fix: use os.availableParallelism() for parallelism when it is available (#623)

Author: alexander-akait

README.md

@@ -167,7 +167,7 @@ type parallel = boolean | number;
 Default: `true`
 
 Use multi-process parallel running to improve the build speed.
-Default number of concurrent runs: `os.cpus().length - 1`.
+Default number of concurrent runs: `os.cpus().length - 1` or `os.availableParallelism() - 1` (if this function is supported).
 
 > **Note**
 >

src/index.js

@@ -320,11 +320,14 @@ class TerserPlugin {
   static getAvailableNumberOfCores(parallel) {
     // In some cases cpus() returns undefined
     // https://github.com/nodejs/node/issues/19022
-    const cpus = os.cpus() || { length: 1 };
+    const cpus =
+      typeof os.availableParallelism === "function"
+        ? { length: os.availableParallelism() }
+        : os.cpus() || { length: 1 };
 
-    return parallel === true
+    return parallel === true || typeof parallel === "undefined"
       ? cpus.length - 1
-      : Math.min(Number(parallel) || 0, cpus.length - 1);
+      : Math.min(parallel || 0, cpus.length - 1);
   }
 
   /**

test/__snapshots__/parallel-option.test.js.snap

@@ -90,6 +90,19 @@ exports[`parallel option should match snapshot for the "true" value: errors 1`]
 
 exports[`parallel option should match snapshot for the "true" value: warnings 1`] = `Array []`;
 
+exports[`parallel option should match snapshot for the "undefined" value: assets 1`] = `
+Object {
+  "four.js": "(()=>{var r={921:r=>{r.exports=function(){console.log(7)}}},o={};(function t(e){var n=o[e];if(void 0!==n)return n.exports;var s=o[e]={exports:{}};return r[e](s,s.exports,t),s.exports})(921)})();",
+  "one.js": "(()=>{var r={921:r=>{r.exports=function(){console.log(7)}}},o={};(function t(e){var n=o[e];if(void 0!==n)return n.exports;var s=o[e]={exports:{}};return r[e](s,s.exports,t),s.exports})(921)})();",
+  "three.js": "(()=>{var r={921:r=>{r.exports=function(){console.log(7)}}},o={};(function t(e){var n=o[e];if(void 0!==n)return n.exports;var s=o[e]={exports:{}};return r[e](s,s.exports,t),s.exports})(921)})();",
+  "two.js": "(()=>{var r={921:r=>{r.exports=function(){console.log(7)}}},o={};(function t(e){var n=o[e];if(void 0!==n)return n.exports;var s=o[e]={exports:{}};return r[e](s,s.exports,t),s.exports})(921)})();",
+}
+`;
+
+exports[`parallel option should match snapshot for the "undefined" value: errors 1`] = `Array []`;
+
+exports[`parallel option should match snapshot for the "undefined" value: warnings 1`] = `Array []`;
+
 exports[`parallel option should match snapshot when a value is not specify: assets 1`] = `
 Object {
   "four.js": "(()=>{var r={921:r=>{r.exports=function(){console.log(7)}}},o={};(function t(e){var n=o[e];if(void 0!==n)return n.exports;var s=o[e]={exports:{}};return r[e](s,s.exports,t),s.exports})(921)})();",

test/parallel-option.test.js

@@ -15,8 +15,11 @@ import {
 
 jest.mock("os", () => {
   const actualOs = jest.requireActual("os");
+  const isAvailableParallelism =
+    typeof actualOs.availableParallelism !== "undefined";
 
   const mocked = {
+    availableParallelism: isAvailableParallelism ? jest.fn(() => 4) : undefined,
     cpus: jest.fn(() => {
       return { length: 4 };
     }),
@@ -53,6 +56,14 @@ jest.mock("jest-worker", () => {
 
 const workerPath = require.resolve("../src/minify");
 
+const getParallelism = () => {
+  if (typeof os.availableParallelism !== "undefined") {
+    return os.availableParallelism();
+  }
+
+  return os.cpus().length;
+};
+
 describe("parallel option", () => {
   let compiler;
 
@@ -77,7 +88,7 @@ describe("parallel option", () => {
     expect(Worker).toHaveBeenCalledTimes(1);
     expect(Worker).toHaveBeenLastCalledWith(workerPath, {
       enableWorkerThreads: ENABLE_WORKER_THREADS,
-      numWorkers: os.cpus().length - 1,
+      numWorkers: getParallelism() - 1,
     });
     expect(workerTransform).toHaveBeenCalledTimes(
       Object.keys(stats.compilation.assets).length
@@ -109,7 +120,27 @@ describe("parallel option", () => {
     expect(Worker).toHaveBeenCalledTimes(1);
     expect(Worker).toHaveBeenLastCalledWith(workerPath, {
       enableWorkerThreads: ENABLE_WORKER_THREADS,
-      numWorkers: Math.min(4, os.cpus().length - 1),
+      numWorkers: getParallelism() - 1,
+    });
+    expect(workerTransform).toHaveBeenCalledTimes(
+      Object.keys(stats.compilation.assets).length
+    );
+    expect(workerEnd).toHaveBeenCalledTimes(1);
+
+    expect(readsAssets(compiler, stats)).toMatchSnapshot("assets");
+    expect(getErrors(stats)).toMatchSnapshot("errors");
+    expect(getWarnings(stats)).toMatchSnapshot("warnings");
+  });
+
+  it('should match snapshot for the "undefined" value', async () => {
+    new TerserPlugin({ parallel: undefined }).apply(compiler);
+
+    const stats = await compile(compiler);
+
+    expect(Worker).toHaveBeenCalledTimes(1);
+    expect(Worker).toHaveBeenLastCalledWith(workerPath, {
+      enableWorkerThreads: ENABLE_WORKER_THREADS,
+      numWorkers: getParallelism() - 1,
     });
     expect(workerTransform).toHaveBeenCalledTimes(
       Object.keys(stats.compilation.assets).length