如今的服务器、PC，甚至是平板电脑和手机，几乎没有单核的CPU。

单线程的node.js代码意味着只能同时使用一个CPU核心，那么node.js如何发挥多核CPU的性能呢？

我想多进程至少是其中一个方案。

child_process是Node.js提供的子进程模块。

child_process模块提供了7个创建子进程的方法，四个异步方法和三个同步方法，它们分别是child_process.exec()、child_process.execFile()、 child_process.fork()、child_process.spawn()和child_process.execSync()、 child_process.execFileSync()、child_process.spawnSync()。

我们首先来看一下这7个方法的源码：

child_process.exec():

exports.exec = function(command /*, options, callback*/) {
  var opts = normalizeExecArgs.apply(null, arguments);
  return exports.execFile(opts.file,
                          opts.args,
                          opts.options,
                          opts.callback);

};

child_process.execFile():

exports.execFile = function(file /*, args, options, callback*/) {
  var args = [], callback;
  var options = {
    encoding: 'utf8',
    timeout: 0,
    maxBuffer: 200 * 1024,
    killSignal: 'SIGTERM',
    cwd: null,
    env: null
  };

  // Parse the optional positional parameters.
  var pos = 1;
  if (pos < arguments.length && Array.isArray(arguments[pos])) {
    args = arguments[pos++];
  } else if (pos < arguments.length && arguments[pos] == null) {
    pos++;
  }

  if (pos < arguments.length && typeof arguments[pos] === 'object') {
    options = util._extend(options, arguments[pos++]);
  } else if (pos < arguments.length && arguments[pos] == null) {
    pos++;
  }

  if (pos < arguments.length && typeof arguments[pos] === 'function') {
    callback = arguments[pos++];
  }

  if (pos === 1 && arguments.length > 1) {
    throw new TypeError('Incorrect value of args option');
  }

  var child = spawn(file, args, {
    cwd: options.cwd,
    env: options.env,
    gid: options.gid,
    uid: options.uid,
    windowsVerbatimArguments: !!options.windowsVerbatimArguments
  });

  var encoding;
  var _stdout;
  var _stderr;
  if (options.encoding !== 'buffer' && Buffer.isEncoding(options.encoding)) {
    encoding = options.encoding;
    _stdout = '';
    _stderr = '';
  } else {
    _stdout = [];
    _stderr = [];
    encoding = null;
  }
  var stdoutLen = 0;
  var stderrLen = 0;
  var killed = false;
  var exited = false;
  var timeoutId;

  var ex = null;

  function exithandler(code, signal) {
    if (exited) return;
    exited = true;

    if (timeoutId) {
      clearTimeout(timeoutId);
      timeoutId = null;
    }

    if (!callback) return;

    // merge chunks
    var stdout;
    var stderr;
    if (!encoding) {
      stdout = Buffer.concat(_stdout);
      stderr = Buffer.concat(_stderr);
    } else {
      stdout = _stdout;
      stderr = _stderr;
    }

    if (ex) {
      // Will be handled later
    } else if (code === 0 && signal === null) {
      callback(null, stdout, stderr);
      return;
    }

    var cmd = file;
    if (args.length !== 0)
      cmd += ' ' + args.join(' ');

    if (!ex) {
      ex = new Error('Command failed: ' + cmd + '\n' + stderr);
      ex.killed = child.killed || killed;
      ex.code = code < 0 ? uv.errname(code) : code;
      ex.signal = signal;
    }

    ex.cmd = cmd;
    callback(ex, stdout, stderr);
  }

  function errorhandler(e) {
    ex = e;

    if (child.stdout)
      child.stdout.destroy();

    if (child.stderr)
      child.stderr.destroy();

    exithandler();
  }

  function kill() {
    if (child.stdout)
      child.stdout.destroy();

    if (child.stderr)
      child.stderr.destroy();

    killed = true;
    try {
      child.kill(options.killSignal);
    } catch (e) {
      ex = e;
      exithandler();
    }
  }

  if (options.timeout > 0) {
    timeoutId = setTimeout(function() {
      kill();
      timeoutId = null;
    }, options.timeout);
  }

  if (child.stdout) {
    if (encoding)
      child.stdout.setEncoding(encoding);

    child.stdout.addListener('data', function(chunk) {
      stdoutLen += chunk.length;

      if (stdoutLen > options.maxBuffer) {
        ex = new Error('stdout maxBuffer exceeded');
        kill();
      } else {
        if (!encoding)
          _stdout.push(chunk);
        else
          _stdout += chunk;
      }
    });
  }

  if (child.stderr) {
    if (encoding)
      child.stderr.setEncoding(encoding);

    child.stderr.addListener('data', function(chunk) {
      stderrLen += chunk.length;

      if (stderrLen > options.maxBuffer) {
        ex = new Error('stderr maxBuffer exceeded');
        kill();
      } else {
        if (!encoding)
          _stderr.push(chunk);
        else
          _stderr += chunk;
      }
    });
  }

  child.addListener('close', exithandler);
  child.addListener('error', errorhandler);

  return child;
};

child_process.fork():

exports.fork = function(modulePath /*, args, options*/) {

  // Get options and args arguments.
  var options, args, execArgv;
  if (Array.isArray(arguments[1])) {
    args = arguments[1];
    options = util._extend({}, arguments[2]);
  } else if (arguments[1] && typeof arguments[1] !== 'object') {
    throw new TypeError('Incorrect value of args option');
  } else {
    args = [];
    options = util._extend({}, arguments[1]);
  }

  // Prepare arguments for fork:
  execArgv = options.execArgv || process.execArgv;

  if (execArgv === process.execArgv && process._eval != null) {
    const index = execArgv.lastIndexOf(process._eval);
    if (index > 0) {
      // Remove the -e switch to avoid fork bombing ourselves.
      execArgv = execArgv.slice();
      execArgv.splice(index - 1, 2);
    }
  }

  args = execArgv.concat([modulePath], args);

  // Leave stdin open for the IPC channel. stdout and stderr should be the
  // same as the parent's if silent isn't set.
  options.stdio = options.silent ? ['pipe', 'pipe', 'pipe', 'ipc'] :
      [0, 1, 2, 'ipc'];

  options.execPath = options.execPath || process.execPath;
  
  return spawn(options.execPath, args, options);

};