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Trace cold request waterfalls and isolate the layout discovery regression

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Jonathan Sykes
2026-10-08 08:45:00 +08:00
parent af6aabc3b7
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perf/README.md
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@@ -335,3 +335,14 @@ No product behavior changes are involved in these measurement corrections.
Keep browser/build/test processes idle during full timing measurements. WebKit's
zero long-task field means unavailable, not zero work. Autoplay-blocked/media-ready
null results do not establish playback performance or iPhone audio continuity.
### Cold waterfall diagnosis
`--source-root perf/.tmp/<historical-worktree>` runs that worktree's matching
frontend **and server**; `--frontend-commit` instead combines archived frontend
with the current backend. Run historical comparisons sequentially to avoid CPU
contention. The result records `sourceCommit`, `sourceRoot` and the served build.
Every cold sample has `waterfalls`: proxy request order/start/headers/end times
and page Resource Timing discovery/start/end times. Proxy times start when
tracking begins; page times start at navigation. Compare within each clock,
then use the page FCP to locate the rendering dependency.