Repro for nodejs/node#9706 on windows (at least). Run like `node master 1111`. 3500 works for me, but 4000 will hang forever.

master.js

'use strict';

const { fork } = require('child_process');
const { join } =  require('path');

const count = process.argv[2] * limit;
// number of cores on your machine - 1
// or whatever you want \_(*_*)_/
const limit = 7;

function log(str) {
  process.stdout.write(`\n${Date.now()} | ${str}`);
}

const processes = [];

function loaded(proc) {
  processes.push(proc);

  if (processes.length >= limit) {
    initialized();
  }
}

for (let i = 0; i < limit; i += 1) {
  const proc = fork(join(__dirname, 'slave.js'), {
    stdio: [0, 1, 2, 'ipc'],
  });

  proc.on('error', err => { throw err; });
  proc.on('message', (response) => {
    if (response === 'initialized') {
      loaded(proc);
    }
  });
}

const responses = [];

function initialized() {
  processes.forEach((proc) => {
    proc.on('message', ({ number, hash }) => {
      responses.push([number, hash]);
      log(`got response ${hash} for ${number}`);

      if (responses.length >= count) {
        done();
      }
    });
  });

  for (let i = 0; i < count; i += 1) {
    log(`sending ${i}`);
    processes[i % limit].send(i);
  }
}

function done() {
  processes.forEach(proc => proc.kill());
}

slave.js

'use strict';

const bcrypt = require('bcrypt');

function log(str) {
  process.stdout.write(`\n${Date.now()} | ${str}`);
}

process.on('message', (number) => {
  log(`hashing ${number}`);
  const hash = bcrypt.hashSync(number.toString(), 12);
  log(`hash of ${number}: ${hash}`);

  process.send({
    number,
    hash,
  });
});

process.send('initialized');