About dispatch_queue_t的线程调度

dispatch_queue_t
A dispatch queue is a lightweight object to which your application submits blocks for subsequent execution.

typedef struct dispatch_queue_s *dispatch_queue_t;
Discussion
A dispatch queue invokes blocks submitted to it serially in FIFO order. A serial queue invokes only one block at a time, but independent queues may each invoke their blocks concurrently with respect to each other.

The global concurrent queues invoke blocks in FIFO order but do not wait for their completion, allowing multiple blocks to be invoked concurrently.

The system manages a pool of threads that process dispatch queues and invoke blocks submitted to them. Conceptually, a dispatch queue may have its own thread of execution, and interaction between queues is highly asynchronous.

Dispatch queues are reference counted via calls to dispatch_retain and dispatch_release. Pending blocks submitted to a queue also hold a reference to the queue until they have finished. Once all references to a queue have been released, the queue will be deallocated by the system.

Availability
Available in iOS 4.0 and later.
Declared In
dispatch/queue.h

关于dispatch_queue_t的系统线程处理，系统维护了一个线程池，dispatch queues中的blocks会提交给线程池中的线程进行处理。一个dispatch queue可能拥有自己的线程执行任务，不同queues之间是高度异步的。