lib/bench.js
function runAsync () {
    if (runIndex++ < sampleSize) {
      fn.call(child, function () {
        setTimeout(runAsync, 0)
      })
    } else {
      setTimeout(finishChild, 0)
    }
  }
function runAsync () {
    if (runIndex++ < sampleSize) {
      fn.call(child, function () {
        setTimeout(runAsync, 0)
      })
    } else {
      setTimeout(finishChild, 0)
    }
  }
Run the current child's test function asynchronously.