feat: optimize getMaxWorkers using os.availableParallelism (#1378)

Summary:
Optimize  the maximum number of worker parallelism using os.availableParallelism

`os.cpus().length` returns the number of CPUs in the physical device, which is static. It should be replaced with the maximum number of computations the system can execute simultaneously. In Node.js, this is provided by `os.availableParallelism()`.

Currently, Metro requires Node.js version 18.18 or higher, which includes `os.availableParallelism`.

Changelog: [Internal]

Pull Request resolved: #1378

Test Plan: Green CI

Reviewed By: vzaidman

Differential Revision: D65277683

Pulled By: robhogan

fbshipit-source-id: 69c22a28626a89588082ca309dda6ebc9d08413a

docs/Configuration.md

@@ -143,7 +143,7 @@ If `true`, Metro will use a stable mapping from files to transformer workers, so
 
 Type: `number`
 
-The number of workers to use for parallel processing in Metro. Defaults to approximately half of the number of cores available on the machine, as reported by [`os.cpus()`](https://nodejs.org/api/os.html#oscpus).
+The number of workers to use for parallel processing in Metro. Defaults to approximately half of the number of cores available on the machine, as reported by [`os.availableParallelism()`](https://nodejs.org/api/os.html#osavailableparallelism).
 
 :::note
 1. Values exceeding the number of available cores have no effect.

packages/metro/src/lib/__tests__/getMaxWorkers-test.js

@@ -20,17 +20,17 @@ test('calculates the number of max workers', () => {
   /* $FlowFixMe(>=0.99.0 site=react_native_fb) This comment suppresses an error
    * found when Flow v0.99 was deployed. To see the error, delete this comment
    * and run Flow. */
-  os.cpus.mockReturnValue({length: 1});
+  os.availableParallelism.mockReturnValue(1);
   expect(getMaxWorkers()).toBe(1);
   /* $FlowFixMe(>=0.99.0 site=react_native_fb) This comment suppresses an error
    * found when Flow v0.99 was deployed. To see the error, delete this comment
    * and run Flow. */
-  os.cpus.mockReturnValue({length: 8});
+  os.availableParallelism.mockReturnValue(8);
   expect(getMaxWorkers()).toBe(6);
   /* $FlowFixMe(>=0.99.0 site=react_native_fb) This comment suppresses an error
    * found when Flow v0.99 was deployed. To see the error, delete this comment
    * and run Flow. */
-  os.cpus.mockReturnValue({length: 24});
+  os.availableParallelism.mockReturnValue(24);
   expect(getMaxWorkers()).toBe(14);
   expect(getMaxWorkers(5)).toBe(5);
 });

packages/metro/src/lib/getMaxWorkers.js

@@ -14,7 +14,8 @@
 const os = require('os');
 
 module.exports = (workers: ?number): number => {
-  const cores = os.cpus().length;
+  // $FlowFixMe[prop-missing] Missing Flow lib def for availableParallelism
+  const cores = os.availableParallelism();
   return typeof workers === 'number' && Number.isInteger(workers)
     ? Math.min(cores, workers > 0 ? workers : 1)
     : Math.max(1, Math.ceil(cores * (0.5 + 0.5 * Math.exp(-cores * 0.07)) - 1));