为什么在for循环中声明的变量的最后一次迭代没有得到垃圾回收?

我的问题是这是否是一个nodejs垃圾收集器错误?或者这是意料之中的事?

在Windows上运行node v14.15.0。

在为你的 this question 在涉及WeakRef对象时,我发现了一个关于垃圾收集的奇怪的东西,它似乎是一个可能的bug。分配给在一个变量中声明的变量的对象 for let 变量超出了 对于 循环。这里感兴趣的变量命名为 element 这是它的循环。只是循环最后一次迭代的对象没有得到GCed(即 要素 最后指向):

// fill all the arrays and the cache
// and put everything into the holding array too
for (let i = 0; i < numElements; i++) {
    let arr = new Array(lenArrays);
    arr.fill(i);
    let element = { id: i, data: arr };

    // temporarily hold onto each element by putting a
    // full reference (not a weakRef) into an array
    holding.push(element);

    // add a weakRef to the Map
    cache.set(i, new WeakRef(element));
}

holding 有了这个:

holding.length = 0;

所有的值 要素 从那个循环中应该可以得到GC。唯一提到他们的是via WeakRef 对象(不阻止GC)。

事实上,如果我让nodejs有一些空闲时间,那么除了由 对于 循环确实是GCed。但是,最后一个不是。如果我加上 element = null 到最后 对于 循环,最后一个就可以拿到GCed了。所以,nodejs没有清除 要素 要素

因此,您可以在这里看到整个代码(您可以将其放到一个文件中,自己在nodejs中运行):

'use strict';

// to make memory usage output easier to read
function addCommas(str) {
    var parts = (str + "").split("."),
        main = parts[0],
        len = main.length,
        output = "",
        i = len - 1;

    while (i >= 0) {
        output = main.charAt(i) + output;
        if ((len - i) % 3 === 0 && i > 0) {
            output = "," + output;
        }
        --i;
    }
    // put decimal part back
    if (parts.length > 1) {
        output += "." + parts[1];
    }
    return output;
}

function delay(t, v) {
    return new Promise(resolve => {
        setTimeout(resolve, t, v);
    });
}

function logUsage() {
    let usage = process.memoryUsage();
    console.log(`heapUsed: ${addCommas(usage.heapUsed)}`);
}

const numElements = 10000;
const lenArrays = 10000;

async function run() {

    const cache = new Map();
    const holding = [];

    function checkItem(n) {
        let item = cache.get(n).deref();
        console.log(item);
    }

    // fill all the arrays and the cache
    // and put everything into the holding array too
    for (let i = 0; i < numElements; i++) {
        let arr = new Array(lenArrays);
        arr.fill(i);
        let element = { id: i, data: arr };

        // temporarily hold onto each element by putting a
        // full reference (not a weakRef) into an array
        holding.push(element);

        // add a weakRef to the Map
        cache.set(i, new WeakRef(element));
    }

    // should have a big Map holding lots of data
    // all items should still be available
    checkItem(numElements - 1);
    logUsage();

    await delay(5000);
    logUsage();

    // make whole holding array contents eligible for GC
    holding.length = 0;

    // pause for GC, then see if items are available
    // and what memory usage is
    await delay(5000);
    checkItem(0);
    checkItem(1);
    checkItem(numElements - 1);

    // count how many items are still in the Map
    let cnt = 0;
    for (const [index, item] of cache) {
        if (item.deref()) {
            ++cnt;
            console.log(`Index item ${index} still in cache`);
        }
    }
    console.log(`There are ${cnt} items that haven't been GCed in the map`);
    logUsage();
}

run();

当我运行这个时,我得到以下输出:

{
  id: 9999,
  data: [
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    ... 9900 more items
  ]
}
heapUsed: 805,544,472
heapUsed: 805,582,072
undefined
undefined
{
  id: 9999,
  data: [
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999, 9999,
    ... 9900 more items
  ]
}
Index item 9999 still in cache
There are 1 items that haven't been GCed in the map
heapUsed: 3,490,168

两个 undefined 需要行。id:9999对象的第二个记录输出不应出现。也应该是 . 而且,不希望找到仍在缓存中的id:9999对象。它应该是合格的GC。

一种可能的理论是V8优化器正在 要素 对于 循环,以避免在循环中一次又一次地创建它,但在循环完成后又不能使它符合GC的条件—实际上是将它提升到更高的范围。

另一种理论是GC并不总是块作用域粒度。

有没有虫子?