feat: add --max-workers option & limit Rspack workers by default (#1257)

Author: jbroma

.changeset/heavy-toes-lose.md

@@ -0,0 +1,5 @@
+---
+"@callstack/repack": minor
+---
+
+Add `--max-workers` option to start & bundle commands. By default the number of workers is derived from available cores. This option is only effective for Rspack configurations

packages/repack/src/commands/common/getMaxWorkers.ts

@@ -0,0 +1,16 @@
+import os from 'node:os';
+
+export function getMaxWorkers(): number {
+  const cores = os.availableParallelism();
+
+  const decayCoefficient = 0.07;
+  const decay = Math.exp(-cores * decayCoefficient);
+
+  // Scale transitions from 1.0 (low cores) toward 0.5 (many cores)
+  const scale = 0.5 + 0.5 * decay;
+
+  // Adjust down slightly to leave headroom for main thread and I/O
+  const adjusted = cores * scale - 1;
+
+  return Math.max(1, Math.ceil(adjusted));
+}

packages/repack/src/commands/common/index.ts

@@ -1,4 +1,5 @@
 export * from './getDevMiddleware.js';
+export * from './getMaxWorkers.js';
 export * from './getMimeType.js';
 export * from './parseUrl.js';
 export * from './resetPersistentCache.js';

packages/repack/src/commands/common/setupEnvironment.ts

@@ -1,6 +1,8 @@
 import {
   ASSETS_DEST_ENV_KEY,
   BUNDLE_FILENAME_ENV_KEY,
+  RSPACK_RAYON_THREADS_ENV_KEY,
+  RSPACK_TOKIO_THREADS_ENV_KEY,
   SOURCEMAP_FILENAME_ENV_KEY,
   VERBOSE_ENV_KEY,
 } from '../../env.js';
@@ -24,3 +26,8 @@ export function setupEnvironment(args: EnvironmentArgs): void {
   setEnvVar(SOURCEMAP_FILENAME_ENV_KEY, args.sourcemapOutput);
   setEnvVar(ASSETS_DEST_ENV_KEY, args.assetsDest);
 }
+
+export function setupRspackEnvironment(maxWorkers: string): void {
+  setEnvVar(RSPACK_TOKIO_THREADS_ENV_KEY, maxWorkers);
+  setEnvVar(RSPACK_RAYON_THREADS_ENV_KEY, maxWorkers);
+}

packages/repack/src/commands/options.ts

@@ -59,6 +59,12 @@ export const startCommandOptions = [
     name: '--verbose',
     description: 'Enables verbose logging',
   },
+  {
+    name: '--max-workers <number>',
+    description:
+      '(Rspack only) The maximum number of workers to use for transformation parallelization ',
+    parse: (val: string) => Number(val),
+  },
   {
     name: '--config <path>',
     description: 'Path to a bundler config file, e.g webpack.config.js',
@@ -149,6 +155,12 @@ export const bundleCommandOptions = [
     name: '--watch',
     description: 'Watch for file changes',
   },
+  {
+    name: '--max-workers <number>',
+    description:
+      '(Rspack only) The maximum number of workers to use for transformation parallelization ',
+    parse: (val: string) => Number(val),
+  },
   {
     name: '--config <path>',
     description: 'Path to a bundler config file, e.g webpack.config.js',

packages/repack/src/commands/rspack/bundle.ts

@@ -3,11 +3,13 @@ import type { Stats } from '@rspack/core';
 import { CLIError } from '../../helpers/index.js';
 import { makeCompilerConfig } from '../common/config/makeCompilerConfig.js';
 import {
+  getMaxWorkers,
   normalizeStatsOptions,
   resetPersistentCache,
+  setupEnvironment,
+  setupRspackEnvironment,
   writeStats,
 } from '../common/index.js';
-import { setupEnvironment } from '../common/setupEnvironment.js';
 import type { BundleArguments, CliConfig } from '../types.js';
 
 /**
@@ -35,6 +37,9 @@ export async function bundle(
   // expose selected args as environment variables
   setupEnvironment(args);
 
+  const maxWorkers = args.maxWorkers ?? getMaxWorkers();
+  setupRspackEnvironment(maxWorkers.toString());
+
   if (!args.entryFile && !config.entry) {
     throw new CLIError("Option '--entry-file <path>' argument is missing");
   }

packages/repack/src/commands/rspack/start.ts

@@ -12,15 +12,17 @@ import {
 import { makeCompilerConfig } from '../common/config/makeCompilerConfig.js';
 import {
   getDevMiddleware,
+  getMaxWorkers,
   getMimeType,
   parseUrl,
   resetPersistentCache,
   resolveProjectPath,
+  runAdbReverse,
+  setupEnvironment,
   setupInteractions,
+  setupRspackEnvironment,
 } from '../common/index.js';
-import { runAdbReverse } from '../common/index.js';
 import logo from '../common/logo.js';
-import { setupEnvironment } from '../common/setupEnvironment.js';
 import type { CliConfig, StartArguments } from '../types.js';
 import { Compiler } from './Compiler.js';
 
@@ -58,6 +60,9 @@ export async function start(
   // expose selected args as environment variables
   setupEnvironment(args);
 
+  const maxWorkers = args.maxWorkers ?? getMaxWorkers();
+  setupRspackEnvironment(maxWorkers.toString());
+
   const isVerbose = isTruthyEnv(process.env[VERBOSE_ENV_KEY]);
   const devServerOptions = configs[0].devServer ?? {};
   const showHttpRequests = isVerbose || args.logRequests;

packages/repack/src/commands/types.ts

@@ -13,6 +13,7 @@ export interface BundleArguments {
   resetCache?: boolean;
   verbose?: boolean;
   watch?: boolean;
+  maxWorkers?: number;
   config?: string;
   webpackConfig?: string;
 }
@@ -31,6 +32,7 @@ export interface StartArguments {
   resetCache?: boolean;
   reversePort?: boolean;
   verbose?: boolean;
+  maxWorkers?: number;
   config?: string;
   webpackConfig?: string;
 }

packages/repack/src/env.ts

@@ -5,4 +5,7 @@ export const BUNDLE_FILENAME_ENV_KEY = 'REPACK_BUNDLE_FILENAME';
 export const SOURCEMAP_FILENAME_ENV_KEY = 'REPACK_SOURCEMAP_FILENAME';
 export const ASSETS_DEST_ENV_KEY = 'REPACK_ASSETS_DEST';
 
+export const RSPACK_TOKIO_THREADS_ENV_KEY = 'TOKIO_WORKER_THREADS';
+export const RSPACK_RAYON_THREADS_ENV_KEY = 'RAYON_NUM_THREADS';
+
 export const EXPERIMENTAL_CACHE_ENV_KEY = 'REPACK_EXPERIMENTAL_CACHE';

website/src/latest/api/cli/bundle.mdx

@@ -120,6 +120,12 @@ Enables verbose logging.
 
 Watch for file changes.
 
+### `--max-workers`
+
+- Type: `number`
+
+(Rspack only) The maximum number of workers to use for transformation parallelization. By default, the number of workers is derived from available CPU cores.
+
 ### `--config`
 
 - Type: `string`