Support percent values in maxProcesses

README.md

@@ -146,8 +146,9 @@ concurrently [options] <command ...>
 
 General
   -m, --max-processes          How many processes should run at once.
+                               Could be an exact number or a percent of CPUs available.
                                New processes only spawn after all restart tries
-                               of a process.                            [number]
+                               of a process.                            [string]
   -n, --names                  List of custom names to be used in prefix
                                template.
                                Example names: "main,browser,server"     [string]

bin/concurrently.ts

@@ -30,8 +30,9 @@ const args = yargs(argsBeforeSep)
             alias: 'm',
             describe:
                 'How many processes should run at once.\n' +
-                'New processes only spawn after all restart tries of a process.',
-            type: 'number',
+                'New processes only spawn after all restart tries of a process.\n' +
+                'Could be an exact number or a percent of CPUs available (for example "50%")',
+            type: 'string',
         },
         names: {
             alias: 'n',

src/concurrently.spec.ts

@@ -1,4 +1,5 @@
 import { createMockInstance } from 'jest-create-mock-instance';
+import os from 'os';
 import { Writable } from 'stream';
 
 import { ChildProcess, KillProcess, SpawnCommand } from './command';
@@ -16,6 +17,8 @@ const create = (commands: ConcurrentlyCommandInput[], options: Partial<Concurren
     concurrently(commands, Object.assign(options, { controllers, spawn, kill }));
 
 beforeEach(() => {
+    jest.resetAllMocks();
+
     processes = [];
     spawn = jest.fn(() => {
         const process = createFakeProcess(processes.length);
@@ -79,6 +82,35 @@ it('spawns commands up to configured limit at once', () => {
     expect(spawn).toHaveBeenCalledTimes(4);
 });
 
+it('spawns commands up to percent based limit at once', () => {
+    const cpusSpy = jest.spyOn(os, 'cpus');
+    cpusSpy.mockReturnValue(
+        new Array(4).fill({
+            model: 'Intel',
+            speed: 0,
+            times: { user: 0, nice: 0, sys: 0, idle: 0, irq: 0 },
+        })
+    );
+
+    create(['foo', 'bar', 'baz', 'qux'], { maxProcesses: '50%' });
+    expect(spawn).toHaveBeenCalledTimes(2);
+    expect(spawn).toHaveBeenCalledWith('foo', expect.objectContaining({}));
+    expect(spawn).toHaveBeenCalledWith('bar', expect.objectContaining({}));
+
+    // Test out of order completion picking up new processes in-order
+    processes[1].emit('close', 1, null);
+    expect(spawn).toHaveBeenCalledTimes(3);
+    expect(spawn).toHaveBeenCalledWith('baz', expect.objectContaining({}));
+
+    processes[0].emit('close', null, 'SIGINT');
+    expect(spawn).toHaveBeenCalledTimes(4);
+    expect(spawn).toHaveBeenCalledWith('qux', expect.objectContaining({}));
+
+    // Shouldn't attempt to spawn anything else.
+    processes[2].emit('close', 1, null);
+    expect(spawn).toHaveBeenCalledTimes(4);
+});
+
 it('runs controllers with the commands', () => {
     create(['echo', '"echo wrapped"']);
 

src/concurrently.ts

@@ -1,5 +1,6 @@
 import assert from 'assert';
 import _ from 'lodash';
+import { cpus } from 'os';
 import spawn from 'spawn-command';
 import { Writable } from 'stream';
 import treeKill from 'tree-kill';
@@ -67,11 +68,12 @@ export type ConcurrentlyOptions = {
 
     /**
      * Maximum number of commands to run at once.
+     * Could be an exact number or a percent of CPUs available.
      *
      * If undefined, then all processes will start in parallel.
      * Setting this value to 1 will achieve sequential running.
      */
-    maxProcesses?: number;
+    maxProcesses?: number | string;
 
     /**
      * Whether commands should be spawned in raw mode.
@@ -187,7 +189,12 @@ export function concurrently(
     }
 
     const commandsLeft = commands.slice();
-    const maxProcesses = Math.max(1, Number(options.maxProcesses) || commandsLeft.length);
+    const maxProcesses = Math.max(
+        1,
+        (typeof options.maxProcesses === 'string' && options.maxProcesses.endsWith('%')
+            ? Math.round(cpus().length * percentToNumber(options.maxProcesses))
+            : Number(options.maxProcesses)) || commandsLeft.length
+    );
     for (let i = 0; i < maxProcesses; i++) {
         maybeRunMore(commandsLeft);
     }
@@ -245,3 +252,7 @@ function maybeRunMore(commandsLeft: Command[]) {
         maybeRunMore(commandsLeft);
     });
 }
+
+function percentToNumber(percent: string): number {
+    return Number(percent.replace('%', '')) / 100;
+}