code: function destroy_next() {
  		if (old_retired_workers.length <= 0) {
  			return deferred.resolve();
  		}
  		destroyWorker.call(_self, old_retired_workers.shift().pid, reason, 0);
  		setTimeout(destroy_next, _self.options.spawn_delay);
  	}
docs: we kill the retired workers with delay, to dampen the migrating hurd effect