Master and worker processes in Node.js - Performance & Optimization

Performance: Master and worker processes

HIGH IMPACT

This concept affects how Node.js handles CPU-intensive tasks and concurrent requests, impacting server responsiveness and throughput.

Handling CPU-intensive tasks in a Node.js server

Node.js

const cluster = require('cluster');
const http = require('http');
const numCPUs = require('os').cpus().length;

if (cluster.isMaster) {
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }
} else {
  http.createServer((req, res) => {
    // Worker handles request without blocking master
    let count = 0;
    for (let i = 0; i < 1e9; i++) count += i;
    res.end('Done ' + count);
  }).listen(3000);
}

Spreads workload across multiple worker processes, preventing any single event loop from blocking and improving concurrency.

📈 Performance GainReduces event loop blocking per process, improves INP by parallelizing CPU work across cores.

Handling CPU-intensive tasks in a Node.js server

Node.js

const http = require('http');
http.createServer((req, res) => {
  // Heavy computation blocking event loop
  let count = 0;
  for (let i = 0; i < 1e9; i++) count += i;
  res.end('Done ' + count);
}).listen(3000);

Heavy computation blocks the single main event loop, causing slow or unresponsive server during processing.

📉 Performance CostBlocks event loop for hundreds of milliseconds per request, causing high INP and poor responsiveness.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Single process heavy CPU task	N/A	N/A	N/A	[X] Bad
Master with multiple worker processes	N/A	N/A	N/A	[OK] Good

Rendering Pipeline

Master process delegates tasks to worker processes, which run independently. This avoids blocking the main event loop and keeps the server responsive.

→Event Loop

→Task Scheduling

→CPU Utilization

⚠️ BottleneckSingle-threaded event loop blocking due to CPU-heavy tasks

Core Web Vital Affected

INP

This concept affects how Node.js handles CPU-intensive tasks and concurrent requests, impacting server responsiveness and throughput.

Optimization Tips

1Offload CPU-intensive tasks to worker processes to keep the main event loop free.

2Use as many worker processes as CPU cores for best parallelism.

3Avoid blocking the event loop in the master process to improve server responsiveness.

Performance Quiz - 3 Questions

What is the main performance benefit of using worker processes in Node.js?

A They improve CSS rendering speed in the browser.

B They reduce the size of the JavaScript bundle sent to the browser.

C They prevent the main event loop from blocking by parallelizing CPU work.

D They eliminate the need for asynchronous code.

DevTools: Performance

How to check: Record a CPU profile while sending requests to the server. Look for long blocking tasks in the main thread.

What to look for: Long tasks blocking the event loop indicate poor use of worker processes; multiple smaller tasks across processes indicate good parallelism.