perf(coverage): speed up report generation (bounded-memory merge + precompiled globs) by toxik · Pull Request #10506 · vitest-dev/vitest

toxik · 2026-06-02T13:18:44Z

Description

Two independent, self-contained performance fixes to coverage report generation (generateCoverage / BaseCoverageProvider), measured on a real 1124-test-file app (~39 MB of raw V8 coverage across the run). Numbers below are best-of-3 runs, baseline vs this PR on the same machine:

Phase	Before	After
read + merge	~20.8s	~0.30s
convert (incl. `isIncluded` filtering)	~22.2s	~2.6s
total `generateCoverage`	~43s	~2.9s

Peak heap during coverage generation drops from ~2.5 GB → ~1.15 GB, and resident raw coverage is now bounded to ≤16 MB regardless of suite size.

isIncluded runs per covered file inside convertCoverage, so the glob fix's savings land in the convert row above; the pure astV8ToIstanbul transform (~2.6s) is itself unchanged.

Tests

No behavior change. Both fixes are verified against the existing test/coverage-test suites (v8 + istanbul), which assert coverage output and the set of included files. pnpm typecheck and pnpm lint pass. A backport to the v4 release line will follow.

🤖 Generated with Claude Code

github-actions · 2026-06-02T13:19:01Z

Hello @toxik. Your PR has been labeled maybe automated because it appears to have been fully generated by AI with no human involvement.

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toxik · 2026-06-02T13:21:45Z

Hello @toxik. Your PR has been labeled maybe automated because it appears to have been fully generated by AI with no human involvement.

To keep your PR open, please follow these steps:

Confirm that you are a real human. If you are an automated agent, disclose that

Confirm you've read, reviewed and stand behind its content

Confirm you've read the full issue along with all of its comments, as well as any linked issues and their comments

Make sure it follows our contribution guidelines and uses the correct GitHub template

Disclose any AI tools you used (e.g. Claude, Copilot, Codex)

Please, do not generate or format the response with AI. If you do not speak English, reply in your native language or use translation software like Google Translate or Deepl. If the response is generated, the PR will be closed automatically.

These measures help us reduce maintenance burden and keep the team's work efficient. See our AI contributions policy for more context.

Hello,

I am indeed a human and there was indeed coding agent help with this PR from Claude Code.
I've reviewed and tested the code prior to opening the PR, as well as tested it in a large project to confirm the numbers.

vitest-ecosystem-ci · 2026-06-03T06:18:42Z

📝 Ran ecosystem CI: Open

suite	result
vitest-coverage-large	✅ success

AriPerkkio

The early coverage map merging is there intentionally. We cannot hold all coverage results in memory at once, as it would lead to out-of-memory crashes. That's the reason why we do file-system round-trip in the first place, instead of just holding everything in memory.

Related issue:

#4476

AriPerkkio · 2026-06-03T06:08:43Z

-
-        // mergeProcessCovs sometimes loses autoAttachSubprocess
-        const fromExtendedContext = autoAttachSubprocess ? coverage.result.filter(r => r.isExtendedContext) : []
+        coverages.push(coverage)


We cannot hold these in-memory as it would cause out-of-memory issues in large projects.

Thanks for the review!
I'll look into what can be done to reduce the memory footprint.
In the meantime, would it make sense to open a separate PR for the regex optimization?

Yup sounds good! Also if it makes sense to optimize the startOffset and autoAttachSubprocess handling separately, feel free to include them there.

left things in the same PR though 😅
batched the files into ~16MB chunks to still keep O(n) instead of O(n^2)
01c2c27

AriPerkkio · 2026-06-03T07:14:40Z

+
+      this._globMatchers = {
+        matchExclude: exclude.length ? pm(exclude, { dot: true }) : () => false,
+        matchInclude: pm(include, { dot: true, ignore: exclude }),


This could probably also check this.options.include, and default to () => true when it's falsy. 🤔

addressed in 43c3903f76e14c51b578dab4a9090e05c21be04a

generateCoverage() folded mergeProcessCovs([merged, coverage]) once per coverage file and re-scanned the growing merged.result on every file to repair startOffset/isExtendedContext, making the read phase O(n^2) in the number of coverage files. Collect the raw coverages and merge them in a single pass, restoring the dropped fields via lookup maps built while reading. Profiled on a real 1124-test-file app: the read+merge phase dropped from ~24.5s to ~0.3s (single merge ~95ms). Together with the isIncluded fix, total coverage generation went from 52.8s to 7.1s. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

isIncluded() called picomatch.isMatch(file, patterns, options) per file, which recompiles every glob into a regex on each call. The globCache does not help during coverage filtering because each result entry has a unique filename, so every call paid the full compile cost. Compile the include/exclude matchers once and reuse them. Profiled on a real 1124-test-file app: the include/exclude filtering step dropped from ~23.3s to ~0.38s (~61x), with identical results. Together with the single-pass merge, total coverage generation went from 52.8s to 7.1s. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Co-authored-by: Ari Perkkiö <ari.perkkio@gmail.com>

github-actions Bot added the maybe automated User is likely an AI agent, or the content was generated by an AI assistant without user control label Jun 2, 2026

toxik mentioned this pull request Jun 2, 2026

perf(coverage): speed up report generation (single-pass merge + precompiled globs) [backport to v4] #10507

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sheremet-va removed the maybe automated User is likely an AI agent, or the content was generated by an AI assistant without user control label Jun 2, 2026

vitest-dev deleted a comment from vitest-ecosystem-ci Bot Jun 3, 2026

AriPerkkio requested changes Jun 3, 2026

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AriPerkkio reviewed Jun 3, 2026

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toxik and others added 3 commits June 3, 2026 11:03

fixup! perf(coverage): merge V8 coverage in a single pass

01c2c27

toxik force-pushed the perf/coverage-generation branch from 14a9977 to 65dd2ad Compare June 3, 2026 08:08

fixup! perf(coverage): precompile include/exclude globs in isIncluded

43c3903

Co-authored-by: Ari Perkkiö <ari.perkkio@gmail.com>

toxik force-pushed the perf/coverage-generation branch from 65dd2ad to 43c3903 Compare June 3, 2026 08:10

toxik changed the title ~~perf(coverage): speed up report generation (single-pass merge + precompiled globs)~~ perf(coverage): speed up report generation (bounded-memory merge + precompiled globs) Jun 3, 2026

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perf(coverage): speed up report generation (bounded-memory merge + precompiled globs)#10506

perf(coverage): speed up report generation (bounded-memory merge + precompiled globs)#10506
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