vllm.benchmarks.forward.batch

Build a SchedulerOutput for an exact batch, without the scheduler.

Classes:

• BenchBatch –

Functions:

• build_batch –

  Allocate disjoint blocks and build NewRequestData for every request.

• kv_block_size –

  Tokens per block as the scheduler counts them: DCP spreads each

BenchBatch dataclass

Methods:

• finish –

  A step that only removes this batch's requests.

Source code in vllm/benchmarks/forward/batch.py

@dataclass
class BenchBatch:
    scheduler_output: SchedulerOutput
    # req_id -> token IDs of the query tokens, in order.
    query_token_ids: dict[str, np.ndarray]
    # Per KV cache group: allocated (begin, end) block ranges.
    group_block_ranges: list[list[tuple[int, int]]]
    num_blocks: int
    # req_id -> the runner's expected (scheduled, computed, is_decode).
    expected: dict[str, tuple[int, int, bool]]

    @property
    def num_tokens(self) -> int:
        return self.scheduler_output.total_num_scheduled_tokens

    @property
    def req_ids(self) -> list[str]:
        return list(self.scheduler_output.num_scheduled_tokens)

    def finish(self) -> SchedulerOutput:
        """A step that only removes this batch's requests."""
        output = SchedulerOutput.make_empty()
        output.finished_req_ids = set(self.req_ids)
        return output

finish()

A step that only removes this batch's requests.

Source code in vllm/benchmarks/forward/batch.py

def finish(self) -> SchedulerOutput:
    """A step that only removes this batch's requests."""
    output = SchedulerOutput.make_empty()
    output.finished_req_ids = set(self.req_ids)
    return output

_num_blocks(spec, computed, kv_len, dcp_size)

Return (leading null blocks, live blocks) for one request.

Source code in vllm/benchmarks/forward/batch.py

def _num_blocks(
    spec: KVCacheSpec, computed: int, kv_len: int, dcp_size: int
) -> tuple[int, int]:
    """Return (leading null blocks, live blocks) for one request."""
    block_size = kv_block_size(spec, dcp_size)
    if isinstance(spec, UniformTypeKVCacheSpecs):
        spec = spec.first_spec
    if isinstance(spec, (CircularBufferSpec, MambaSpec)):
        return 0, 1
    total = -(-kv_len // block_size)
    skipped = 0
    if isinstance(spec, SlidingWindowSpec):
        # Blocks wholly before the window are freed to the null block.
        skipped = max(0, computed - spec.sliding_window + 1) // block_size
    return skipped, total - skipped

build_batch(case, kv_cache_groups, num_kv_blocks, vocab_size, rng, dcp_size=1)

Allocate disjoint blocks and build NewRequestData for every request.

Groups share one block pool, so block IDs are unique across groups. Block 0 is the null block.

With prefix P = K - Q (KV length K, query length Q):

Kind	prompt tokens	num_computed_tokens	spec tokens
Prefill	K	P	-
Decode	P	P	-
Verify (D drafts)	P	P	D

Unless chunked, a prefill's prefix is a prefix hit, which replays its last R = min(window, P) tokens on models with SWA bounded replay (window 0 otherwise), like the scheduler: it schedules R + Q tokens from replay_start = num_computed_tokens = P - R.

Source code in vllm/benchmarks/forward/batch.py

def build_batch(
    case: BenchCase,
    kv_cache_groups,
    num_kv_blocks: int,
    vocab_size: int,
    rng: np.random.Generator,
    dcp_size: int = 1,
) -> BenchBatch:
    """Allocate disjoint blocks and build `NewRequestData` for every request.

    Groups share one block pool, so block IDs are unique across groups. Block
    0 is the null block.

    With prefix P = K - Q (KV length K, query length Q):

    | Kind               | prompt tokens | num_computed_tokens | spec tokens |
    | ------------------ | ------------- | ------------------- | ----------- |
    | Prefill            | K             | P                   | -           |
    | Decode             | P             | P                   | -           |
    | Verify (D drafts)  | P             | P                   | D           |

    Unless chunked, a prefill's prefix is a prefix hit, which replays its last
    R = min(window, P) tokens on models with SWA bounded replay (window 0
    otherwise), like the scheduler: it schedules R + Q tokens from
    replay_start = num_computed_tokens = P - R.
    """
    replay_window = max(
        (group.kv_cache_spec.prefix_replay_tokens for group in kv_cache_groups),
        default=0,
    )
    expected: dict[str, tuple[int, int, bool]] = {}
    cursor = 1
    new_reqs = []
    query_token_ids = {}
    group_block_ranges: list[list[tuple[int, int]]] = [[] for _ in kv_cache_groups]
    output = SchedulerOutput.make_empty()
    for i, req in enumerate(case.requests):
        req_id = f"bench-{i}"
        replay = min(replay_window, req.prefix_len) if req.prefix_hit else 0
        computed = req.prefix_len - replay
        block_ids = []
        for group, ranges in zip(kv_cache_groups, group_block_ranges):
            skipped, live = _num_blocks(
                group.kv_cache_spec, computed, req.kv_len, dcp_size
            )
            block_ids.append([0] * skipped + list(range(cursor, cursor + live)))
            ranges.append((cursor, cursor + live))
            cursor += live
        tokens = rng.integers(0, vocab_size, req.kv_len, dtype=np.int64)
        prompt = tokens[: req.prefix_len] if req.is_decode else tokens
        query_token_ids[req_id] = tokens[computed:]
        expected[req_id] = (req.query_len + replay, computed, req.is_decode)
        new_reqs.append(
            NewRequestData(
                req_id=req_id,
                prompt_token_ids=prompt.tolist(),
                prefill_token_ids=prompt.tolist(),
                mm_features=[],
                sampling_params=SamplingParams(max_tokens=16, temperature=0.0),
                pooling_params=None,
                block_ids=tuple(block_ids),
                num_computed_tokens=computed,
                lora_request=None,
                replay_start=computed if replay else 0,
            )
        )
        output.num_scheduled_tokens[req_id] = req.query_len + replay
        if req.num_draft_tokens:
            output.scheduled_spec_decode_tokens[req_id] = tokens[
                req.prefix_len + 1 :
            ].tolist()
    if cursor > num_kv_blocks:
        raise ValueError(
            f"Case {case.name!r} needs {cursor} KV blocks, but only "
            f"{num_kv_blocks} exist. Increase the KV cache size."
        )
    output.scheduled_new_reqs = new_reqs
    output.total_num_scheduled_tokens = sum(output.num_scheduled_tokens.values())
    return BenchBatch(output, query_token_ids, group_block_ranges, cursor, expected)

kv_block_size(spec, dcp_size)

Tokens per block as the scheduler counts them: DCP spreads each block's tokens over the DCP ranks.

Source code in vllm/benchmarks/forward/batch.py

def kv_block_size(spec: KVCacheSpec, dcp_size: int) -> int:
    """Tokens per block as the scheduler counts them: DCP spreads each
    block's tokens over the DCP ranks."""
    return spec.block_size * (dcp_size if spec.dcp_sharded else 1)