fork.js

var n = +(process.argv[2] || 10000)

var cp = require('child_process')
var child = cp.fork(__dirname + '/worker.js')

console.log('trying', n, 'messages')

var total = n
child.on('message', function () {
  if (!--total) console.log('completed %d roundtrips', n - total)
  else if (!(total % 1000)) console.log('%d roundtrips', n - total)
})

for (var i = n; i--;) {
  child.send('a')
}

worker.js

var r = 0
process.on('message', function () {
  process.send('b')
  if (!(r % 1000)) console.log('received: %d', ++r)
})