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Summary

When a Rolldown session is opened for the first time, the DevTools server parses logs.json and then writes the .cache/ sidecars. Writing those sidecars blocked the event loop in large chunks: building the module metrics and plugin summaries ran in one synchronous pass, and each sidecar was serialized with a single JSON.stringify (41 MB of module metrics on a large project). When DevTools runs inside the Vite dev server, this shares the process with Vite, so page loads and HMR stalled too.

This PR keeps the cache output identical and splits that work so the server stays responsive.

Changes

• Build the cache data in slices. One manager.snapshot() is shared across the sidecars (it was taken three to four times), and module metrics compaction and plugin summaries are built entry by entry, yielding about every 8 ms.
• Accumulate plugin summaries while parsing. RolldownEventsManager keeps plugin_build_summaries in step with plugin_build_metrics, so writing the plugin summary no longer rescans every call. Summaries are rebuilt on restore and cleared on dispose; metrics hydrated from the index fall back to computing from calls.
• Serialize sidecars in slices. stringifyInSlices produces the same output as JSON.stringify, but serializes large arrays item by item and yields in between.

Impact

Measured locally on a lobehub SPA build log (~29k modules, 6.9M events, 4 GB), cold open with no cache, 5 runs each:

	Before	After
Longest event-loop stall	182–675 ms (median 450 ms)	54–157 ms (median 90 ms)
Stalls over 100 ms	1–3	0–1
Cold open total	15.8–17.5 s	16.6–17.8 s (within noise)

The total cold-open time is unchanged; it's dominated by parsing the log.

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commit: 7c6cd8b

[Copilot]

🟡 Changes recommended

Active issues cause excessive small filesystem writes that slow cold opens and incomplete metric summaries that omit hydrated counts and durations when logs grow.

2 open issues

• Seed summaries from hydrated metrics to preserve cached totals · New
• Coalesce serializer fragments to avoid excessive filesystem writes · New

What changed in this PR

This PR reduces event-loop stalls when opening uncached Rolldown sessions, helping keep the shared Vite dev-server process responsive during cache generation.

Changes:

• Reuses one manager snapshot and builds cache summaries in slices.
• Accumulates plugin summaries during parsing and rebuilds them on restore.
• Adds sliced JSON serialization and tests for output equivalence and summary consistency.

File	Description
packages/​rolldown/​src/​node/​rolldown/​log-cache.ts	Shares snapshots and slices cache preparation and serialization.
packages/​rolldown/​src/​node/​rolldown/​events-manager.ts	Tracks incremental plugin summaries through parsing, restore, and disposal.
packages/​rolldown/​src/​node/​rolldown/​__tests__/​cold-open-cache.test.ts	Tests plugin summary consistency and serialized JSON equivalence.

_{Review effort: Balanced}