Use named functions to avoid closures!

  for (var i = 0; i < 3; ++i) {
    namedFunction(i);
  }

  function namedFunction(id) {
    fs.readdir('.', function () {
      var end = process.hrtime(start);
      console.log(util.format('readdir %d finished in %ds', id, end[0] + end[1] / 1e9));
    });
  };