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fix(pylon): exclude cached prompt tokens from fallback TPS - #2296
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Included review availability: This review used your included allowance. Your plan provides up to 12 included reviews per hour; 11 remain after this review. 📝 WalkthroughWalkthroughMock OpenAI responses now report cached input tokens. Pylon parses cached-token usage, records uncached input-token counts in request observations, and uses ordered request windows to calculate fallback input rates and eligible maximum rates. ChangesCached-token usage and fallback throughput
Priority: ➖ Normal Estimated code review effort: 4 (Complex) | ~60 minutes Change: Bug fix · Severity of issue fixed: Medium Suggested reviewers: Merge Risk: ⚪ Minimal · up to Cached prompt tokens no longer inflate the fallback input-throughput estimate. Total-token observations update the mean but not the generation maximum. No outstanding defects were identified, so the change appears ready to merge. Pre-merge checks |
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The title follows Conventional Commits format with the required scoped fix type and accurately describes the primary change: excluding cached prompt tokens from fallback TPS. |
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Issue [#2294] requires cache-aware fallback input TPS for Chat Completions and Responses. The changes parse cached-token usage, compute uncached input tokens, defer expected-usage samples until termin… |
| Out of Scope Changes check | |
The changes to usage parsing, request observation, fallback projection and aggregation, MockDynamo responses, tests, calibration, and runtime statistics documentation support issue [#2294]. No unrelat… |
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@src/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rs:
- Line 780: Replace saturating subtraction in the cached-token calculation with
checked subtraction, and treat a cached count greater than the input total as
unavailable for calibration rather than as zero uncached tokens.
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src/libraries/rust/stargate/crates/mock-dynamo/src/openai.rssrc/libraries/rust/stargate/crates/mock-dynamo/src/tests.rssrc/libraries/rust/stargate/crates/pylon-lib/src/quic_http_tunnel/core.rssrc/libraries/rust/stargate/crates/pylon-lib/src/quic_http_tunnel/tests.rssrc/libraries/rust/stargate/crates/pylon-lib/src/request_observer.rssrc/libraries/rust/stargate/crates/pylon-lib/src/runtime_state.rssrc/libraries/rust/stargate/crates/pylon-lib/src/sse_message_stream.rssrc/libraries/rust/stargate/crates/pylon-lib/src/stats/collector.rssrc/libraries/rust/stargate/crates/pylon-lib/src/stats/projection.rssrc/libraries/rust/stargate/docs/runtime-stats-interface.md
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barrygreengus
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Thanks for this. The usage parsing looks solid: both Chat and Responses formats are handled, cached counts that are invalid or exceed the total fall back to total tokens and mark calibration ineligible, and the tests cover those cases. Two issues limit how much the fix helps in practice, plus one question.
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The correction arrives after the inflated rate is already published.
The fallback window records each request's interval at first output with the total prompt tokens. The cached count only arrives in end-of-stream usage, and
request_input_intervals.observe(stats/projection.rs, the call changed in this PR) then replaces the entry and recomputes. Means published between first output and end of stream still carry the cached tokens, and anything that keeps a high-water mark of the published rate, such as the maximum input TPS that Pulsar weights by in #1002, keeps the inflated value after the correction.I modeled this in the routing simulator from #2223 (a local experiment, not part of either PR), with four ways of counting prompt tokens in the fallback window: total tokens (current), this PR (total at first output, replaced by uncached tokens at completion), holding each interval until usage arrives and recording it once with uncached tokens, and an ideal that knows the uncached count at first output. Pulsar-WaW goodput in requests/s, range over seeds, with the lowest SLO attainment:
Fleet and rate Current This PR Held until usage Ideal Mixed backend sizes, 450 RPS (half-scale engine) 371-383 (96.4%) 366-391 (95.8%) 431-439 (100%) 431-439 (100%) Equal backends, 550 RPS (half-scale engine) 303-400 (82.4%) 301-400 (81.9%) 434-487 (96.4%) 434-487 (96.3%) Mixed backend sizes, 900 RPS 861-876 (100%) 798-857 (99.9%) 889-909 (100%) 889-909 (100%) Mixed backend sizes, 1,100 RPS 493-545 (69.0%) 530-547 (73.2%) 1,081-1,103 (100%) 1,082-1,103 (100%) Equal backends, 1,100 RPS 733-1,088 (86.5%) 998-1,020 (98.4%) 1,086-1,112 (100%) 1,086-1,112 (100%) In this model the PR as written changes little, while holding the interval until usage arrives recovers the full benefit. Suggestion: when the request will report usage, defer adding its interval to the window until the terminal usage event, and record it once with the uncached count. Requests without usage keep today's behavior.
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The fix only applies when the stream carries usage with cached details.
Chat streams include usage only when the client sets
stream_options.include_usageor Pylon runs with--force-chat-completions-include-usage, which defaults to off. Without either, behavior is unchanged and nothing signals it. It would help to state this requirement indocs/runtime-stats-interface.md, and to decide whether deployments that rely on fallback stats should enable the forcing flag. -
Question: do the production engines we route to populate
prompt_tokens_details.cached_tokens(Chat) andinput_tokens_details.cached_tokens(Responses) by default? Some engines only report prompt token details behind a server option, so it is worth confirming before relying on this path.
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Fixed on remote HEAD |
FamousDirector
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Usage parsing for Chat and Responses, the checked subtraction, and the MockDynamo changes look right, and cargo test --locked -p pylon-lib -p mock-dynamo passes on 414a29a (541 and 34+2 tests). Two issues in the fallback projection, both inline. The PR merges cleanly with current main, but main now has max_input_tps from #1002, which changes what this path needs to do.
Signed-off-by: Efe Gökdemir <gokdemirefe1903@gmail.com>
Signed-off-by: Efe Gökdemir <gokdemirefe1903@gmail.com>
Signed-off-by: Efe Gökdemir <gokdemirefe1903@gmail.com>
Signed-off-by: Efe Gökdemir <gokdemirefe1903@gmail.com>
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Requests that expect usage now reserve a pending slot in first-output order instead of joining the window at completion. The published rate covers the latest resolved entries before the oldest pending one, so long decodes no longer leave gaps that inflate the interval union and under-read the rate. A retained-entry cap provisionally resolves the oldest pending entry with its total tokens so one stuck request cannot freeze the window forever. Calibration observations remain max-eligible, so a calibrated backend still publishes max_input_tps without cached-usage traffic. Eligibility is not downgraded by a later observation without cached-token data, and evicted requests cannot re-enter the window. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
Add collector tests for total-token observations that move the mean without raising max_input_tps, windows that wait for total-token entries to leave before raising the maximum, deferred requests whose usage never arrives, calibration windows that still raise the maximum, a pending request holding newer completions out of the mean, and the concurrent long-decode schedule where the deferred rate must match the undeferred 500 TPS. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
…eans Document that requests expecting usage hold a pending slot in first-output order, that the published rate covers resolved requests before the oldest pending one with a retained-entry cap, and which means may raise max_input_tps, including calibration. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
At serving concurrency the retained-entry cap fired on every arrival and resolved pending entries with total prompt tokens, so the published mean counted cached tokens again and max_input_tps never rose. A lost terminal event could also freeze the mean until the cap filled. Bounds now drop the oldest pending entry instead of resolving it, and its late usage is ignored. The count cap holds 1024 entries (or 8 windows if larger), and a pending entry is also dropped once newer first outputs are more than 120 seconds past its own. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
Describe the 120 second lag bound and the retained-entry cap that drop the oldest pending request from the fallback input rate, and note that engines without cached-token details do not raise max_input_tps. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
Calibration sent the same "1"-repeated prompt to every request in a batch, and each ramp step repeated or extended the previous prompt. With prefix caching enabled (vLLM automatic prefix caching, SGLang radix cache, KV-aware routing), nearly every calibration prefill after the first was a cache hit, so the calibrated input rate overstated uncached prefill throughput. Calibration windows are max-eligible, so the inflated rate could also set max_input_tps. Each calibration request now starts with a random per-request prefix followed by filler, keeping the prompt exactly prompt_units characters so the input header estimate and ramp semantics are unchanged. The non-streaming calibration path also parsed only completion_tokens, so calibration windows used the character-count header estimate. It now parses usage.prompt_tokens and prompt_tokens_details.cached_tokens and feeds them to the request observer, matching the streaming parser: malformed cached counts or cached counts above the prompt total leave the uncached count unavailable. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
When every usage-reporting request outlived the lag bound or the retained cap, each pending slot was dropped before its usage arrived and the late usage was rejected, so no runtime mean was published while traffic flowed. Bound drops now record the request instead of advancing the eviction guard. Later live events for it cannot reserve a new slot, and its usage re-enters at its first-output position unless newer resolved entries have already left the window. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Signed-off-by: jcameron <jcameron@nvidia.com>
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TL;DR
Exclude cached prompt tokens from fallback input TPS when OpenAI usage reports them. Keep total-token accounting when cache details are absent, and include cache counts in MockDynamo usage responses.
Additional Details
Covers Chat Completions and Responses usage formats.
For the Reviewer
Please review the usage parsing and fallback projection path.
For QA
cargo test --locked -p pylon-libcargo test --locked -p mock-dynamocargo clippy --locked -p pylon-lib -p mock-dynamo --all-targets -- -D warningsrustfmt --checkandgit diff --checkIssues
Fixes #2294
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