[Metrics][Spec Decode] Preserve acceptance counts by draft length

[jiangularity]

Purpose

With variable drafting budgets, (drafted, accepted) = (1, 1), (3, 2) and (2, 1), (2, 2) have identical existing summary metrics, although full acceptance occurs at different budgets. Add acceptance_histogram_by_draft_length to the existing opt-in response, preserving this distinction without retaining every verification step.

Each observed draft length k stores k + 1 acceptance buckets. Storage is bounded by configured draft length, independently of sequence length.

This extends merged #48915; #43310, #44487 and #54748 cover native-response, Prometheus and scheduled-budget metrics respectively. The emitted-token corrections in #56137 / #56195 are separate from this raw-verifier histogram.

Validation

PYTHONPATH=. .venv/bin/python -m pytest --noconftest tests/v1/spec_decode/test_request_acceptance.py -q

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[mergify]

Documentation preview: https://vllm--56278.org.readthedocs.build/en/56278/

[jiangularity]

@khluu @youkaichao @Isotr0py

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[jiangularity]

@liulanze Thanks for the review! Could a maintainer trigger /ci run?

cc @DarkLight1337 @markmc @benchislett

[seongyun1104]

Thanks for drawing the map in the description — the split between #43310, #44487 and #54748 is how I'd draw it too, and this histogram is the piece none of them cover. Two notes from reading the diff against main (a9271c750f), both about what the new rows can and cannot contain rather than about the shape itself.

The k = 0 row is unreachable in serving, so the histogram counts steps that drafted, not steps that could have.

RequestSpecDecodeMetrics.observe is only reached through vllm/v1/core/sched/scheduler.py:2002, which sits under the guard at :1970:

scheduled_spec_token_ids = (
    scheduler_output.scheduled_spec_decode_tokens.get(req_id)
)
if scheduled_spec_token_ids and (
    generated_token_ids or self.num_sampled_tokens_per_step == 0
):

and the only decode-path write to that dict is guarded by if request.spec_token_ids: at :808. When a variable budget resolves to zero — num_speculative_tokens_per_batch_size sending a step to K = 0, or any future scheduler that declines to draft — nothing is proposed, no key is written, and the step short-circuits before observe. histogram_by_draft_length can represent a zero row, but no serving path can produce one.

This is visible in the new tests: test_budget_histograms_match_detailed_pairs_without_storing_steps gets {0: [...]} because _metrics calls observe directly, which the scheduler cannot do. So the assertion holds while the serving behaviour it describes does not.

It matters because of exactly the distinction the description opens with. (1, 1), (3, 2) versus (2, 1), (2, 2) are now separable, but a policy that drafts on 20% of steps and one that drafts on every step are still identical in this response — both show only the steps where drafting happened. With a fixed k that is fine, since the two are the same number. With variable budgets the skipped steps are the policy.

I don't think that has to be solved here. Documenting the boundary in acceptance_metrics.md — that rows cover steps with at least one proposed draft, and the count of skipped steps is not in this response — would keep the field from being read as a per-step budget distribution. If you'd rather have the zero row for real, the count exists at scheduler level; #54748 records the resolved per-step K there including zero, and I'm happy to match naming or shape to whatever you land here, or to leave the whole area to this PR if you'd rather own it.

Second, smaller: the rows are keyed after the grammar adjustment, so one configured budget can fragment into several.

observe is called with adj_draft_tokens, i.e. num_draft_tokens minus that request's num_invalid_spec_tokens. Under structured output, steps issued at the same budget land in different rows depending on how many drafts the grammar invalidated, and a row key is then a post-adjustment proposed length rather than a budget. The docs line says "excludes grammar-invalidated drafts", which is accurate, but the PR title and the by_draft_length name both read as budget. One clause — that keys are post-adjustment and may therefore split a single configured budget — would remove the ambiguity for anyone comparing rows across a structured-output run.

Neither of these changes the design; the storage bound argument holds either way.

[jiangularity]

Thanks for taking a close look! I agree with your point(if I understand it correctly) that the docs should make both boundaries clearer. I've clarified that steps where the scheduler chooses not to draft aren’t included, and that the row keys are post-grammar lengths, so one configured budget can contribute to several rows.

On k=0, I think it can still occur when a non-empty scheduled draft is entirely invalidated by the grammar. The padded list remains non-empty, while adj_draft_tokens becomes zero. The accumulator test covers zero-length input, though it doesn’t establish scheduler reachability by itself.

[seongyun1104]

You're right, and my claim was wrong. I traced the path you describe on main (9a35c081e8) and it does reach observe with zero.

update_draft_token_ids_in_output pads rather than drops, so the dict entry survives full invalidation:

# vllm/v1/core/sched/scheduler.py:2451-2466
orig_num_spec_tokens = len(placeholder_spec_tokens)
...
    spec_token_ids = metadata.grammar.validate_tokens(spec_token_ids)   # may return []
num_invalid_tokens = orig_num_spec_tokens - len(spec_token_ids)
if num_invalid_tokens:
    spec_token_ids.extend([-1] * num_invalid_tokens)
    num_invalid_spec_tokens[req_id] = num_invalid_tokens
sched_spec_tokens[req_id] = spec_token_ids

scheduled_spec_token_ids at :1967 is then [-1, ...], which is truthy, so the guard at :1970 passes and adj_draft_tokens at :1997 subtracts the full count down to zero. I had only traced the :808 write and missed that this path mutates scheduled_spec_decode_tokens in place afterwards, so "no serving path can produce a zero row" does not hold.

The narrower thing that does hold is that the two zeros are not the same event, and only one of them is representable: a k = 0 row means drafts were scheduled and all were invalidated, while steps where the scheduler proposed nothing write no key at :808 and so contribute no row at all. So an empty k = 0 bucket is not evidence that drafting was never skipped.

Both of the paragraphs you added in ba0c5da say exactly that, and the k = 0 sentence is more accurate than what I asked for. Nothing further needed from my side — thanks for checking it rather than taking my word for it.