NodeJSのWriteStreamで、細かいチャンク（というか空）を大量に書き込む前後で、rssとheapTotalの差が36MBから2.5GBに広がる話。

README.md

実行例

$ uname -a
Linux docker-desktop 5.15.133.1-microsoft-standard-WSL2 #1 SMP Thu Oct 5 21:02:42 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux
$ node --version
v20.14.0
$ node mem.js
before write: rss: 40.05MB, heapTotal: 4.20MB, heapUsed: 3.53MB, external: 1.33MB
writing 0: rss: 42.77MB, heapTotal: 4.45MB, heapUsed: 3.87MB, external: 1.45MB
writing 5000000: rss: 2249.27MB, heapTotal: 1316.65MB, heapUsed: 1268.95MB, external: 1.45MB
writing 10000000: rss: 3962.07MB, heapTotal: 2235.37MB, heapUsed: 2169.62MB, external: 1.45MB
writing 15000000: rss: 3963.01MB, heapTotal: 2210.23MB, heapUsed: 2147.29MB, external: 1.45MB
writing 20000000: rss: 4011.84MB, heapTotal: 2243.73MB, heapUsed: 2147.31MB, external: 1.45MB
writing 25000000: rss: 4774.08MB, heapTotal: 3005.14MB, heapUsed: 2113.35MB, external: 1.45MB
writing 30000000: rss: 4790.29MB, heapTotal: 3020.89MB, heapUsed: 2113.53MB, external: 1.45MB
writing 35000000: rss: 4925.86MB, heapTotal: 3155.89MB, heapUsed: 2113.31MB, external: 1.45MB
writing 40000000: rss: 6049.37MB, heapTotal: 3517.00MB, heapUsed: 3406.85MB, external: 1.45MB
writing 45000000: rss: 6030.36MB, heapTotal: 2752.64MB, heapUsed: 2113.42MB, external: 1.45MB
writing 50000000: rss: 6030.34MB, heapTotal: 2606.64MB, heapUsed: 2113.43MB, external: 1.45MB
writing 55000000: rss: 6032.05MB, heapTotal: 2925.14MB, heapUsed: 2113.35MB, external: 1.45MB
writing 60000000: rss: 6032.02MB, heapTotal: 3478.17MB, heapUsed: 3407.50MB, external: 1.45MB
writing 65000000: rss: 6032.27MB, heapTotal: 3477.92MB, heapUsed: 3407.79MB, external: 1.45MB
writing 70000000: rss: 6040.72MB, heapTotal: 3478.67MB, heapUsed: 3406.11MB, external: 1.45MB
writing 75000000: rss: 6067.33MB, heapTotal: 2930.64MB, heapUsed: 2113.26MB, external: 1.45MB
writing 80000000: rss: 6033.58MB, heapTotal: 2790.64MB, heapUsed: 2113.26MB, external: 1.45MB
writing 85000000: rss: 6034.83MB, heapTotal: 2941.64MB, heapUsed: 2113.26MB, external: 1.45MB
writing 90000000: rss: 6033.95MB, heapTotal: 2903.64MB, heapUsed: 2113.26MB, external: 1.45MB
writing 95000000: rss: 6033.57MB, heapTotal: 3002.64MB, heapUsed: 2113.45MB, external: 1.45MB
after write: rss: 5996.64MB, heapTotal: 3439.52MB, heapUsed: 3367.73MB, external: 1.45MB
onclose: rss: 6034.87MB, heapTotal: 2916.64MB, heapUsed: 2113.49MB, external: 1.45MB
onclose + 1s: rss: 5474.29MB, heapTotal: 2916.64MB, heapUsed: 2121.69MB, external: 1.45MB
onclose + 60s: rss: 2550.00MB, heapTotal: 4.45MB, heapUsed: 3.02MB, external: 1.45MB

test.js

const fs = require('fs')
const ws = fs.createWriteStream("/dev/null");

const fmt = n => (n / 1024 / 1024).toFixed(2) + "MB";
const wait = ms => new Promise(resolve => setTimeout(resolve, ms));
const log = msg => {
    globalThis.gc?.();
    const usage = process.memoryUsage();
    console.log(`${msg}: rss: ${fmt(usage.rss)}, heapTotal: ${fmt(usage.heapTotal)}, heapUsed: ${fmt(usage.heapUsed)}, external: ${fmt(usage.external)}`);
};

(async () => {
    log("before write");
    for (let i = 0; i < 100 * 1000 * 1000; i++) {
    ws.write("");
    if (i % (5 * 1000 * 1000) === 0) {
        await wait(0);
        log("writing " + i);
    }
    }
    ws.end();
    log("after write");
    ws.on("close", () => {
        log("onclose");
        setTimeout(() => log("onclose + 1s"), 1000);
        setTimeout(() => log("onclose + 60s"), 60000);
    });
})();