[Spec Decode] Introduce DraftModelRunner (vllm-project#5799)

tests/spec_decode/test_multi_step_worker.py

@@ -7,6 +7,7 @@
 
 from vllm.model_executor.utils import set_random_seed
 from vllm.sequence import ExecuteModelRequest, Logprob, SamplerOutput
+from vllm.spec_decode.draft_model_runner import TP1DraftModelRunner
 from vllm.spec_decode.multi_step_worker import MultiStepWorker
 from vllm.spec_decode.top1_proposer import Top1Proposer
 from vllm.worker.worker import Worker
@@ -85,6 +86,7 @@ def test_same_output_for_single_step():
         block_size,
         num_gpu_blocks,
         seed,
+        model_runner_cls=TP1DraftModelRunner,
     )
     worker = create_worker(
         Worker,
@@ -168,6 +170,7 @@ def test_same_output_for_multi_step():
         block_size,
         num_gpu_blocks,
         seed,
+        model_runner_cls=TP1DraftModelRunner,
     )
 
     worker = create_worker(

tests/spec_decode/utils.py

@@ -14,6 +14,7 @@
                            SequenceOutput)
 from vllm.utils import get_distributed_init_method, get_ip, get_open_port
 from vllm.worker.cache_engine import CacheEngine
+from vllm.worker.model_runner import ModelRunner
 from vllm.worker.worker import Worker
 
 T = TypeVar("T", bound=Worker)
@@ -66,7 +67,8 @@ def create_worker(cls: Callable[..., T],
                   num_gpu_blocks: int,
                   seed: int,
                   is_driver_worker: bool = True,
-                  enforce_eager: bool = True) -> T:
+                  enforce_eager: bool = True,
+                  model_runner_cls: Optional[ModelRunner] = None) -> T:
     engine_args = EngineArgs(
         model=model_name,
         seed=seed,
@@ -89,6 +91,7 @@ def create_worker(cls: Callable[..., T],
         rank=0,
         distributed_init_method=distributed_init_method,
         is_driver_worker=is_driver_worker,
+        model_runner_cls=model_runner_cls,
     )
 
     worker.init_device()

vllm/sequence.py

@@ -880,6 +880,8 @@ class ExecuteModelRequest:
     running_queue_size: int = 0
     # Optional hidden states from prior step.
     previous_hidden_states: Optional[HiddenStates] = None
+    # The number of forward steps to run.
+    num_steps: int = 1
 
     def clone(
         self, seq_group_metadata_list: List[SequenceGroupMetadata]
@@ -893,4 +895,5 @@ def clone(
             num_lookahead_slots=self.num_lookahead_slots,
             running_queue_size=self.running_queue_size,
             previous_hidden_states=self.previous_hidden_states,
+            num_steps=self.num_steps,
         )

vllm/spec_decode/draft_model_runner.py

@@ -0,0 +1,170 @@
+from typing import List, Optional
+
+import torch
+
+from vllm.config import (CacheConfig, DeviceConfig, LoadConfig, LoRAConfig,
+                         ModelConfig, ParallelConfig, SchedulerConfig,
+                         VisionLanguageConfig)
+from vllm.logger import init_logger
+from vllm.sequence import SamplerOutput, SequenceGroupMetadata
+from vllm.worker.model_runner import (ModelInputForGPUWithSamplingMetadata,
+                                      ModelRunner)
+
+logger = init_logger(__name__)
+
+
+class TP1DraftModelRunner(ModelRunner):
+    """Specialized model runner for speculative decoding draft model.
+    Since the draft model always execute k forward passes consecutively to
+    generate k speculative tokens in a single speculative decoding step,
+    we could get rid of most CPU-GPU synchronization and data transfer
+    overheads by keeping model input and output tensors on GPU all the time.
+
+    This runner is still under development so there's no performance gain
+    at this moment. Currently we adopt a temporary solution that caches the
+    seq_group_metadata_list for multi-step execution, so that we can
+    leverage existing prepare_model_input to be compatible with the current
+    execution flow, but we plan to remove this cache and avoid calling
+    prepare_model_input in execute_model at all.
+    
+    The detail development plan includes:
+    1. Use "update_model_input" to update existing model_input without
+       creating a new one.
+    2. Improve the performance of "update_model_input" with a GPU kernel.
+    3. Support TP > 1 (this requires some designs because we do not expect
+       any broadcasting inside execute_model).
+    """
+
+    def __init__(
+        self,
+        model_config: ModelConfig,
+        parallel_config: ParallelConfig,
+        scheduler_config: SchedulerConfig,
+        device_config: DeviceConfig,
+        cache_config: CacheConfig,
+        load_config: LoadConfig,
+        lora_config: Optional[LoRAConfig],
+        kv_cache_dtype: Optional[str] = "auto",
+        is_driver_worker: bool = False,
+        vision_language_config: Optional[VisionLanguageConfig] = None,
+        return_hidden_states: bool = False,
+    ):
+        if return_hidden_states:
+            raise ValueError(
+                "return_hidden_states is not supported for TP1DraftModelRunner."
+            )
+
+        super().__init__(
+            model_config=model_config,
+            parallel_config=parallel_config,
+            scheduler_config=scheduler_config,
+            device_config=device_config,
+            cache_config=cache_config,
+            load_config=load_config,
+            lora_config=lora_config,
+            kv_cache_dtype=kv_cache_dtype,
+            is_driver_worker=is_driver_worker,
+            vision_language_config=vision_language_config,
+            return_hidden_states=return_hidden_states,
+        )
+
+        # TODO: Remove this cache when we are able to update model_input
+        # directly in advance_step.
+        self.cached_seq_group_metadata_list: Optional[
+            List[SequenceGroupMetadata]] = None
+
+    def prepare_model_input(
+        self,
+        seq_group_metadata_list: List[SequenceGroupMetadata],
+    ) -> ModelInputForGPUWithSamplingMetadata:
+        """A temporary solution that caches the seq_group_metadata_list
+        for multi-step execution.
+        TODO: In-place update model_input and remove this function.
+        """
+        self.cached_seq_group_metadata_list = seq_group_metadata_list
+        return super().prepare_model_input(seq_group_metadata_list)
+
+    def update_model_input(
+            self, model_input: ModelInputForGPUWithSamplingMetadata,
+            last_output: SamplerOutput
+    ) -> ModelInputForGPUWithSamplingMetadata:
+        """Prepare the model inputs for the next step.
+        TODO: In-place update model_input instead of calling
+        prepare_model_input.
+        """
+
+        # Append the output token to the sequence data.
+        assert self.cached_seq_group_metadata_list is not None
+        for seq_group_metadata, sequence_group_outputs in zip(
+                self.cached_seq_group_metadata_list, last_output.outputs):
+            seq_group_metadata.is_prompt = False
+
+            for seq_output in sequence_group_outputs.samples:
+                seq = seq_group_metadata.seq_data[seq_output.parent_seq_id]
+
+                token_id = seq_output.output_token
+                token_logprob = seq_output.logprobs[token_id]
+
+                seq.append_token_id(token_id, token_logprob.logprob)
+                seq.update_num_computed_tokens(1)
+
+        return self.prepare_model_input(self.cached_seq_group_metadata_list)
+
+    @torch.inference_mode()
+    def execute_model(
+        self,
+        model_input: ModelInputForGPUWithSamplingMetadata,
+        kv_caches: List[torch.Tensor],
+        num_steps: int = 1,
+    ) -> Optional[List[SamplerOutput]]:
+        # Since we do not broadcast data inside execute_model anymore,
+        # we need to figure out the best way to support TP > 1 in this
+        # case, because we will at least need to broadcast the sampled
+        # tokens to all workers.
+        if not self.is_driver_worker:
+            raise ValueError("TP1DraftModelRunner only supports TP=1.")
+
+        if self.lora_config:
+            assert model_input.lora_requests is not None
+            assert model_input.lora_mapping is not None
+            self.set_active_loras(model_input.lora_requests,
+                                  model_input.lora_mapping)
+
+        outputs: List[SamplerOutput] = []
+        for step in range(num_steps):
+            # Currently cuda graph is only supported by the decode phase.
+            assert model_input.attn_metadata is not None
+            prefill_meta = model_input.attn_metadata.prefill_metadata
+            decode_meta = model_input.attn_metadata.decode_metadata
+            if prefill_meta is None and decode_meta.use_cuda_graph:
+                assert model_input.input_tokens is not None
+                graph_batch_size = model_input.input_tokens.shape[0]
+                model_executable = self.graph_runners[graph_batch_size]
+            else:
+                model_executable = self.model
+
+            multi_modal_kwargs = model_input.multi_modal_kwargs or {}
+            hidden_states = model_executable(
+                input_ids=model_input.input_tokens,
+                positions=model_input.input_positions,
+                kv_caches=kv_caches,
+                attn_metadata=model_input.attn_metadata,
+                **multi_modal_kwargs,
+            )
+
+            # Compute the logits.
+            logits = self.model.compute_logits(hidden_states,
+                                               model_input.sampling_metadata)
+
+            # Sample the next token.
+            outputs.append(
+                self.model.sample(
+                    logits=logits,
+                    sampling_metadata=model_input.sampling_metadata,
+                ))
+
+            # Prepare the inputs for the next step.
+            if step != num_steps - 1:
+                model_input = self.update_model_input(model_input, outputs[-1])
+
+        return outputs

vllm/spec_decode/multi_step_worker.py

@@ -6,6 +6,7 @@
 
 from vllm.sequence import (ExecuteModelRequest, SamplerOutput, SequenceData,
                            SequenceGroupMetadata)
+from vllm.spec_decode.draft_model_runner import TP1DraftModelRunner
 from vllm.spec_decode.interfaces import (SpeculativeProposals,
                                          SpeculativeProposer)
 from vllm.spec_decode.proposer_worker_base import ProposerWorkerBase
@@ -67,22 +68,24 @@ def sampler_output(
         copied_execute_model_req = execute_model_req.clone(
             copied_seq_group_metadata_list)
 
-        # Assert enough KV space for sample_len tokens per sequence.
-        self._assert_enough_kv_space(execute_model_req.seq_group_metadata_list,
-                                     sample_len)
-
         # Run model sample_len times.
         model_outputs: List[SamplerOutput] = []
-        for _ in range(sample_len):
-            model_output: List[SamplerOutput] = super().execute_model(
+        if isinstance(self.model_runner, TP1DraftModelRunner):
+            copied_execute_model_req.num_steps = sample_len
+            model_outputs = self.execute_model(
                 execute_model_req=copied_execute_model_req)
-            assert (len(model_output) == 1
-                    ), "composing multistep workers not supported"
-            model_output = model_output[0]
-
-            self._append_new_tokens(model_output,
-                                    copied_seq_group_metadata_list)
-            model_outputs.append(model_output)
+        else:
+            # TODO: Remove this branch once DraftModelRunner supports TP>1.
+            for _ in range(sample_len):
+                model_output: List[SamplerOutput] = super().execute_model(
+                    execute_model_req=copied_execute_model_req)
+                assert (len(model_output) == 1
+                        ), "composing multistep workers not supported"
+                model_output = model_output[0]
+
+                self._append_new_tokens(model_output,
+                                        copied_seq_group_metadata_list)
+                model_outputs.append(model_output)
 
         return model_outputs, True
 

vllm/spec_decode/spec_decode_worker.py

@@ -11,6 +11,7 @@
                            HiddenStates, SamplerOutput, SequenceGroupMetadata,
                            get_all_seq_ids)
 from vllm.spec_decode.batch_expansion import BatchExpansionTop1Scorer
+from vllm.spec_decode.draft_model_runner import TP1DraftModelRunner
 from vllm.spec_decode.interfaces import (SpeculativeProposals,
                                          SpeculativeScorer, SpeculativeScores)
 from vllm.spec_decode.metrics import AsyncMetricsCollector
@@ -117,6 +118,8 @@ def create_worker(
             draft_tp = draft_parallel_config.tensor_parallel_size
             target_tp = scorer_worker.parallel_config.tensor_parallel_size
 
+            if draft_tp == 1:
+                draft_worker_kwargs["model_runner_cls"] = TP1DraftModelRunner
             proposer_worker = MultiStepWorker(**draft_worker_kwargs)
             proposer_worker = SmallerTpProposerWorker.maybe_wrap_worker(
                 proposer_worker, draft_tp, target_tp)

vllm/worker/cpu_model_runner.py

@@ -351,7 +351,12 @@ def execute_model(
         self,
         model_input: CPUModelInput,
         kv_caches: List[torch.Tensor],
-    ) -> Optional[SamplerOutput]:
+        num_steps: int = 1,
+    ) -> Optional[List[SamplerOutput]]:
+        if num_steps > 1:
+            raise ValueError(
+                "CPU worker does not support multi-step execution.")
+
         model_executable = self.model
         execute_model_kwargs = {
             "input_ids": model_input.input_tokens,
@@ -371,11 +376,11 @@ def execute_model(
 
         # Only perform sampling in the driver worker.
         if not self.is_driver_worker:
-            return None
+            return []
 
         # Sample the next token.
         output = self.model.sample(
             logits=logits,
             sampling_metadata=model_input.sampling_metadata,
         )
-        return output
+        return [output]

vllm/worker/embedding_model_runner.py

@@ -57,7 +57,12 @@ def execute_model(
         self,
         model_input: ModelInputForGPUWithPoolingMetadata,
         kv_caches: List[torch.Tensor],
-    ) -> Optional[PoolerOutput]:
+        num_steps: int = 1,
+    ) -> Optional[List[PoolerOutput]]:
+        if num_steps > 1:
+            raise ValueError(
+                "EmbeddingModelRunner does not support multi-step execution.")
+
         if self.lora_config:
             assert model_input.lora_requests is not None
             assert model_input.lora_mapping is not None
@@ -91,10 +96,12 @@ def execute_model(
 
         # Only perform pooling in the driver worker.
         if not self.is_driver_worker:
-            return None
+            return []
 
-        return self.model.pooler(hidden_states=hidden_states,
-                                 pooling_metadata=model_input.pooling_metadata)
+        return [
+            self.model.pooler(hidden_states=hidden_states,
+                              pooling_metadata=model_input.pooling_metadata)
+        ]
 
     def make_model_input_from_broadcasted_tensor_dict(
             self,

vllm/worker/model_runner.py

@@ -959,7 +959,11 @@ def execute_model(
         self,
         model_input: ModelInputForGPUWithSamplingMetadata,
         kv_caches: List[torch.Tensor],
-    ) -> SamplerOutput:
+        num_steps: int = 1,
+    ) -> Optional[List[SamplerOutput]]:
+        if num_steps > 1:
+            raise ValueError("num_steps > 1 is not supported in ModelRunner")
+
         if self.lora_config:
             assert model_input.lora_requests is not None
             assert model_input.lora_mapping is not None
@@ -992,7 +996,7 @@ def execute_model(
 
         # Only perform sampling in the driver worker.
         if not self.is_driver_worker:
-            return None
+            return []
 
         # Sample the next token.
         output: SamplerOutput = self.model.sample(
@@ -1011,7 +1015,7 @@ def execute_model(
 
             output.hidden_states = hidden_states
 
-        return output
+        return [output]
 
 
 class CUDAGraphRunner:

vllm/worker/model_runner_base.py

@@ -150,7 +150,8 @@ def execute_model(
         self,
         model_input: T,
         kv_caches: Optional[List[torch.Tensor]],
-    ) -> Optional[SamplerOutput]:
+        num_steps: int = 1,
+    ) -> Optional[List[SamplerOutput]]:
         """
         Execute the model on the given input.
         """