[Perf][V1] Fully overlap model execution (#23569)

Signed-off-by: Benjamin Chislett <benjamin.chislett@centml.ai>

vllm/v1/executor/multiproc_executor.py

@@ -3,6 +3,7 @@
 import multiprocessing
 import os
 import pickle
+import queue
 import signal
 import threading
 import time
@@ -33,7 +34,8 @@ from vllm.utils import (decorate_logs, get_distributed_init_method,
                         get_loopback_ip, get_mp_context, get_open_port,
                         set_process_title)
 from vllm.v1.executor.abstract import Executor, FailureCallback
-from vllm.v1.outputs import DraftTokenIds, ModelRunnerOutput
+from vllm.v1.outputs import (AsyncModelRunnerOutput, DraftTokenIds,
+                             ModelRunnerOutput)
 from vllm.worker.worker_base import WorkerWrapperBase
 
 logger = init_logger(__name__)
@@ -414,6 +416,16 @@ class WorkerProc:
         # Initializes a message queue for sending the model output
         self.worker_response_mq = MessageQueue(1, 1)
 
+        scheduler_config = vllm_config.scheduler_config
+        self.use_async_scheduling = scheduler_config.async_scheduling
+        if self.use_async_scheduling:
+            self.async_output_queue: queue.Queue = queue.Queue()
+            self.async_output_copy_thread = Thread(
+                target=self.async_output_busy_loop,
+                daemon=True,
+                name="WorkerAsyncOutputCopy")
+            self.async_output_copy_thread.start()
+
         # Initialize device and loads weights
         self.worker.init_device()
         self.worker.load_model()
@@ -595,6 +607,36 @@ class WorkerProc:
         SUCCESS = auto()
         FAILURE = auto()
 
+    def enqueue_output(self, output: Any):
+        """Prepares output from the worker and enqueues it to the
+        worker_response_mq. If the output is an Exception, it is
+        converted to a FAILURE response.
+        """
+        if isinstance(output, AsyncModelRunnerOutput):
+            output = output.get_output()
+
+        if isinstance(output, Exception):
+            result = (WorkerProc.ResponseStatus.FAILURE, str(output))
+        else:
+            result = (WorkerProc.ResponseStatus.SUCCESS, output)
+        self.worker_response_mq.enqueue(result)
+
+    def handle_output(self, output: Any):
+        """Handles output from the worker. If async scheduling is enabled,
+        it is passed to the async_output_busy_loop thread. Otherwise, it is
+        enqueued directly to the worker_response_mq.
+        """
+        if self.use_async_scheduling:
+            self.async_output_queue.put(output)
+        else:
+            self.enqueue_output(output)
+
+    def async_output_busy_loop(self):
+        """Entrypoint for the thread which handles outputs asynchronously."""
+        while True:
+            output = self.async_output_queue.get()
+            self.enqueue_output(output)
+
     def worker_busy_loop(self):
         """Main busy loop for Multiprocessing Workers"""
         while True:
@@ -614,10 +656,8 @@ class WorkerProc:
                 # exception might not be serializable, so we convert it to
                 # string, only for logging purpose.
                 if output_rank is None or self.rank == output_rank:
-                    self.worker_response_mq.enqueue(
-                        (WorkerProc.ResponseStatus.FAILURE, str(e)))
+                    self.handle_output(e)
                 continue
 
             if output_rank is None or self.rank == output_rank:
-                self.worker_response_mq.enqueue(
-                    (WorkerProc.ResponseStatus.SUCCESS, output))
+                self.handle_output(output)

vllm/v1/outputs.py

 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
+from abc import ABC, abstractmethod
 from dataclasses import dataclass
 from typing import NamedTuple, Optional
 
@@ -114,6 +115,20 @@ class ModelRunnerOutput:
     num_nans_in_logits: Optional[dict[str, int]] = None
 
 
+# ModelRunnerOutput wrapper for async scheduling.
+class AsyncModelRunnerOutput(ABC):
+
+    @abstractmethod
+    def get_output(self) -> ModelRunnerOutput:
+        """Get the ModelRunnerOutput for this async output.
+        
+        This is a blocking call that waits until the results are ready, which
+        might involve copying device tensors to the host.
+        This method should only be called once per AsyncModelRunnerOutput.
+        """
+        pass
+
+
 @dataclass
 class DraftTokenIds:
 

vllm/v1/worker/gpu_input_batch.py

@@ -250,6 +250,11 @@ class InputBatch:
 
         self.pooling_params: dict[str, PoolingParams] = {}
 
+        # Cached reference to the GPU tensor of previously sampled tokens
+        self.prev_sampled_token_ids: Optional[torch.Tensor] = None
+        self.prev_sampled_token_ids_invalid_indices: Optional[set[int]] = None
+        self.prev_req_id_to_index: Optional[dict[str, int]] = None
+
     @property
     def req_ids(self) -> list[str]:
         # None elements should only be present transiently

vllm/v1/worker/gpu_model_runner.py

@@ -67,8 +67,8 @@ from vllm.v1.kv_cache_interface import (AttentionSpec,
                                         FullAttentionSpec, KVCacheConfig,
                                         KVCacheGroupSpec, KVCacheSpec,
                                         MambaSpec, SlidingWindowSpec)
-from vllm.v1.outputs import (EMPTY_MODEL_RUNNER_OUTPUT, DraftTokenIds,
-                             LogprobsTensors, ModelRunnerOutput)
+from vllm.v1.outputs import (EMPTY_MODEL_RUNNER_OUTPUT, AsyncModelRunnerOutput,
+                             DraftTokenIds, LogprobsTensors, ModelRunnerOutput)
 from vllm.v1.pool.metadata import PoolingMetadata
 from vllm.v1.sample.logits_processor import LogitsProcessors, build_logitsprocs
 from vllm.v1.sample.metadata import SamplingMetadata
@@ -100,6 +100,53 @@ else:
 logger = init_logger(__name__)
 
 
+# Wrapper for ModelRunnerOutput to support overlapped execution.
+class AsyncGPUModelRunnerOutput(AsyncModelRunnerOutput):
+
+    def __init__(
+        self,
+        model_runner_output: ModelRunnerOutput,
+        sampled_token_ids: torch.Tensor,
+        invalid_req_indices: list[int],
+        async_output_copy_stream: torch.cuda.Stream,
+    ):
+        self._model_runner_output = model_runner_output
+        self._invalid_req_indices = invalid_req_indices
+
+        # Event on the copy stream so we can synchronize the non-blocking copy.
+        self._async_copy_ready_event = torch.cuda.Event()
+
+        # Keep a reference to the device tensor to avoid it being
+        # deallocated until we finish copying it to the host.
+        self._sampled_token_ids = sampled_token_ids
+
+        # Initiate the copy on a separate stream, but do not synchronize it.
+        default_stream = torch.cuda.current_stream()
+        with torch.cuda.stream(async_output_copy_stream):
+            async_output_copy_stream.wait_stream(default_stream)
+            self._sampled_token_ids_cpu = self._sampled_token_ids.to(
+                'cpu', non_blocking=True)
+            self._async_copy_ready_event.record()
+
+    def get_output(self) -> ModelRunnerOutput:
+        """Copy the device tensors to the host and return a ModelRunnerOutput.
+        
+        This function blocks until the copy is finished.
+        """
+        self._async_copy_ready_event.synchronize()
+
+        # Release the device tensor once the copy has completed
+        del self._sampled_token_ids
+
+        valid_sampled_token_ids = self._sampled_token_ids_cpu.tolist()
+        for i in self._invalid_req_indices:
+            valid_sampled_token_ids[i].clear()
+
+        output = self._model_runner_output
+        output.sampled_token_ids = valid_sampled_token_ids
+        return output
+
+
 class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
 
     def __init__(
@@ -230,6 +277,10 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
             is_pooling_model=self.is_pooling_model,
         )
 
+        self.use_async_scheduling = self.scheduler_config.async_scheduling
+        self.async_output_copy_stream = torch.cuda.Stream() if \
+            self.use_async_scheduling else None
+
         # TODO(woosuk): Provide an option to tune the max cudagraph batch size.
         # The convention is different.
         # self.cudagraph_batch_sizes sorts in ascending order.
@@ -654,6 +705,73 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
 
         return cu_num_tokens, arange
 
+    def _prepare_input_ids(self, total_num_scheduled_tokens: int,
+                           cu_num_tokens: np.ndarray) -> None:
+        """Prepare the input IDs for the current batch.
+        
+        Carefully handles the `prev_sampled_token_ids` which can be cached
+        from the previous engine iteration, in which case those tokens on the
+        GPU need to be copied into the corresponding slots into input_ids."""
+
+        if self.input_batch.prev_sampled_token_ids is None:
+            # Normal scheduling case
+            self.input_ids.copy_to_gpu(total_num_scheduled_tokens)
+            return
+
+        # Async scheduling case, where some decode requests from the previous
+        # iteration won't have entries in input_ids_cpu and need to be copied
+        # on the GPU from prev_sampled_token_ids.
+        prev_req_id_to_index = self.input_batch.prev_req_id_to_index
+        assert prev_req_id_to_index is not None
+        flattened_indices = []
+        prev_common_req_indices = []
+        indices_match = True
+        max_flattened_index = -1
+        for req_id, cur_index in self.input_batch.req_id_to_index.items():
+            if (prev_index := prev_req_id_to_index.get(req_id)) is not None:
+                prev_common_req_indices.append(prev_index)
+                # We need to compute the flattened input_ids index of the
+                # last token in each common request.
+                flattened_index = cu_num_tokens[cur_index].item() - 1
+                flattened_indices.append(flattened_index)
+                indices_match &= (prev_index == flattened_index)
+                max_flattened_index = max(max_flattened_index, flattened_index)
+        num_commmon_tokens = len(flattened_indices)
+        if num_commmon_tokens < total_num_scheduled_tokens:
+            # If not all requests are decodes from the last iteration,
+            # We need to copy the input_ids_cpu to the GPU first.
+            self.input_ids.copy_to_gpu(total_num_scheduled_tokens)
+        if num_commmon_tokens == 0:
+            # No requests in common with the previous iteration
+            # So input_ids_cpu will have all the input ids.
+            return
+        if indices_match and max_flattened_index == (num_commmon_tokens - 1):
+            # Common-case optimization: the batch is unchanged
+            # and no reordering happened.
+            # The indices are both the same permutation of 0..N-1 so
+            # we can copy directly using a single slice.
+            self.input_ids.gpu[:num_commmon_tokens].copy_(
+                self.input_batch.prev_sampled_token_ids[:num_commmon_tokens,
+                                                        0],
+                non_blocking=True)
+            return
+        # Upload the index tensors asynchronously
+        # so the scatter can be non-blocking.
+        input_ids_index_tensor = torch.tensor(flattened_indices,
+                                              dtype=torch.int64,
+                                              pin_memory=self.pin_memory).to(
+                                                  self.device,
+                                                  non_blocking=True)
+        prev_common_req_indices_tensor = torch.tensor(
+            prev_common_req_indices,
+            dtype=torch.int64,
+            pin_memory=self.pin_memory).to(self.device, non_blocking=True)
+        self.input_ids.gpu.scatter_(
+            dim=0,
+            index=input_ids_index_tensor,
+            src=self.input_batch.prev_sampled_token_ids[
+                prev_common_req_indices_tensor, 0])
+
     def _prepare_inputs(
         self,
         scheduler_output: "SchedulerOutput",
@@ -740,7 +858,8 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         max_seq_len = self.seq_lens.np[:num_reqs].max().item()
 
         # Copy the tensors to the GPU.
-        self.input_ids.copy_to_gpu(total_num_scheduled_tokens)
+        self._prepare_input_ids(total_num_scheduled_tokens, cu_num_tokens)
+
         if self.uses_mrope:
             # Only relevant for models using M-RoPE (e.g, Qwen2-VL)
             self.mrope_positions.gpu[:, :total_num_scheduled_tokens].copy_(
@@ -1458,7 +1577,7 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         self,
         scheduler_output: "SchedulerOutput",
         intermediate_tensors: Optional[IntermediateTensors] = None,
-    ) -> Union[ModelRunnerOutput, IntermediateTensors]:
+    ) -> Union[ModelRunnerOutput, AsyncModelRunnerOutput, IntermediateTensors]:
         self._update_states(scheduler_output)
         if not scheduler_output.total_num_scheduled_tokens:
             if not has_kv_transfer_group():
@@ -1673,6 +1792,12 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                 # so that we could clear the sampled tokens before returning.
                 discard_sampled_tokens_req_indices.append(i)
 
+        # Copy some objects so they don't get modified after returning.
+        # This is important when using async scheduling.
+        req_ids_output_copy = self.input_batch.req_ids.copy()
+        req_id_to_index_output_copy = \
+            self.input_batch.req_id_to_index.copy()
+
         # NOTE: GPU -> CPU Sync happens here.
         # Move as many CPU operations as possible before this sync point.
         logprobs_tensors = sampler_output.logprobs_tensors
@@ -1685,21 +1810,41 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
             scheduler_output.num_scheduled_tokens,
         )
 
-        # Get the valid generated tokens.
+        num_sampled_tokens = sampler_output.sampled_token_ids.shape[0]
         sampled_token_ids = sampler_output.sampled_token_ids
-        max_gen_len = sampled_token_ids.shape[-1]
-        if max_gen_len == 1:
-            # No spec decode tokens.
-            valid_sampled_token_ids = self._to_list(sampled_token_ids)
+        if not self.use_async_scheduling:
+            # Get the valid generated tokens.
+            max_gen_len = sampled_token_ids.shape[-1]
+            if max_gen_len == 1:
+                # No spec decode tokens.
+                valid_sampled_token_ids = self._to_list(sampled_token_ids)
+            else:
+                # Includes spec decode tokens.
+                valid_sampled_token_ids = self.rejection_sampler.parse_output(
+                    sampled_token_ids,
+                    self.input_batch.vocab_size,
+                )
+            # Mask out the sampled tokens that should not be sampled.
+            for i in discard_sampled_tokens_req_indices:
+                valid_sampled_token_ids[i].clear()
         else:
-            # Includes spec decode tokens.
-            valid_sampled_token_ids = self.rejection_sampler.parse_output(
-                sampled_token_ids,
-                self.input_batch.vocab_size,
-            )
-        # Mask out the sampled tokens that should not be sampled.
-        for i in discard_sampled_tokens_req_indices:
-            valid_sampled_token_ids[i].clear()
+            valid_sampled_token_ids = []
+            invalid_req_indices = list(discard_sampled_tokens_req_indices)
+            invalid_req_indices_set = set(invalid_req_indices)
+            assert sampled_token_ids.shape[-1] == 1
+
+            # Cache the sampled tokens on the GPU and avoid CPU sync.
+            # These will be copied into input_ids in the next step
+            # when preparing inputs.
+            self.input_batch.prev_sampled_token_ids = \
+                sampled_token_ids
+            self.input_batch.prev_sampled_token_ids_invalid_indices = \
+                invalid_req_indices_set
+            self.input_batch.prev_req_id_to_index = {
+                req_id: i
+                for i, req_id in enumerate(self.input_batch.req_ids)
+                if i not in invalid_req_indices_set
+            }
 
         # Cache the sampled tokens in the model runner, so that the scheduler
         # doesn't need to send them back.
@@ -1707,7 +1852,12 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
         # the sampled tokens back, because there's no direct communication
         # between the first-stage worker and the last-stage worker.
         req_ids = self.input_batch.req_ids
-        for req_idx, sampled_ids in enumerate(valid_sampled_token_ids):
+        for req_idx in range(num_sampled_tokens):
+            if self.use_async_scheduling:
+                sampled_ids = [-1] if \
+                    req_idx not in invalid_req_indices_set else None
+            else:
+                sampled_ids = valid_sampled_token_ids[req_idx]
             if not sampled_ids:
                 continue
 
@@ -1722,6 +1872,7 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
                                            start_idx:end_idx] = sampled_ids
             self.input_batch.num_tokens_no_spec[req_idx] = end_idx
             self.input_batch.num_tokens[req_idx] = end_idx
+
             req_id = req_ids[req_idx]
             req_state = self.requests[req_id]
             req_state.output_token_ids.extend(sampled_ids)
@@ -1741,9 +1892,9 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
 
         self.eplb_step()
 
-        return ModelRunnerOutput(
-            req_ids=self.input_batch.req_ids,
-            req_id_to_index=self.input_batch.req_id_to_index,
+        output = ModelRunnerOutput(
+            req_ids=req_ids_output_copy,
+            req_id_to_index=req_id_to_index_output_copy,
             sampled_token_ids=valid_sampled_token_ids,
             logprobs=logprobs_lists,
             prompt_logprobs_dict=prompt_logprobs_dict,
@@ -1752,6 +1903,16 @@ class GPUModelRunner(LoRAModelRunnerMixin, KVConnectorModelRunnerMixin):
             num_nans_in_logits=num_nans_in_logits,
         )
 
+        if not self.use_async_scheduling:
+            return output
+
+        return AsyncGPUModelRunnerOutput(
+            model_runner_output=output,
+            sampled_token_ids=sampled_token_ids,
+            invalid_req_indices=invalid_req_indices,
+            async_output_copy_stream=self.async_output_copy_stream,
+        )
+
     def take_draft_token_ids(self) -> Optional[DraftTokenIds]:
         if self._draft_token_ids is None:
             return None

vllm/v1/worker/gpu_worker.py

@@ -5,7 +5,7 @@ import copy
 import gc
 import os
 from contextlib import AbstractContextManager, nullcontext
-from typing import TYPE_CHECKING, Any, Optional
+from typing import TYPE_CHECKING, Any, Optional, Union
 
 import torch
 import torch.distributed
@@ -28,8 +28,8 @@ from vllm.tasks import SupportedTask
 from vllm.utils import GiB_bytes, MemorySnapshot, memory_profiling
 from vllm.v1.engine import ReconfigureDistributedRequest, ReconfigureRankType
 from vllm.v1.kv_cache_interface import KVCacheConfig, KVCacheSpec
-from vllm.v1.outputs import (EMPTY_MODEL_RUNNER_OUTPUT, DraftTokenIds,
-                             ModelRunnerOutput)
+from vllm.v1.outputs import (EMPTY_MODEL_RUNNER_OUTPUT, AsyncModelRunnerOutput,
+                             DraftTokenIds, ModelRunnerOutput)
 from vllm.v1.utils import report_usage_stats
 from vllm.v1.worker.gpu_model_runner import GPUModelRunner
 from vllm.v1.worker.worker_base import WorkerBase
@@ -355,7 +355,7 @@ class Worker(WorkerBase):
     def execute_model(
         self,
         scheduler_output: "SchedulerOutput",
-    ) -> Optional[ModelRunnerOutput]:
+    ) -> Optional[Union[ModelRunnerOutput, AsyncModelRunnerOutput]]:
         intermediate_tensors = None
         forward_pass = scheduler_output.total_num_scheduled_tokens > 0
         if forward_pass and not get_pp_group().is_first_rank:
@@ -365,7 +365,7 @@ class Worker(WorkerBase):
 
         output = self.model_runner.execute_model(scheduler_output,
                                                  intermediate_tensors)
-        if isinstance(output, ModelRunnerOutput):
+        if isinstance(output, (ModelRunnerOutput, AsyncModelRunnerOutput)):
             return output
 
         assert isinstance(output, IntermediateTensors)