It's useful to note that from iOS 8 the creation of queues and their abilities have changed. You would use the following to get a serial queue with the lowest priority.
dispatch_queue_attr_t queueAttributes = dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_BACKGROUND, 0);
self.queue = dispatch_queue_create("com.abc.myQueue", queueAttributes);