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bench-regression

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Investigate a Helion benchmark dashboard regression (helionlang.com/dashboard) — find the cause and classify it. Auto-activate when the user reports a perf drop/spike on a dashboard platform (e.g. b200 cute) around a given date.

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Goal: explain a dashboard move and sort it into one of four causes — real regression, benchmark method change, hardware issue, or noise. Don't blame a commit until the data points at one.

1. Pull the data, isolate the platform. Numbers are per-nightly, not live — fetch the JSON, don't WebFetch the page (too big to page through):

curl -s https://helionlang.com/dashboard/dashboard-data.json -o /tmp/dash.json

Each summary[] entry has platform_short (e.g. b200_cute) and a history[] of nightlies with helion_speedup_geomean, triton_speedup_geomean, torch_compile_speedup_geomean, helion_latency_avg_ms, sha. Filter to the platform+kernel and print the series around the date.

For the Pretuned tab, use the sibling URL — same structure, different summary[] fields (geomean, best_speedup, helion_wins/total, baselines, cudagraph):

curl -s https://helionlang.com/dashboard/pretuned-dashboard-data.json -o /tmp/pretuned.json

2. Classify — read the whole series, not one point, and sort the move into one of four causes:

  • Real regression → Helion genuinely got slower. A code change made the kernel (or the config the autotuner picks) worse. Find the commit (step 3).
  • Benchmark method change → the kernel is unchanged; how it's measured changed. A change to the harness, timer, launcher, autotune settings, input shapes, or baseline flips the reported number without touching real performance. The pre- or post-change value is an artifact — decide which is the honest one.
  • Hardware issue → the environment changed, not Helion. Runner swap, GPU/driver change, thermal throttling — affects everything running on that machine, not just Helion.
  • Noise → nothing changed; it's measurement scatter. The move isn't a persistent step, just normal run-to-run variance.

Useful cross-checks when deciding: is the move Helion-only or shared with triton_*/torch_compile_* (shared → environment/baseline, not Helion)? Does it persist or revert (revert → noise)? Do independent metrics agree — e.g. helion_latency_avg_ms and speedup come from different timers (speedup uses --cudagraph; latency uses --latency-measure-mode triton_do_bench, see benchmark.yml), so if one moves and the other doesn't, suspect a measurement change, not real perf. Don't reconstruct baseline = speedup * latency — they're different modes; the product is meaningless.

3. Find the commit (real regression only). Window = between last-good and first-bad nightly sha:

git log --first-parent --pretty="%h %cI %s" <good_sha>..<bad_sha>

Match the suspect to the affected slice: a b200_cute-only drop comes from a [cute] commit, not Pallas/Triton/TPU. Check which kernels the platform runs (kernels_cute default in .github/workflows/benchmark_dispatch.yml) and whether they keep --cudagraph (remove_flags in benchmarks/run.py) before trusting the cudagraph-timed speedup. Confirm by reading the diff for a plausible mechanism.

Report: the cause (one of the four), the first-bad sha + culprit commit if real, and the true speedup.