feat: add `maxParallel` option to limit workspace build concurrency by jpzwarte · Pull Request #940 · rolldown/tsdown

jpzwarte · 2026-05-09T10:01:02Z

Fixes #929.

Problem

When using workspace mode with dts.tsgo: true, buildWithConfigs starts all package builds concurrently via Promise.all(). Each package spawns a tsgo subprocess, so with 64 workspace packages you can end up with 64 concurrent native processes — exhausting CPU/memory and crashing macOS.

Solution

Add a maxParallel option that limits how many workspace builds run at the same time. When set, a simple worker-pool (mapWithConcurrency) is used instead of Promise.all(). When omitted, existing unbounded concurrent behavior is preserved.

Changes

src/config/types.ts: Add maxParallel?: number to UserConfig; add it to the MarkPartial keys so it stays optional in ResolvedConfig
src/cli.ts: Add --max-parallel <number> CLI flag with numeric coercion
src/build.ts: Add normalizeMaxParallel, resolveMaxParallel, and mapWithConcurrency helpers; update buildWithConfigs to use the pool when maxParallel is set

Usage

// tsdown.config.ts
export default {
  workspace: 'packages/*',
  maxParallel: 4,  // at most 4 package builds run at the same time
  dts: { tsgo: true },
}

Or via CLI:

tsdown --workspace packages/* --max-parallel 4

Verification

pnpm typecheck ✅
pnpm test run ✅ (446 passed, 4 expected fail, 4 skipped)
pnpm lint ✅

Fixes rolldown#929 by introducing a `maxParallel` config option that limits how many workspace package builds can run concurrently. Without a limit, `buildWithConfigs` uses `Promise.all()` which starts all workspace builds simultaneously. When combined with `dts.tsgo: true`, each package spawns a `tsgo` subprocess, which can result in dozens of concurrent native processes and exhaust system resources. When `maxParallel` is set to a positive integer, at most that many builds run at the same time via a simple worker-pool implementation. When omitted, existing unbounded concurrent behavior is preserved. Changes: - Add `maxParallel?: number` to `UserConfig` and keep it optional in `ResolvedConfig` (added to `MarkPartial` keys) - Add `--max-parallel <number>` CLI flag; parse it as a number in the action handler so the string from argv is correctly coerced - Add `normalizeMaxParallel`, `resolveMaxParallel`, and `mapWithConcurrency` helpers in `src/build.ts` - Update `buildWithConfigs` to use the concurrency-limited pool when `maxParallel` is specified Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

The per-option validation (positive integer check) belongs in options.ts alongside all other single-config validation, not in build.ts. build.ts still owns the cross-config resolveMaxParallel and the mapWithConcurrency utility since those operate on the already-resolved config array. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

maxParallel is a root-level option; workspace merging ensures all resolved configs have the same value. Reading configs[0].maxParallel is sufficient and simpler. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

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autofix-ci · 2026-05-09T10:01:55Z

Hi! I'm autofix.ci, a bot that automatically fixes trivial issues such as code formatting in pull requests.

I would like to apply some automated changes to this pull request, but it looks like I don't have the necessary permissions to do so. To get this pull request into a mergeable state, please do one of the following two things:

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Pull request overview

Adds a maxParallel configuration/CLI option to cap concurrent builds when running in workspace mode, preventing excessive parallel tsgo subprocesses that can exhaust system resources (Fixes #929).

Changes:

Extend config schema with maxParallel?: number and keep it optional in ResolvedConfig.
Add --max-parallel <number> CLI flag.
Introduce a simple worker-pool (mapWithConcurrency) and use it in buildWithConfigs when maxParallel is set.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
src/config/types.ts	Adds `maxParallel` to `UserConfig` docs/types and keeps it optional in `ResolvedConfig`.
src/config/options.ts	Parses/validates `maxParallel` from user config during resolution.
src/cli.ts	Adds `--max-parallel` CLI option.
src/build.ts	Adds concurrency-limited mapping and uses it to limit config builds.

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+  const maxParallel =
+    maxParallelRaw == null ? undefined : Number(maxParallelRaw)
+


  globalLogger.info('Build start')
-  const bundles = await Promise.all(
-    configs.map((options) => {
-      const isDualFormat = options.pkg
-        ? configChunksByPkg[options.pkg.packageJsonPath].formats.size > 1
-        : true
-      return buildSingle(
-        options,
-        configDeps,
-        isDualFormat,
-        clean,
-        restart,
-        done,
-      )
-    }),
-  )
+  const { maxParallel } = configs[0] ?? {}
+  const buildConfig = (options: ResolvedConfig) => {
+    const isDualFormat = options.pkg
+      ? configChunksByPkg[options.pkg.packageJsonPath].formats.size > 1
+      : true
+    return buildSingle(options, configDeps, isDualFormat, clean, restart, done)
+  }
+  const bundles = maxParallel
+    ? await mapWithConcurrency(configs, maxParallel, buildConfig)


+  const bundles = maxParallel
+    ? await mapWithConcurrency(configs, maxParallel, buildConfig)
+    : await Promise.all(configs.map(buildConfig))


okikio · 2026-07-05T09:23:30Z

any updates on this...

jpzwarte · 2026-07-21T09:05:43Z

@sxzz I haven't done anything further on this PR because nobody responded on Discord. Now that i see that you pushed main to it, do you want to review it? Should i continue work on this?

sxzz · 2026-07-21T09:40:05Z

Thanks for following up. I haven’t reviewed this PR because it is not currently in a reviewable state:

The required CI checks are not green.
The PR description does not follow the repository’s PR template.
The commits are co-authored by Copilot, but the required AI-generated code disclosure and confirmation of line-by-line human review were omitted.
I’m not satisfied with the AI-generated implementation. It is unnecessarily complex, and the API does not appear to have been carefully designed. In particular, maxParallel should be scoped under the workspace option rather than introduced as a top-level option, and rename it to concurrency.

Please don’t continue polishing the current implementation as-is. If you would like to keep working on this, please start over with a simpler design and place maxParallel under the workspace option. If you continue using AI assistance, please use a more capable model and carefully review and understand every generated change yourself.

Please also restore and complete the PR template, rebase the branch, and get CI green before requesting another review. Once those are addressed, feel free to ping me again.

P.S. I’m not opposed to the use of AI. My point is that the PR author—not the AI—remains fully responsible for the contribution. This comment was also drafted with AI assistance, but I take full responsibility for it.

jpzwarte · 2026-07-23T15:37:13Z

Closing this PR in favor of #1030

jpzwarte and others added 4 commits May 9, 2026 11:45

refactor: move maxParallel number coercion from cli.ts to options.ts

a2f898a

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot AI review requested due to automatic review settings May 9, 2026 10:01

jpzwarte mentioned this pull request May 9, 2026

feat: add maxParallel option to limit workspace build concurrency jpzwarte/tsdown#1

Closed

Copilot started reviewing on behalf of jpzwarte May 9, 2026 10:01 View session

Copilot AI reviewed May 9, 2026

View reviewed changes

fix: address maxParallel PR review feedback

91e683a

sxzz force-pushed the main branch from 1320826 to 10e6fe1 Compare July 6, 2026 11:07

okikio mentioned this pull request Jul 6, 2026

Add support for JSR solidjs-community/solid-primitives#973

Merged

sxzz force-pushed the main branch 2 times, most recently from cccf4ff to e0266c1 Compare July 22, 2026 10:58

jpzwarte closed this Jul 23, 2026

jpzwarte deleted the fix/issue-929-max-parallel branch July 23, 2026 15:37

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feat: add `maxParallel` option to limit workspace build concurrency - #940

feat: add `maxParallel` option to limit workspace build concurrency#940
jpzwarte wants to merge 5 commits into
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		const maxParallel =
		maxParallelRaw == null ? undefined : Number(maxParallelRaw)

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jpzwarte commented May 9, 2026

Problem

Solution

Changes

Usage

Verification

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