To take Advantage of Multi-Processor in Node.js, why the number of clusters we fork is CPU cores count?

Question

I know maybe it's a stupid question. I'm not good at OS knowledge.

Usually the number of cluster/process we fork is the CPU cores count, like the Nodejs offical Doc display.

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

if (cluster.isMaster) {
  console.log(`Master ${process.pid} is running`);

  // Fork workers.
  for (let i = 0; i < numCPUs; i++) {
    cluster.fork();
  }

  cluster.on('exit', (worker, code, signal) => {
    console.log(`worker ${worker.process.pid} died`);
  });
} else {
  // Workers can share any TCP connection
  // In this case it is an HTTP server
  http.createServer((req, res) => {
    res.writeHead(200);
    res.end('hello world\n');
  }).listen(8000);

  console.log(`Worker ${process.pid} started`);
}

But I'm confused what if we fork the cluster more than the number of CPU cores, what it gonna happen?

Is it the empirical value, best practice or any other reason?

Answer 1

But I'm confused what if we fork the cluster more than the number of CPU cores, what it gonna happen?

Nothing will happen, everything will still work, but putting more than one CPU-intensive thread or process per core will not make the application any faster - if anything, it may only make is slower because the core will waste some portion of the time on the context switches between the threads.

If you have CPU-bound operations then the optimal number of threads/processes per core is always 1. For I/O-bound operations it shouldn't really matter as most of the time the processes will not do any computations anyway.