perf(replay): reuse mutation serialization options per emission - #4807
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marandaneto wants to merge 1 commit into
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perf(replay): reuse mutation serialization options per emission#4807marandaneto wants to merge 1 commit into
marandaneto wants to merge 1 commit into
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Replay incident risk checkThis diff touches code involved in past incidents. This is a heads-up, not a verdict: read the matched sections of INCIDENTS.md and answer their review questions before merging. For a judgment on whether this diff has the same failure mode, run the |
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Size Change: +1.29 kB (+0.01%) Total Size: 20.5 MB 📦 View Changed
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marandaneto
marked this pull request as ready for review
September 6, 2026 06:17
marandaneto
requested review from
TueHaulund,
arnohillen,
fasyy612 and
ksvat
and removed request for
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September 6, 2026 06:17
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Reviews (1): Last reviewed commit: "perf(replay): reuse mutation serializati..." | Re-trigger Greptile |
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Problem
Large session replay mutations still block the main thread on node-dense pages. The mutation emitter allocates a serialization options object and four callbacks for every serialized node, adding avoidable allocation work during subtree moves and repeated rebuilds.
This PR is stacked on #4801. It contains only the follow-up optimization and benchmark extensions. It is another partial improvement for #4217, not a complete fix.
Changes
Performance results
Local medians from three runs per build on an Apple M4 Pro, Chromium 136.0.7103.25, approximately 50k nodes, compression enabled and no CPU throttling. Both builds used the same benchmark source, with alternating baseline/candidate runs. These are descriptive samples, not customer-level guarantees or significance estimates.
Single rebuild performance was essentially flat/slightly worse. Startup and full-snapshot paths are not optimized. All six comparison runs passed replay, input, privacy and drop/recovery checks. A separate 100k-node candidate run still showed 451–618 ms tasks, so substantial blocking remains.
The deterministic test failed before the change with 500 options objects for 500 fixture nodes and passed afterward with one. The recorder artifact increased by 17 raw bytes and 15 gzip bytes.
Validation
c9b2c2216743cd177b2958aee6af7549538ae055against the parent branch reported no actionable findings.No changes to lazy-load signatures, masking policy, mirror IDs, queue ordering, sampling, session rotation or unload behavior. No recording-volume change is expected. This PR does not add workers or asynchronous DOM capture.
Release info Sub-libraries affected
Libraries affected
Checklist
If releasing new changes
posthog-jspatch changeset🤖 Agent context
Autonomy: Human-driven (agent-assisted)
Implemented with Pi using repository inspection, Chromium profiling, Vitest, Playwright and isolated autoreview. Profiling found that preprocessing was only part of the stall. The change reduces repeated configuration allocation without caching live DOM or masking decisions.
The code formatter hook was skipped to preserve the existing rrweb formatting rather than reformat entire files. Targeted formatting/lint checks and the other commit hooks passed. No hook configuration was changed. Human review is required.