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[Bugfix][Metrics] Restart the logging interval clock on empty intervals - #55474
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Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review. WalkthroughMetric loggers now reset interval state after empty intervals. Deterministic tests verify that idle time does not skew later throughput, rate, duration, or debug-overhead calculations. ChangesIdle interval accounting
Estimated code review effort: 2 (Simple) | ~15 minutes Merge Risk: 🟡 Moderate · up to Idle performance-metric intervals now clear debug observations along with resetting timing. This can omit prior debug-window data and alter MFU-overhead reporting, so the intended debug accumulation behavior should be reconciled before merge. 🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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In `@vllm/v1/metrics/perf.py`:
- Line 1522: Align the debug accumulation window with the retained
DebugPerfStats when a zero-FLOP batch is preserved: update the logic around
self.last_log_time and PerfMetricsDebugLogging.log() to track a separate debug
start time or reset the retained debug data at that boundary. Ensure a
subsequent non-empty sample calculates duration and mfu_calc_overhead across the
full retained window, and add a regression test covering a non-empty sample
after the preserved debug-only interval.
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tests/v1/metrics/test_logging_intervals.pyvllm/v1/metrics/perf.pyvllm/v1/spec_decode/metrics.py
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Verified locally (head 1d59a4ca vs base bc96d76) — the bug is real and the fix is correct.
Bug confirmed by code reading: SpecDecodingLogging.log() and PerfMetricsLogging.log() both return early on empty intervals WITHOUT advancing last_log_time; the clock is only refreshed by reset() at the end of the non-empty path. The outer LoggingStatLogger.log() (loggers.py:315/:320) calls them unconditionally on a fixed VLLM_LOG_STATS_INTERVAL timer, so the first busy interval after any idle stretch divides its counters by idle + active time — throughput under-reported by 1 + idle_intervals (exactly the 3.33 vs 20.0 tok/s and MFU 6.5 vs 200 TF/s examples in the PR body).
Fix semantics verified correct:
- Empty branch now restarts only the clock (
self.last_log_time = time.monotonic()), NOTreset(). That distinction matters forPerfMetricsLogging:reset()also zeroesdebug_logging, whoseobserve()counts batches that produce no flops/bytes (debug stats are gated onVLLM_DEBUG_MFU_METRICS, independent of the flop/byte totals that gate the early return). Callingreset()there would silently discard debug observations from the still-accumulating window — the chosen approach is the right one. (For SpecDecodingLogging the two are equivalent sinceobserve()appends all four lists together, and the code documents this.) - Normal path unchanged: computes
delta_timefrom the restarted clock thenreset()s — no double-counting, no clock rollback.last_log_timehas no other consumers (only the two log paths + reset, confirmed by grep).
Regression-proven (head test file transplanted onto base code): 3/4 tests FAIL on base / PASS at head:
- test_spec_decoding_empty_interval_does_not_skew_next_throughput (300 accepted/400 drafted over ACTIVE_SECONDS, not IDLE+ACTIVE)
- test_perf_metrics_empty_interval_does_not_skew_next_rates (2.0 TF/s/GPU, 3.0 GB/s/GPU over active time only)
- test_perf_metrics_empty_interval_keeps_debug_accumulators (clock restarted to IDLE_SECONDS while total_num_batches/prefill/decode/calc_duration survive — locks the no-
reset()design) - Positive control test_spec_decoding_busy_interval_unaffected passes on BOTH (no behavior change to back-to-back intervals).
Test mechanics: FakeClock monkeypatched over the module time (deterministic, no sleeps), _fail_if_called guards the no-log contract, exact pytest.approx throughput assertions. Full file 4/4 pass at head (teardown errors are the standard CPU-host cleanup_dist_env_and_memory fixture noise — torch.accelerator.empty_cache() needs an accelerator; identical on base, not PR-caused).
Only bot reviews (claude/coderabbit) existed; no human verification, no competing PR. This is the missing piece — happy to re-check if the maintainer wants the debug-logging path exercised under real VLLM_DEBUG_MFU_METRICS traffic.
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Good catch, this was right and worse than the flag suggested. I had restarted Reproduced against the version that was on the branch — 9.0s of calc accumulated during an idle interval, then a 1.0s busy interval: Correct answer is 10.0%. An overhead fraction above 100%.
Test suite is now five tests: both loggers on an idle-then-busy sequence, the busy-to-busy control, the debug window resetting with the clock, and |
| real interval. | ||
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| These tests drive both loggers with a deterministic fake clock and assert that an | ||
| idle interval does not contaminate the interval that follows it. |
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IMO, this is going overboard with testing something so non-critical - at most I'd expect to see a small test added alongside other existing tests
| # to stay together: PerfMetricsDebugLogging.log() is handed | ||
| # delta_time and reports total_calc_duration / delta_time, so debug | ||
| # totals carried across a restarted clock would be divided by a | ||
| # window they did not occur in. |
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I don't think these large comments have enough value to warrant this amount of space - this line of code is pretty straightforward, and I think is understandable and intuitive enough without any comment
(Yes, it wasn't so obvious that it was missing ... but with the line of code now there, I'm not sure any reader would spend more than a few seconds confirming its correctness)
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Thank you for your contribution While Claude may be helpful resolving bugs like this, I think you should try to take control of its verbosity, if only for the sake of us reviewers! If you made this contribution without the help of an AI assistant, the PR description, commit message, and code itself would be a tiny fraction of the size. Hope that make sense 👍 |
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Fair, and useful to hear it said rather than left in the diff. Trimmed and force-pushed:
On what went: the busy-to-busy control passed on base and at head, so it was not discriminating anything and it had no business being there. The The two that are left are the two that fail on main. For a 100s idle stretch before a 1s busy one, spec-decode reports 2.970 tok/s where 300.0 is correct, and perf reports 0.0198 TF/s/GPU where 2.0 is correct. One thing I would rather you decided than me: the |
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Honestly, I'd prefer to feel like I'm working with a human here. I'd merge the PR if it simply added the two reset lines without comments or tests |
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Trimmed to the two reset lines. The tests and the comments are gone and it is a single commit now. |
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✅ @WatchTree-19, CI is now available for this PR.
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/ci run |
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✅ Triggered Buildkite CI #89326 for commit |
SpecDecodingLogging.log() and PerfMetricsLogging.log() return early when nothing was observed during the interval, without advancing last_log_time. The clock is only moved by reset(), at the end of the non-empty path. LoggingStatLogger.log() calls both unconditionally on a fixed VLLM_LOG_STATS_INTERVAL timer, so the first busy interval after any quiet stretch divides its counters by the whole idle gap and under-reports throughput by a factor of 1 + idle_intervals. Call reset() on the empty branch in both. Signed-off-by: WatchTree-19 <119982314+WatchTree-19@users.noreply.github.com>
Head branch was pushed to by a user without write access
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/ci run |
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✅ Triggered Buildkite CI #90101 for commit |
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Rebased onto current main, same two lines. The earlier build failed on the h200 spec-decode-draft-model job with exit status -1 and the branch was behind, so it should get a clean run now. Whenever you have a minute, the changes-requested from the 14th is still sitting on it. |
SpecDecodingLogging.log()andPerfMetricsLogging.log()return early when nothing was observed during the interval, without advancinglast_log_time. The clock is only moved byreset(), at the end of the non-empty path.LoggingStatLogger.log()calls both unconditionally on a fixedVLLM_LOG_STATS_INTERVALtimer, so the first busy interval after any quiet stretch divides its counters by the whole idle gap.The under-report factor is
1 + idle_intervals. After 100s idle at a 10s cadence, 300 accepted tokens in a 1s busy interval report 2.970 tok/s instead of 300.0, and MFU reports 0.0198 TF/s/GPU instead of 2.0. Not a crash and not a NaN, just a wrong throughput printed next to correct totals.The fix calls
reset()on the empty branch in both.reset()rather than assigninglast_log_time, becausePerfMetricsDebugLoggingdividestotal_calc_durationby the samedelta_time, so its accumulators have to restart with the clock ormfu_calc_overheadgoes above 100%.Two tests in
tests/v1/metrics/test_perf_metrics.py, both failing on main.I do not have a GPU. The tests were run against the genuine upstream modules with only their heavy transitive imports stubbed; the classes under test are pure Python and touch none of the stubs on the exercised paths. Worth someone running them in a real environment.
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