Allow % based configuration of max workers

CHANGELOG.md

@@ -35,6 +35,7 @@
 - `[expect]` `expect(Infinity).toBeCloseTo(Infinity)` Treats `Infinity` as equal in toBeCloseTo matcher ([#7405](https://github.com/facebook/jest/pull/7405))
 - `[jest-worker]` Add node worker-thread support to jest-worker ([#7408](https://github.com/facebook/jest/pull/7408))
 - `[jest-config]` Allow `bail` setting to be configured with a number allowing tests to abort after `n` of failures ([#7335](https://github.com/facebook/jest/pull/7335))
+- `[jest-config]` Allow % based configuration of `--max-workers` ([#7494](https://github.com/facebook/jest/pull/7494))
 
 ### Fixes
 

docs/CLI.md

@@ -193,10 +193,12 @@ Lists all tests as JSON that Jest will run given the arguments, and exits. This
 
 Logs the heap usage after every test. Useful to debug memory leaks. Use together with `--runInBand` and `--expose-gc` in node.
 
-### `--maxWorkers=<num>`
+### `--maxWorkers=<num>|<string>`
 
 Alias: `-w`. Specifies the maximum number of workers the worker-pool will spawn for running tests. This defaults to the number of the cores available on your machine. It may be useful to adjust this in resource limited environments like CIs but the default should be adequate for most use-cases.
 
+For environments with variable CPUs available, you can use percentage based configuration: `--maxWorkers=50%`
+
 ### `--noStackTrace`
 
 Disables stack trace in test results output.

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

@@ -34,4 +34,21 @@ describe('getMaxWorkers', () => {
     expect(getMaxWorkers({})).toBe(3);
     expect(getMaxWorkers({watch: true})).toBe(2);
   });
+
+  describe('% based', () => {
+    it('50% = 2 workers', () => {
+      const argv = {maxWorkers: '50%'};
+      expect(getMaxWorkers(argv)).toBe(2);
+    });
+
+    it('< 0 workers should become 1', () => {
+      const argv = {maxWorkers: '1%'};
+      expect(getMaxWorkers(argv)).toBe(1);
+    });
+
+    it("0% shouldn't break", () => {
+      const argv = {maxWorkers: '0%'};
+      expect(getMaxWorkers(argv)).toBe(1);
+    });
+  });
 });

packages/jest-config/src/getMaxWorkers.js

@@ -15,7 +15,20 @@ export default function getMaxWorkers(argv: Argv): number {
   if (argv.runInBand) {
     return 1;
   } else if (argv.maxWorkers) {
-    return parseInt(argv.maxWorkers, 10);
+    const parsed = parseInt(argv.maxWorkers, 10);
+
+    if (
+      typeof argv.maxWorkers === 'string' &&
+      argv.maxWorkers.trim().endsWith('%') &&
+      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;
   } else {
     const cpus = os.cpus() ? os.cpus().length : 1;
     return Math.max(argv.watch ? Math.floor(cpus / 2) : cpus - 1, 1);