_backend != "external_launcher" \
-            and not get_pp_group().is_last_rank:
         assert isinstance(output, IntermediateTensors)
+        parallel_config = self.vllm_config.parallel_config
+        assert parallel_config.distributed_executor_backend != (
+            "external_launcher") and not get_pp_group().is_last_rank
+
         get_pp_group().send_tensor_dict(output.tensors,
                                         all_gather_group=get_tp_group())
 
@@ -383,9 +387,6 @@ class Worker(WorkerBase):
         output.kv_connector_output = kv_connector_output
         return output
 
-        assert isinstance(output, ModelRunnerOutput)
-        return output
-
     def take_draft_token_ids(self) -> Optional[DraftTokenIds]:
         return self.model_runner.take_draft_token_ids()