The Libuv Thread Pool.

In Simple Words: By default, Libuv maintains a thread pool of 4 worker threads for operations that the OS cannot handle asynchronously non-blockingly:

• File system operations (fs.*).
• DNS lookup operations (dns.lookup).
• Cryptographic functions (crypto.pbkdf2, crypto.randomBytes, crypto.scrypt).
• Compression (zlib.*). Note: Network sockets (TCP/UDP, HTTP) do not use the thread pool; they use native non-blocking OS primitives (epoll on Linux, kqueue on macOS).

const crypto = require('crypto');

// Measuring thread pool saturation (default 4 threads):
const start = Date.now();
for (let i = 0; i < 4; i++) {
  crypto.pbkdf2('pass', 'salt', 100000, 512, 'sha512', () => {
    console.log(`Hash ${i + 1} finished in ${Date.now() - start}ms`);
  });
}