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89 lines
4.0 KiB
Markdown
89 lines
4.0 KiB
Markdown
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# Renderer performance probes
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Measuring what a session switch costs in the running Desktop app, over CDP. The
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findings in #4109 were produced with these; they live here so the next
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measurement is a command rather than a rebuild.
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They are ad-hoc tools, not a benchmark suite: they attach to a dev app you
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started, and they answer "how much" only for the interaction they drive.
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## Running
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Start the dev app with the debugger open:
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```bash
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npm run dev -w @maka/desktop -- --remote-debugging-port=9334
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```
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Then, with a populated sidebar (about 30 rows is what the #4109 numbers used):
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```bash
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node scripts/perf/session-switch-commits.mjs before
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node scripts/perf/session-switch-busy-js.mjs
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```
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- `session-switch-commits.mjs` — React commits per switch, how many are
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full-tree renders, and how many fibers actually re-rendered. Reloads the page
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first, so `react-commit-probe.js` is in place before React boots.
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- `session-switch-busy-js.mjs` — renderer busy JS per switch, with the top
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self-time entries. It reports the running build, one configuration. A
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before/after needs both configurations in one instance; see below.
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- `react-commit-probe.js` — the in-page half: a minimal
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`__REACT_DEVTOOLS_GLOBAL_HOOK__` plus a `Function.prototype.bind` wrapper that
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catches React creating a `dispatchSetState`, so a commit can be attributed to
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the `setState` that caused it.
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- `cdp-client.mjs` — the protocol client and the row-clicking helper.
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## The one rule
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**Never compare numbers from two app launches.** Restarting the app shifts these
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metrics by orders of magnitude, while the spread inside a single running
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instance is small. An A/B means: one instance, a `globalThis` switch to select
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the configuration, at least three repetitions per configuration, alternating,
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compared pairwise. Two runs of "before" and "after" against two launches will
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produce a confident number that means nothing.
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The switch is a throwaway, not a shipped affordance. Branch a worktree, put a
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`globalThis` flag in the one place that reads it — one that restores the old
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behaviour when set, so the regressed configuration is reachable without
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rebuilding the defect by hand — alternate the flag inside the running instance,
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then delete the worktree. Measurement discipline is ours; product code carries
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none of it. A flag that reaches `main` is a branch nothing executes and a
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defect replica that has to be maintained against the code it replicates.
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Two smaller ones that cost time to rediscover:
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- Clicks must be `Input.dispatchMouseEvent`. A synthesised `element.click()`
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does not activate an Astryx `SideNavItem`, so the app does nothing and the
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measurement describes it.
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- Disarm the commit probe (`__MAKA_PROBE__.arm = false`) before sampling. Its
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fiber walk otherwise shows up in the profile as the app's own work;
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`session-switch-busy-js.mjs` does this for you.
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## What these do not tell you
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The commit count is unweighted, and busy JS on this workload is spread across
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about a thousand cheap fibers rather than concentrated in one component. A
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change that lowers the commit count has not necessarily lowered the time, and a
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change that moves the top self-time entry has not necessarily lowered anything.
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Read both, and read the medians.
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