Allow % based configuration of max workers

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### Features
 
+- `[jest-config]` Allow % based configuration of `--max-workers` ([#7494](https://github.com/facebook/jest/pull/7494))
 - `[jest-cli]` [**BREAKING**] Only set error process error codes when they are non-zero ([#7363](https://github.com/facebook/jest/pull/7363))
 - `[jest-config]` [**BREAKING**] Deprecate `setupTestFrameworkScriptFile` in favor of new `setupFilesAfterEnv` ([#7119](https://github.com/facebook/jest/pull/7119))
 - `[jest-worker]` [**BREAKING**] Add functionality to call a `setup` method in the worker before the first call and a `teardown` method when ending the farm ([#7014](https://github.com/facebook/jest/pull/7014))

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 which could be unknown or changable 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);