parallel_sampling.py

# SPDX-License-Identifier: Apache-2.0
# SPDX-FileCopyrightText: Copyright contributors to the vLLM project

from copy import copy
from typing import Optional, cast

from vllm.outputs import CompletionOutput
from vllm.sampling_params import RequestOutputKind, SamplingParams
from vllm.v1.metrics.stats import IterationStats


class ParentRequest:
    """Info, state & processing for parallel sampling request.

    Store parent request ID and sampling params.
    Facilitate generating child request sampling params.
    """

    request_id: str
    sampling_params: SamplingParams

    # To track the completion of child requests
    child_requests: set[str]

    # To aggregate child completions when not streaming
    output_aggregator: list[CompletionOutput]

    # To find the max number of generated tokens across all children
    max_num_generation_tokens: int

    # To efficiently obtain child sampling params
    cached_child_sampling_params: SamplingParams | None

    def __init__(self, request_id: str, sampling_params: SamplingParams) -> None:
        self.request_id = request_id
        self.sampling_params = sampling_params

        self.child_requests = set()
        self.output_aggregator = (
            [cast(CompletionOutput, None)] * sampling_params.n
            if (sampling_params.output_kind == RequestOutputKind.FINAL_ONLY)
            else []
        )
        self.max_num_generation_tokens = 0
        self.cached_child_sampling_params = None

    def _get_child_sampling_params(
        self,
        index: int,
    ) -> SamplingParams:
        """Efficiently obtain child `sampling_params`

        If `sampling_params.seed` is not `None` then
        each child request requires a unique clone of
        parent `sampling_params` with a unique seed.

        Args:
          index: index within `n` child requests

        Returns:
          Child `sampling_params` instance.
        """
        seed = self.sampling_params.seed
        if self.cached_child_sampling_params:
            # Reuse child sampling_params data structure
            return self.cached_child_sampling_params
        # Build child sampling_params
        child_sampling_params = copy(self.sampling_params)
        child_sampling_params.n = 1
        if seed is None:
            # Cache child sampling_params for later reuse
            self.cached_child_sampling_params = child_sampling_params
        else:
            # Each child gets a clone with a unique seed
            child_sampling_params.seed = seed + index
        return child_sampling_params

    def get_child_info(self, index: int) -> tuple[str, SamplingParams]:
        """Get child request ID and sampling params.

        Args:
          index: index within `n` child requests.

        Returns:
          (request ID, sampling_params) tuple
        """
        child_req_id = f"{index}_{self.request_id}"
        self.child_requests.add(child_req_id)
        return child_req_id, self._get_child_sampling_params(index)

    @property
    def n(self) -> int:
        return self.sampling_params.n

    def get_outputs(
        self,
        child_request_id: str,
        completion_output: CompletionOutput,
    ) -> tuple[str, list[CompletionOutput], bool]:
        already_finished_and_returned: bool = False
        if completion_output.finished():
            if child_request_id in self.child_requests:
                self.child_requests.remove(child_request_id)
            else:
                # child request ID is not available in child_requests
                # which means the request had finished in previous
                # batch step and returned to the client earlier
                already_finished_and_returned = True

        if self.sampling_params.output_kind != RequestOutputKind.FINAL_ONLY:
            # If streaming, just return the current output
            #
            # DO NOT output finished and already returned child request to client again
            outputs = [] if already_finished_and_returned else [completion_output]
        else:
            # If not streaming, aggregate the n final outputs.
            self.output_aggregator[completion_output.index] = completion_output
            outputs = [] if self.child_requests else self.output_aggregator

        finished = not self.child_requests
        return self.request_id, outputs, finished

    def observe_num_generation_tokens(self, num_generation_tokens: int):
        self.max_num_generation_tokens = max(
            num_generation_tokens, self.max_num_generation_tokens
        )
        return self.max_num_generation_tokens

    @staticmethod
    def observe_finished_request(
        parent_req: Optional["ParentRequest"],
        iteration_stats: IterationStats,
        num_generation_tokens: int,
    ):
        n_param = parent_req.n if parent_req is not None else 1

        if parent_req is not None:
            num_generation_tokens = parent_req.observe_num_generation_tokens(
                num_generation_tokens
            )

        # Child requests finished, we can now record to iteration stats
        if parent_req is None or not parent_req.child_requests:
            iteration_stats.max_num_generation_tokens_iter.append(num_generation_tokens)
            iteration_stats.n_params_iter.append(n_param)