dispatch

Dispatch Queues

Dispatch queues are a C-based mechanism for executing custom tasks. A dispatch queueexecutes tasks either serially or concurrently but always in a first-in, first-out order. (In other words, a dispatch queue always dequeues and starts tasks in the same order in which they were added to the queue.) A serial dispatch queue runs only one task at a time, waiting until that task is complete before dequeuing and starting a new one. By contrast, a concurrent dispatch queue starts as many tasks as it can without waiting for already started tasks to finish.

Dispatch queues have other benefits:

• They provide a straightforward and simple programming interface.

• They offer automatic and holistic thread pool management.

• They provide the speed of tuned assembly.

• They are much more memory efficient (because thread stacks do not linger in application memory).

• They do not trap to the kernel under load.

• The asynchronous dispatching of tasks to a dispatch queue cannot deadlock the queue.

• They scale gracefully under contention.

• Serial dispatch queues offer a more efficient alternative to locks and other synchronization primitives.