javascript - 数组(大小)。填充(1): 500x more time and 70x more memory with 10x more data

Question

我正在对一些简单的 javascript 代码进行基准测试，并且在以下示例中看到了一些意外的内存使用情况:

function startTimer() {
  const time = process.hrtime();
  return time;
}

function endTimer(time) {
  function roundTo(decimalPlaces, numberToRound) {
    return +(Math.round(numberToRound + `e+${decimalPlaces}`)  + `e-${decimalPlaces}`);
  }
  const diff = process.hrtime(time);
  const NS_PER_SEC = 1e9;
  const result = (diff[0] * NS_PER_SEC + diff[1]); // Result in Nanoseconds
  const elapsed = result * 0.0000010;
  return roundTo(6, elapsed); // Result in milliseconds
}

SIZE = 10000000

start = startTimer()
let a = Array(SIZE).fill(1)
elapsed = endTimer(start)

const proc = process.memoryUsage()
console.log("rss (MB): ", proc.rss/1000/1000)
console.log("heapTotal (MB): ", proc.heapTotal/1000/1000)
console.log("heapUsed (MB): ", proc.heapUsed/1000/1000)
console.log("elapsed (ms): ", elapsed)

当 SIZE = 10MB 时，我得到:

[ThinkPad-E14-Gen-2:/home/jj/Documents/repos/tests_javascript]% node --max-old-space-size=4192  example.js
SIZE (MB):  10
rss (MB):  113.87
heapTotal (MB):  84.19
heapUsed (MB):  83.29
elapsed (ms):  47.44

当 SIZE = 100MB 时，我的 RSS 内存增加了 66 倍，执行时间增加了 500 倍:

[ThinkPad-E14-Gen-2:/home/jj/Documents/repos/tests_javascript]% node --max-old-space-size=4192  example.js
SIZE (MB):  100
rss (MB):  7529.71
heapTotal (MB):  4083.26
heapUsed (MB):  4008.59
elapsed (ms):  23135.14

为什么会这样？我如何进行时间/内存分配的基准测试？

Answer 1

您可能会发现这很有用:

https://v8.dev/blog/elements-kinds

https://bugs.chromium.org/p/v8/issues/detail?id=6892

https://2ality.com/2018/12/creating-arrays.html

性能损失可能是由于 Array(SIZE) 产生的多孔阵列类型所致。构造函数。
看完上面的文章，你可能会发现比较填充 10_000_00在内存和时间上的差异很有趣。和(说) 50_000_000用这种方式数组:

let a = Array(SIZE).fill(1)

let a = Array.from({ length: SIZE }).fill(1)

let a = [1]; for (let i = 0; i < SIZE; i++) a.push(1);

let a = [1]; for (let i = 0; i < SIZE; i++) a[i] = 1;

let a = new Uint8Array(SIZE).fill(1) // Wow!