Max workers

CHANGELOG.md

@@ -39,6 +39,7 @@
 
 - `[jest-runtime]` Fix module registry memory leak ([#8282](https://github.com/facebook/jest/pull/8282))
 - `[jest-resolve]` optimize resolve module path ([#8388](https://github.com/facebook/jest/pull/8388))
+- `[jest-config]` Adjust `--maxWorkers` behavior to use the lesser of provided number and number of cores. ([#8410](https://github.com/facebook/jest/pull/8410))
 
 ## 24.7.1
 

packages/jest-config/src/__tests__/getMaxWorkers.test.ts

@@ -25,9 +25,9 @@ describe('getMaxWorkers', () => {
     expect(getMaxWorkers({})).toBe(1);
   });
 
-  it('Returns the `maxWorkers` when specified', () => {
-    const argv = {maxWorkers: 8};
-    expect(getMaxWorkers(argv)).toBe(8);
+  it('Returns the lesser of `maxWorkers` and (the number of cpus - 1)', () => {
+    expect(getMaxWorkers({maxWorkers: 2})).toBe(2);
+    expect(getMaxWorkers({maxWorkers: 50})).toBe(3);
   });
 
   it('Returns based on the number of cpus', () => {

packages/jest-config/src/getMaxWorkers.ts

@@ -11,6 +11,7 @@ import {Config} from '@jest/types';
 export default function getMaxWorkers(
   argv: Partial<Pick<Config.Argv, 'maxWorkers' | 'runInBand' | 'watch'>>,
 ): number {
+  const cpus = os.cpus() ? os.cpus().length : 1;
   if (argv.runInBand) {
     return 1;
   } else if (argv.maxWorkers) {
@@ -24,15 +25,13 @@ export default function getMaxWorkers(
       parsed > 0 &&
       parsed <= 100
     ) {
-      const cpus = os.cpus().length;
       const workers = Math.floor((parsed / 100) * cpus);
       return workers >= 1 ? workers : 1;
     }
 
-    return parsed > 0 ? parsed : 1;
+    return Math.max(Math.min(cpus - 1, parsed), 1);
   } else {
     // In watch mode, Jest should be unobtrusive and not use all available CPUs.
-    const cpus = os.cpus() ? os.cpus().length : 1;
     return Math.max(argv.watch ? Math.floor(cpus / 2) : cpus - 1, 1);
   }
 }

packages/jest-transform/src/ScriptTransformer.ts

@@ -573,15 +573,15 @@ const calcIgnorePatternRegExp = (config: Config.ProjectConfig) => {
     !config.transformIgnorePatterns ||
     config.transformIgnorePatterns.length === 0
   ) {
-    return;
+    return undefined;
   }
 
   return new RegExp(config.transformIgnorePatterns.join('|'));
 };
 
 const calcTransformRegExp = (config: Config.ProjectConfig) => {
   if (!config.transform.length) {
-    return;
+    return undefined;
   }
 
   const transformRegexp: Array<[RegExp, string]> = [];