Unlike the main queue or queues allocated with the named DispatchQueue constructor, the global concurrent queues schedule blocks as soon as threads become available (non-FIFO completion order). The global concurrent queues represent three priority bands: DispatchQueuePriority.High, DispatchQueuePriority.Default and DispatchQueuePriority.Low.

Tasks submitted to the high priority global queue will be invoked before those submitted to the default or low priority global queues. Blocks submitted to the low priority global queue will only be invoked if no blocks are pending on the default or high priority queues.