Add output streaming support to multi-step + async while ensuring RequestOutput obj reuse (#8335)

tests/entrypoints/openai/test_accuracy.py

@@ -19,7 +19,11 @@ FILTER = "exact_match,strict-match"
 RTOL = 0.03
 EXPECTED_VALUE = 0.58
 DEFAULT_ARGS = ["--max-model-len", "4096", "--disable-log-requests"]
-MORE_ARGS_LIST = [["--enable-chunked-prefill"], ["--num-scheduler-steps", "8"]]
+MORE_ARGS_LIST = [
+    ["--enable-chunked-prefill"],  # Chunked
+    ["--num-scheduler-steps", "8"],  # MS
+    ["--num-scheduler-steps", "8", "--multi-step-stream-outputs"]  # MS+Stream
+]
 
 
 @pytest.mark.parametrize("more_args", MORE_ARGS_LIST)

vllm/config.py

@@ -960,6 +960,7 @@ class SchedulerConfig:
                  is_multimodal_model: bool = False,
                  preemption_mode: Optional[str] = None,
                  num_scheduler_steps: int = 1,
+                 multi_step_stream_outputs: bool = False,
                  send_delta_data: bool = False) -> None:
         if max_num_batched_tokens is None:
             if enable_chunked_prefill:
@@ -1000,6 +1001,7 @@ class SchedulerConfig:
         self.embedding_mode = embedding_mode
         self.preemption_mode = preemption_mode
         self.num_scheduler_steps = num_scheduler_steps
+        self.multi_step_stream_outputs = multi_step_stream_outputs
         self.send_delta_data = send_delta_data
         self._verify_args()
 

vllm/engine/arg_utils.py

@@ -145,6 +145,7 @@ class EngineArgs:
     max_cpu_loras: Optional[int] = None
     device: str = 'auto'
     num_scheduler_steps: int = 1
+    multi_step_stream_outputs: bool = False
     ray_workers_use_nsight: bool = False
     num_gpu_blocks_override: Optional[int] = None
     num_lookahead_slots: int = 0
@@ -595,6 +596,10 @@ class EngineArgs:
                             help=('Maximum number of forward steps per '
                                   'scheduler call.'))
 
+        parser.add_argument(
+            '--multi-step-stream-outputs',
+            action='store_true',
+            help='If True, then multi-step will stream outputs for every step')
         parser.add_argument(
             '--scheduler-delay-factor',
             type=float,
@@ -999,6 +1004,7 @@ class EngineArgs:
             is_multimodal_model=model_config.is_multimodal_model,
             preemption_mode=self.preemption_mode,
             num_scheduler_steps=self.num_scheduler_steps,
+            multi_step_stream_outputs=self.multi_step_stream_outputs,
             send_delta_data=(envs.VLLM_USE_RAY_SPMD_WORKER
                              and parallel_config.use_ray),
         )

vllm/engine/llm_engine.py

@@ -95,7 +95,7 @@ class OutputData(NamedTuple):
 
 class SchedulerContext:
 
-    def __init__(self):
+    def __init__(self, multi_step_stream_outputs: bool = False):
         self.output_queue: Deque[OutputData] = deque()
         self.request_outputs: List[Union[RequestOutput,
                                          EmbeddingRequestOutput]] = []
@@ -103,6 +103,8 @@ class SchedulerContext:
             List[SequenceGroupMetadata]] = None
         self.scheduler_outputs: Optional[SchedulerOutputs] = None
 
+        self.multi_step_stream_outputs: bool = multi_step_stream_outputs
+
     def append_output(self, outputs: List[SamplerOutput],
                       seq_group_metadata_list: List[SequenceGroupMetadata],
                       scheduler_outputs: SchedulerOutputs, is_async: bool,
@@ -219,6 +221,7 @@ class LLMEngine:
         usage_context: UsageContext = UsageContext.ENGINE_CONTEXT,
         stat_loggers: Optional[Dict[str, StatLoggerBase]] = None,
         input_registry: InputRegistry = INPUT_REGISTRY,
+        use_cached_outputs: bool = False,
     ) -> None:
         logger.info(
             "Initializing an LLM engine (v%s) with config: "
@@ -234,8 +237,9 @@ class LLMEngine:
             "quantization_param_path=%s, device_config=%s, "
             "decoding_config=%r, observability_config=%r, "
             "seed=%d, served_model_name=%s, use_v2_block_manager=%s, "
-            "num_scheduler_steps=%d, enable_prefix_caching=%s, "
-            "use_async_output_proc=%s, mm_processor_kwargs=%s)",
+            "num_scheduler_steps=%d, multi_step_stream_outputs=%s, "
+            "enable_prefix_caching=%s, use_async_output_proc=%s, "
+            "use_cached_outputs=%s, mm_processor_kwargs=%s)",
             VLLM_VERSION,
             model_config.model,
             speculative_config,
@@ -266,8 +270,10 @@ class LLMEngine:
             model_config.served_model_name,
             scheduler_config.use_v2_block_manager,
             scheduler_config.num_scheduler_steps,
+            scheduler_config.multi_step_stream_outputs,
             cache_config.enable_prefix_caching,
             model_config.use_async_output_proc,
+            use_cached_outputs,
             model_config.mm_processor_kwargs,
         )
         # TODO(woosuk): Print more configs in debug mode.
@@ -287,6 +293,7 @@ class LLMEngine:
         self.observability_config = observability_config or ObservabilityConfig(
         )
         self.log_stats = log_stats
+        self.use_cached_outputs = use_cached_outputs
 
         if not self.model_config.skip_tokenizer_init:
             self.tokenizer = self._init_tokenizer()
@@ -379,7 +386,8 @@ class LLMEngine:
         ]
 
         self.scheduler_contexts = [
-            SchedulerContext()
+            SchedulerContext(multi_step_stream_outputs=self.scheduler_config.
+                             multi_step_stream_outputs)
             for _ in range(self.parallel_config.pipeline_parallel_size)
         ]
 
@@ -998,7 +1006,8 @@ class LLMEngine:
 
             seq_group = scheduled_seq_group.seq_group
             seq_group.maybe_set_first_token_time(now)
-            request_output = RequestOutputFactory.create(seq_group)
+            request_output = RequestOutputFactory.create(
+                seq_group, use_cache=self.use_cached_outputs)
             if request_output:
                 ctx.request_outputs.append(request_output)
 
@@ -1019,8 +1028,8 @@ class LLMEngine:
             for scheduler in self.scheduler:
                 scheduler.free_finished_seq_groups()
 
-        # For multi-step, do not create outputs each iteration
-        if not is_last_step:
+        # For multi-step without streaming, don't create outputs each iteration
+        if not is_last_step and not ctx.multi_step_stream_outputs:
             # Immediately process request outputs here (if callback is given)
             if (finished_now
                     and self.process_request_outputs_callback is not None):
@@ -1037,17 +1046,27 @@ class LLMEngine:
 
             seq_group = scheduled_seq_group.seq_group
             seq_group.maybe_set_first_token_time(now)
-            request_output = RequestOutputFactory.create(seq_group)
+            request_output = RequestOutputFactory.create(
+                seq_group, use_cache=self.use_cached_outputs)
             if request_output:
                 ctx.request_outputs.append(request_output)
 
+        # For multi-step with streaming, create outputs each iteration
+        if not is_last_step and ctx.multi_step_stream_outputs:
+            # Immediately process request outputs here (if callback is given)
+            if self.process_request_outputs_callback is not None:
+                self.process_request_outputs_callback(ctx.request_outputs)
+                ctx.request_outputs.clear()
+            return
+
         for seq_group in scheduler_outputs.ignored_seq_groups:
             params = seq_group.sampling_params
             if params is not None and params.output_kind == (
                     RequestOutputKind.DELTA) and not seq_group.is_finished():
                 continue
 
-            request_output = RequestOutputFactory.create(seq_group)
+            request_output = RequestOutputFactory.create(
+                seq_group, use_cache=self.use_cached_outputs)
             if request_output:
                 ctx.request_outputs.append(request_output)
 

vllm/engine/multiprocessing/engine.py

@@ -66,7 +66,14 @@ class MQLLMEngine:
                  *args,
                  log_requests: bool = True,
                  **kwargs) -> None:
-        self.engine = LLMEngine(*args, **kwargs)
+        # For MQLLMEngine, we can use cached outputs, since each new request
+        # output is immediately pickled and send over the socket, which frees
+        # the python object to be reused again.
+        use_cached_outputs = True
+
+        self.engine = LLMEngine(*args,
+                                **kwargs,
+                                use_cached_outputs=use_cached_outputs)
         self.log_requests = log_requests
 
         self.use_async_sockets = use_async_sockets

vllm/outputs.py

@@ -114,17 +114,28 @@ class RequestOutput:
         self.encoder_prompt_token_ids = encoder_prompt_token_ids
 
     @classmethod
-    def from_seq_group(cls,
-                       seq_group: SequenceGroup) -> Optional["RequestOutput"]:
+    def from_seq_group(cls, seq_group: SequenceGroup,
+                       use_cache: bool) -> Optional["RequestOutput"]:
         sampling_params = seq_group.sampling_params
         if sampling_params is None:
             raise ValueError(
                 "Sampling parameters are missing for a CompletionRequest.")
+
         finished = seq_group.is_finished()
         if sampling_params.output_kind == RequestOutputKind.FINAL_ONLY and (
                 not finished):
             return None
 
+        # Init cache (if needed)
+        if use_cache and seq_group.cached_request_output is None:
+            seq_group.cached_request_output = RequestOutput(  # type: ignore
+                request_id="",
+                prompt=None,
+                prompt_token_ids=[],
+                prompt_logprobs=None,
+                outputs=[],
+                finished=False)
+
         seqs = seq_group.get_seqs()
         if len(seqs) == 1:
             top_n_seqs = seqs
@@ -149,29 +160,66 @@ class RequestOutput:
 
         outputs = []
         include_prompt = True
-        for seq in top_n_seqs:
+        for i, seq in enumerate(top_n_seqs):
             output_text = seq.get_output_text_to_return(
                 text_buffer_length, delta)
+
             output_token_ids = seq.get_output_token_ids_to_return(delta)
+            num_output_tokens = 1 if isinstance(output_token_ids,
+                                                int) else len(output_token_ids)
+
             output_logprobs = seq.output_logprobs if include_logprobs else None
 
             if delta:
                 # Slice logprobs delta if applicable
                 if output_logprobs:
-                    output_logprobs = output_logprobs[-len(output_token_ids):]
+                    output_logprobs = output_logprobs[-num_output_tokens:]
                 # Don't include prompt if this is after the first output
                 # containing decode token ids
-                if include_prompt and seq.get_output_len() > len(
-                        output_token_ids):
+                if include_prompt and seq.get_output_len() > num_output_tokens:
                     include_prompt = False
 
-            outputs.append(
-                CompletionOutput(
-                    seqs.index(seq), output_text, output_token_ids,
+            if use_cache:
+                # Get cached output object
+                cached_outputs = seq_group.cached_request_output.outputs  # type: ignore
+                if i >= len(cached_outputs):
+                    cached_outputs.append(
+                        CompletionOutput(index=i,
+                                         text="",
+                                         token_ids=[],
+                                         cumulative_logprob=None,
+                                         logprobs=None,
+                                         finish_reason=None,
+                                         stop_reason=None))
+                output = cached_outputs[i]
+
+                # Init cached output object
+                assert output.index == i
+                output.text = output_text
+
+                if isinstance(output_token_ids, int):
+                    output.token_ids.clear()
+                    output.token_ids.append(output_token_ids)
+                else:
+                    output.token_ids = output_token_ids
+
+                output.cumulative_logprob = seq.get_cumulative_logprob() \
+                    if include_logprobs else None
+                output.logprobs = output_logprobs
+                output.finish_reason = SequenceStatus.get_finished_reason(
+                    seq.status)
+                output.stop_reason = seq.stop_reason
+
+            else:
+                output = CompletionOutput(
+                    seqs.index(seq), output_text, [output_token_ids]
+                    if isinstance(output_token_ids, int) else output_token_ids,
                     seq.get_cumulative_logprob() if include_logprobs else None,
                     output_logprobs,
                     SequenceStatus.get_finished_reason(seq.status),
-                    seq.stop_reason))
+                    seq.stop_reason)
+
+            outputs.append(output)
 
         # Every sequence in the sequence group should have the same prompt.
         if include_prompt:
@@ -188,16 +236,20 @@ class RequestOutput:
             prompt_logprobs = None
         finished_time = time.time() if finished else None
         seq_group.set_finished_time(finished_time)
-        return cls(seq_group.request_id,
-                   prompt,
-                   prompt_token_ids,
-                   prompt_logprobs,
-                   outputs,
-                   finished,
-                   seq_group.metrics,
-                   lora_request=seq_group.lora_request,
-                   encoder_prompt=encoder_prompt,
-                   encoder_prompt_token_ids=encoder_prompt_token_ids)
+
+        init_args = (seq_group.request_id, prompt, prompt_token_ids,
+                     prompt_logprobs, outputs, finished, seq_group.metrics,
+                     seq_group.lora_request, encoder_prompt,
+                     encoder_prompt_token_ids)
+
+        if use_cache:
+            request_output = seq_group.cached_request_output
+            request_output.__init__(*init_args)  # type: ignore
+
+        else:
+            request_output = cls(*init_args)
+
+        return request_output
 
     def __repr__(self) -> str:
         return (f"RequestOutput(request_id={self.request_id}, "
@@ -261,10 +313,10 @@ class EmbeddingRequestOutput:
 class RequestOutputFactory:
 
     @staticmethod
-    def create(seq_group):
+    def create(seq_group: SequenceGroup, use_cache: bool = False):
         # Determine the type based on a condition, for example:
         if hasattr(seq_group,
                    'embeddings') and seq_group.embeddings is not None:
             return EmbeddingRequestOutput.from_seq_group(seq_group)
         else:
-            return RequestOutput.from_seq_group(seq_group)
+            return RequestOutput.from_seq_group(seq_group, use_cache)

vllm/sequence.py

@@ -436,7 +436,7 @@ class Sequence:
         self.stop_reason: Union[int, str, None] = None
 
         # These are used to keep track of delta outputs
-        self._last_token_ids_offset: int = 0
+        self._last_output_token_ids_offset: int = 0
         self._last_output_text_offset: int = 0
 
         # Used for incremental detokenization
@@ -499,18 +499,26 @@ class Sequence:
             return self.output_text[last_offset:length]
         return ""
 
-    def get_output_token_ids_to_return(self,
-                                       delta: bool) -> GenericSequence[int]:
+    def get_output_token_ids_to_return(
+            self, delta: bool) -> Union[GenericSequence[int], int]:
         """If delta is True, only new tokens since the last call to
         this method are returned"""
         if not delta:
             return self.get_output_token_ids()
-        length = self.get_output_len()
-        last_offset = self._last_token_ids_offset
-        if last_offset < length:
-            self._last_token_ids_offset = length
-            return self.data._output_token_ids[last_offset:]
-        return ()
+
+        output_len = self.get_output_len()
+
+        # Get the number of new tokens
+        num_new_tokens = output_len - self._last_output_token_ids_offset
+        self._last_output_token_ids_offset = output_len
+
+        # Return new tokens
+        if num_new_tokens == 1:
+            # Optimization for single decode token case
+            # (which is what we have most of the time)
+            return self.data._cached_all_token_ids[-1]
+
+        return self.data._cached_all_token_ids[-num_new_tokens:]
 
     def hash_of_block(self, logical_idx: int) -> int:
         # TODO This can produce incorrect hash when block size > prompt size
@@ -671,6 +679,8 @@ class SequenceGroup:
         self.encoder_seq = encoder_seq
         self.trace_headers = trace_headers
 
+        self.cached_request_output = None
+
     @property
     def prompt(self) -> Optional[str]:
         # All sequences in the group should have the same prompt.