Node libuv Threadpool Tuning: Size UV_THREADPOOL_SIZE With Benchmarks

// Things that queue on UV threadpool (async forms):
// fs.readFile / fs.promises.* (non-sync)
// crypto.pbkdf2, scrypt, randomBytes (async)
// dns.lookup (not dns.resolve*)
// some zlib async APIs
// ❌ DNS resolve via c-ares may NOT use the pool the same way — measure!

// src/bench-pool.ts
import { pbkdf2 } from "node:crypto";
import { monitorEventLoopDelay } from "node:perf_hooks";
import { Worker } from "node:worker_threads"; // not needed — just concurrent pbkdf2

const h = monitorEventLoopDelay({ resolution: 20 });
h.enable();

const N = 200;
const concurrency = 32;
let done = 0;
const t0 = Date.now();

function kick() {
 pbkdf2("pw", "salt", 100_000, 32, "sha512", () => {
 if (++done === N) {
 console.log(JSON.stringify({
 ms: Date.now() - t0,
 elp99: h.percentile(99) / 1e6,
 pool: process.env.UV_THREADPOOL_SIZE ?? "4",
 }));
 process.exit(0);
 } else if (done < N) kick();
 });
}
for (let i = 0; i < concurrency; i++) kick();

# ✅ Sweep from outside — env must be set before node boots
for s in 4 8 16 24; do UV_THREADPOOL_SIZE=$s node dist/bench-pool.js; done

// ❌ Setting UV_THREADPOOL_SIZE inside running JS — too late
process.env.UV_THREADPOOL_SIZE = "16"; // libuv already initialized

ENV UV_THREADPOOL_SIZE=16
CMD ["node", "dist/server.js"]