[Docs] Note pooled aggregation semantics for mean_acceptance_length - #52169
[Docs] Note pooled aggregation semantics for mean_acceptance_length#52169arrdel wants to merge 1 commit into
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The `mean_acceptance_length` metric in `SpecDecodingLogging._log` and the corresponding PromQL recipe in `SpecDecodingProm.__doc__` compute a pooled mean (weighted by draft count, equivalent to `total_generated / total_verify_calls`), not an unweighted mean of per-request means. External benchmarks that report the latter, for example, SpecForge's `average_acceptance_length`, which is `statistics.fmean` over per-request lengths, are not directly comparable to vLLM's number on variable-length workloads, and the two can differ by several points in either direction depending on the length-vs-acceptance correlation. See vllm-project#42508 for the original reproduction and a source-level comparison against SpecForge and SGLang. This is a comment/docstring-only change; the metric definitions and CLI/ Prometheus output are byte-identical to before. Signed-off-by: arrdel <chindahel1@gmail.com>
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FYI to any maintainer with label access: this PR is hitting the standard new-contributor The change is docstring/comment-only (12 additions, 1 deletion, no runtime impact); pre-commit + |
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Purpose
SpecDecodingLogging._logand the PromQL recipe inSpecDecodingProm.__doc__compute themean_acceptance_lengthas a pooled mean (weighted by draft count, equivalent tototal_generated / total_verify_calls), not an unweighted mean of per-request means.Some external benchmarks report the latter. SpecForge's
average_acceptance_length, for example, isstatistics.fmean(acceptance_lengths)over per-request lengths. On variable-length workloads the two aggregations can differ by several points in either direction, depending on the sign of the per-request-length ↔ acceptance-rate correlation. This has confused at least one report already (issue #42508: a 55.08%-vs-44% pooled-vs-mean-of-means gap for EAGLE3 on GSM8K, and 66.57%-vs-72% in the opposite direction for the standalone drafter, same numerator, different aggregation).Changes
Docstring/comment only. Adds a short aggregation note
mean_acceptance_lengthis computed, andSpecDecodingProm.__doc__below the existing PromQL recipe.Both points cross-reference #42508 for the reproduction.
The metric definitions and the CLI/Prometheus output are byte-identical to before.
Test Plan
None (comment/docstring only, no runtime behavior changed).
ruff format --checkclean; pre-commit hooks pass locally.Test Result
n/a
Documentation Update
n/a (this PR is the documentation update).
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