How do you implement a custom event loop monitoring system?

Question

Accepted Answer

**Event loop monitoring** — detect event loop lag.

```javascript
class EventLoopMonitor {
    constructor(options = {}) {
        this.interval = options.interval || 1000;
        this.lags = [];
        this.timer = null;
    }

start() {
        let lastCheck = process.hrtime.bigint();

this.timer = setInterval(() => {
            const now = process.hrtime.bigint();
            const expected = BigInt(this.interval) * 1000000n;
            const actual = now - lastCheck;
            const lag = Number(actual - expected) / 1000000; // ms

this.lags.push({
                timestamp: Date.now(),
                lag,
            });

if (this.lags.length > 300) this.lags.shift();

if (lag > 100) {
                console.warn(`Event loop lag: ${lag.toFixed(2)}ms`);
            }

lastCheck = now;
        }, this.interval);
    }

stop() {
        clearInterval(this.timer);
    }

getStats() {
        if (this.lags.length === 0) return null;
        const lags = this.lags.map(l => l.lag);
        lags.sort((a, b) => a - b);

return {
            avg: lags.reduce((a, b) => a + b, 0) / lags.length,
            min: lags[0],
            max: lags[lags.length - 1],
            p95: lags[Math.floor(lags.length * 0.95)],
            p99: lags[Math.floor(lags.length * 0.99)],
        };
    }
}

const monitor = new EventLoopMonitor();
monitor.start();

setInterval(() => {
    console.log("Event loop stats:", monitor.getStats());
}, 30000);
```

Node.js: How do you implement a custom event loop monitoring system?