leaky.mjs

const server = process.argv.at(-1);
const fmt = new Intl.NumberFormat();
let total = 0;
const batch = 50;
const delay = 0;
while (true) {
  const array = new Array(batch);
  for (let i = 0; i < batch; i++) {
    array[i] = fetch(server);
  }
  await Promise.all(array);
  await new Promise((r) => setTimeout(r, delay));
  console.log(
    "RSS",
    (process.memoryUsage.rss() / 1024 / 1024) | 0,
    "MB after",
    fmt.format((total += batch)) + " fetch() requests"
  );
}

server.js

const blob = new Blob([require("crypto").randomFillSync( new Uint8Array(1024 * 512))]);

Bun.serve({
  fetch(req) {
    return new Response(blob);
  },
  port: process.argv.at(-1),
});

this code will work on deno too. seems to perform pretty much the same as bun, but with some strange occasional stalls on my setup. defo see the same issue as you with node.js versions 20 and 21. memory keeps growing and never gets released, even if i force gc on every iteration of the loop.

import * as process from 'node:process'

const server = (globalThis.Deno? Deno.args : process.argv).at(-1);

const fmt = new Intl.NumberFormat();
let total = 0;
const batch = 50;
const delay = 0;
while (true) {
  const array = new Array(batch);
  for (let i = 0; i < batch; i++) {
    array[i] = fetch(server);
  }
  await Promise.all(array);
  await new Promise((r) => setTimeout(r, delay));
  console.log(
    "RSS",
    (process.memoryUsage.rss() / 1024 / 1024) | 0,
    "MB after",
    fmt.format((total += batch)) + " fetch() requests"
  );
}

Also, if you change the fetch call to await the body of the response, then bun/deno/node20/21 all behave pretty much the same but node.js uses ~30% less memory (150 MB) than deno or bun (both around 210 MB) on ubuntu 22.04.

    array[i] = fetch(server).then(res => res.text());