feat(loader-runner): Allow limiting worker pool size for parallel loaders

[Pablinho]

Summary

This is related to feat: support experimental loader parallelism ( #9807 ). Currently worker threads are spawned on demand and, by default, use all available CPU threads (effectively cpus() - 1, fallback to 1)

Problem

• For very large projects that build multiple targets (for example, browser and node in dev mode), enabling parallel loaders (for project still using babel-loader for example) can cause OOMs or build failures with little diagnostic output. I’ve seen high CPU usage just before the OOM. The root cause appears to be the total number of worker threads tinypool is using.

Solution

• Tools like thread-loader already let you control worker count via a worker option: the number of spawned workers, defaults to (number of cpus - 1) or fallback to 1 when require('os').cpus() is undefined.
• Similarly, the terser-webpack-plugin provides a numberic parallel option in order to enable the multi-process parallel and lets you control how many concurrent workers are used.

As recomended I extended the parallel setting to accept booleann | { maxWorkers?: number }.

• If parallel is an object, user can set maxWorkers. Then, we use that number as the requested thread count for Tinypool (applied to minThreads/maxThreads). Depends on the user to use a valid integer. It should be an integer > 0 (and ideally not larger than your CPU count).
• If parallel: true defaults to (number of cpus - 1) or fallback to 1 (as usual)

This lets users tune Tinypool concurrency per-loader, helping avoid OOMs on constrained machines or very large builds. Any feedback, concerns, or alternative approaches welcome — I’m happy to adjust the implementation. Thanks in advance for the review!

Related links

• https://github.com/webpack/thread-loader/blob/v4.0.4/src/workerPools.js#L19
• https://github.com/webpack/terser-webpack-plugin/blob/v5.3.14/src/index.js#L131
• https://vitest.dev/guide/migration.html#pool-rework

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[codspeed-hq]

CodSpeed Performance Report

Merging #12277 will not alter performance

_{Comparing Pablinho:make-tinypool-number-threads-configurable (e209af4) with main (e2b3889)}

Summary

✅ 17 untouched

[chenjiahan]

LGTM, thank you!