[Metrics] Say what vllm:prompt_tokens and vllm:request_prompt_tokens count - #56923
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…count Both metrics carried the HELP text "Number of prefill tokens processed", which is not what either one measures. The counter is the prompt length summed over every sequence, cache hits included, so it equals the sum of vllm:prompt_tokens_by_source_total and, under parallel sampling, exceeds the client-facing usage.prompt_tokens by a factor of n. The histogram observes the prompt length of each finished sequence once, which is what the design doc already says it does. The Rust frontend mirrors the same strings. Signed-off-by: Guo Cheng <224264187+GuoCheng24@users.noreply.github.com>
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| "Prompt tokens summed over sequences, including prefix-cache hits; " | ||
| "each parallel-sampling child counts its prompt separately. " | ||
| "Equals the sum of vllm:prompt_tokens_by_source_total." |
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For resumable streaming-input requests, this description incorrectly implies that the counter includes the sequence's complete prompt. After the first output, Request.take_prefill_stats() clears prefill_stats; _update_request_as_session() then appends later input chunks and updates num_prompt_tokens without creating new PrefillStats, so IterationStats adds only the initial chunk to this counter. Consequently realtime speech-to-text and other multi-chunk sessions are undercounted relative to the stated “prompt tokens summed over sequences” semantics; either qualify the HELP text as initial-prefill accounting or include continuation chunks in the counter.
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Right, and it holds for the Rust frontend too, since both count from the engine core's prefill_stats. The only place a PrefillStats is populated is the if request.prefill_stats and request.num_preemptions <= 0 branch in the scheduler. take_prefill_stats() sets it to None at the first output, and _update_request_as_session() extends the prompt without creating a new one. So a streaming-input session contributes its first input chunk to vllm:prompt_tokens, and nothing after that.
Pushed 99c771c, which says this in all three places: the Python and Rust HELP strings, and docs/design/metrics.md. The PR only changes documentation, so it describes today's behaviour rather than changing it. Counting later chunks as well would mean creating a fresh PrefillStats in _update_request_as_session(). That is a behaviour change to a counter people already have dashboards on, so if maintainers want it I would rather do it as a separate PR.
…hunk Request.take_prefill_stats() clears prefill_stats at the first output, and _update_request_as_session() extends the prompt without creating a new PrefillStats, so vllm:prompt_tokens (Python and Rust) counts a streaming-input session's first input chunk only. Say so in the HELP text and the design doc. Signed-off-by: Guo Cheng <224264187+GuoCheng24@users.noreply.github.com>
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Purpose
Both
vllm:prompt_tokens(Counter) andvllm:request_prompt_tokens(Histogram) ship with theHELP text
Number of prefill tokens processed., and neither one measures that. This changes thetwo HELP strings (Python and the Rust frontend, which mirrors them verbatim) and the matching
line in
docs/design/metrics.mdto say what the series actually count. No metric name, label,bucket or value changes.
What they count, measured on
vllm==0.28.0with--enable-prefix-cachingand confirmedagainst the accumulation on
main(PrefillTokenStats.update_from_output, which addsnum_prompt_tokenstototalalongsidenum_computed_tokensandnum_cached_tokens):prompt_tokens_totalusage.prompt_tokensprompt_tokens_total == n x prompt_lenandcomputed + cache_hits == prompt_tokens_totalholdexactly in every cell across two runs, and on the same scrape
local_compute + local_cache_hit + external_kv_transfer == prompt_tokens_total(3423 + 7840 + 0 = 11263). So the counter is the prompt length summed over sequences, cache hits
included; it is not "prefill tokens processed", and vLLM already publishes that split under
vllm:prompt_tokens_by_source_total. The same identical-prompt test atn=1gives 310 inprompt_tokens_totalfor 6 tokens of KV computed on a warm cache, which is correct under theper-sequence reading and is why the wording matters.
The histogram observes the full prompt length once per finished sequence (count +4, sum
4 x prompt at
n=4), which is what the design doc entry three lines below it already says(
Histogram of input prompt token counts.); its HELP string was a copy of the counter's.Discussion and measurement scripts: #56361.
Test Plan
No behaviour change.
ruff checkandruff format --checkpass onloggers.py. Both newstrings were registered through
prometheus_clientand rendered withgenerate_latest()toconfirm they are accepted and appear as
# HELPlines.tests/entrypoints/serve/instrumentator/test_metrics.pyasserts on metric names and values only, not HELP text, so it is unaffected; the same
grepfinds no test that reads either HELP string.Test Result
Not changed here, flagged for a follow-up if wanted:
examples/observability/prometheus_grafana/grafana.jsonhas a panel described as "Number of tokens processed per second"; if it plots
vllm:prompt_tokens_total, its description carries the same reading.