public static function checkIfChildRunning()
    {
        foreach (static::$_pidMap as $worker_id => $worker_pid_array) {
            foreach ($worker_pid_array as $pid => $worker_pid) {
                if (!posix_kill($pid, 0)) {
                    unset(static::$_pidMap[$worker_id][$pid]);
                }
            }
        }
    }