Nodejs - 每个函数的 Promise.all 性能与标准 async/await 相比

Question

我仍在尝试理解 Promise.all，因此编写了一个快速简单的脚本来查看 Promise.all 与使用 while 循环等待每个函数相比的性能差异。

我假设 Promise.all 会更快，因为它并行运行函数，但我的脚本似乎告诉我不然。

这是否太简单了，我错过了什么吗？

const p1 = async () => {
  let x = 0
  
  while(x < 100000000) {
    x++
  }
  return x
}

const p2 = async () => {
  let y = 100000000
  
  while(y > 0) {
    y--
  }
  return y
}

const p3 = async () => {
  let z = 0
  
  while(z < 200000000) {
    z = z + 2
  }
  return z
}

const run1 = async () => {
   console.time("Parallel")
   
   const [res1, res2, res3] = await Promise.allSettled([p1(), p2(), p3()])
   console.log(res1.value, res2.value, res3.value)
   
   console.timeEnd("Parallel")
}

const run2 = async () => {
   console.time("Sync")
   
   const res1 = await p1()
   const res2 = await p2()
   const res3 = await p3()
   console.log(res1, res2, res3)
   
   console.timeEnd("Sync")
}

run1()
// run2()

结果

Answer 1

不要忘记 js 是单线程的，因此主线程上的计算是以牺牲其他函数为代价的，例如 p1 和 p2 “并行”运行，但在同一个事件循环上。

const expensiveComputation = async (nb) => {
  let x = 0
  while(x < nb*100*1000*1000) {x++}
  return x
}

const expensiveComputationsParallel = async () => {
   console.time("Expensive Parallel")
   const [res1, res2, res3] = await Promise.all([
       expensiveComputation(2), 
       expensiveComputation(3), 
       expensiveComputation(4)
   ])
   console.timeEnd("Expensive Parallel")
}

const expensiveComputationsSynchronous = async () => {
   console.time("Expensive Synchronous")
   const res1 = await expensiveComputation(2)
   const res2 = await expensiveComputation(3)
   const res3 = await expensiveComputation(4)
   console.timeEnd("Expensive Synchronous")
}

const lightComputation = async (timeMs) => {
  return new Promise(resolve => {
    timeout = setTimeout(() =>  {
      resolve();
    }, timeMs);
  });
}

const lightComputationsParallel = async () => {
   console.time("Light Parallel")
   const [res1, res2, res3] = await Promise.all([
      lightComputation(100), 
      lightComputation(200),
      lightComputation(150)
   ])
   console.timeEnd("Light Parallel")
}

const lightComputationsSynchronous= async () => {
   console.time("Light Synchronous")
   const res1 = await lightComputation(100)
   const res2 = await lightComputation(200)
   const res3 = await lightComputation(150)
   console.timeEnd("Light Synchronous")
}

(async()=>{
  await expensiveComputationsParallel()
  await expensiveComputationsSynchronous()
  await lightComputationsParallel()
  await lightComputationsSynchronous()
})()

正如您在第二个示例中看到的，对于非计算密集型任务 Promise.all 需要的时间与其最长的 Promise 一样长，并且链接的时间与所有 Promise 的总和一样长。