Event loop latency

Learn event loop latency and blocking.

Event loop latency

Event loop latency is how long the event loop takes to complete one turn. High latency means the event loop is blocked; this slows down requests.

Measurement method: you schedule setTimeout(cb, 0) and measure when cb fires. The latency is the difference between when cb fires and when it was scheduled. In Node.js 16+ performance.eventLoopUtilization() is also useful.

The latency measurement pattern

A typical latency measurement: const start = performance.now(); setTimeout(() => { const delay = performance.now() - start; }, 0);. The expected delay is near zero; but if the event loop is busy it grows.

Repeating this pattern at regular intervals gives you a trend. If latency stays high, a CPU-heavy task is blocking the event loop. The fix: move that task to a worker thread.

Exercise Compare elapsed time

Import performance. Assign performance.now() to a start variable. Run a short loop. Log performance.now() >= start.

Show example solution

const { performance } = require("perf_hooks");
const start = performance.now();
let sum = 0;
for (let i = 0; i < 100; i++) sum += i;
console.log(performance.now() >= start);

Test yourself

Complete the method name

const start = performance.____()

Show answer

const start = performance.now()

What does event loop latency measure?
- Dosya boyutunu
- The event loop delay
- The network speed
- The memory usage
Show answer

The event loop delay — Event loop latency measures the event loop's delay.
Match each piece to its role

performance.now()

Start time
latency

Delay
setTimeout

Timer
event loop

Event loop

Next lesson: Memory usage

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies