Event Loop Blockers: Perfetto Traces That Metrics Alone Will Miss

// src/eld.ts — always-on cheap signal
import { monitorEventLoopDelay } from "node:perf_hooks";

const h = monitorEventLoopDelay({ resolution: 10 });
h.enable();

setInterval(() => {
 metrics.gauge("node.eld_p99_ms", h.percentile(99) / 1e6);
 metrics.gauge("node.eld_max_ms", h.max / 1e6);
 h.reset();
}, 10_000);

# ✅ Staging capture (Chrome DevTools protocol via inspector)
node --inspect=0.0.0.0:9229 dist/server.js
# Then: chrome://inspect → Performance → record during repro
# Or export trace events:
node --trace-event-categories=v8,node,node.async_hooks dist/server.js

// ❌ Sync parse on giant payloads in the request path
app.post("/hook", (req, res) => {
 const body = JSON.parse(req.rawBody.toString("utf8")); // 12MB stalls
 res.end("ok");
});

// ✅ Streaming / size-capped parse
import { once } from "node:events";
app.post("/hook", async (req, res) => {
 const chunks: Buffer[] = [];
 let n = 0;
 for await (const c of req) {
 n += c.length;
 if (n > 1_000_000) return res.status(413).end();
 chunks.push(c);
 }
 const body = JSON.parse(Buffer.concat(chunks).toString("utf8"));
 res.end("ok");
});

// ❌ crypto.pbkdf2Sync in login
const hash = crypto.pbkdf2Sync(password, salt, 310000, 32, "sha256");

// ✅ async API — returns to the loop between libuv turns
const hash = await pbkdf2(password, salt, 310000, 32, "sha256");

import { performance } from "node:perf_hooks";

export function withSpan<T>(name: string, fn: () => T): T {
 performance.mark(`${name}:start`);
 try {
 return fn();
 } finally {
 performance.mark(`${name}:end`);
 performance.measure(name, `${name}:start`, `${name}:end`);
 }
}