dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);

 dispatch_queue_t
  dispatch_get_global_queue(long priority, unsigned long flags)
  {
          if (flags & ~(unsigned long)DISPATCH_QUEUE_OVERCOMMIT) {
                  return DISPATCH_BAD_INPUT;
          }    
          dispatch_qos_t qos = _dispatch_qos_from_queue_priority(priority); <---
  #if !HAVE_PTHREAD_WORKQUEUE_QOS
          if (qos == QOS_CLASS_MAINTENANCE) {
                  qos = DISPATCH_QOS_BACKGROUND;
          } else if (qos == QOS_CLASS_USER_INTERACTIVE) {
                  qos = DISPATCH_QOS_USER_INITIATED;
          }    
  #endif
          if (qos == DISPATCH_QOS_UNSPECIFIED) {
                  return DISPATCH_BAD_INPUT;
          }    
          return _dispatch_get_root_queue(qos, flags & DISPATCH_QUEUE_OVERCOMMIT);
  }