Grand Central Dispatch

• GCD schedules blocks for run-time execution by placing them on a dispatch queue.
• When it removes a block from a queue, it assigns the block to an available thread from the thread pool it manages.
• GCD identifies two types of dispatch queues: serial and concurrent.
• Internally, GCD’s thread pool is composed of POSIX threads.

Serial Queue

• Blocks placed on a serial queue are removed in FIFO order
• Once a block has been removed from the queue, it must complete execution before another block is removed.
• Each process has its own serial queue (known as its main queue).
• Developers can create additional serial queues that are local to particular processes.
• Serial queues are useful for ensuring the sequential execution of several tasks.

Concurrent Queue

• Blocks placed on a concurrent queue are also removed in FIFO order, but several blocks may be removed at a time, thus allowing multiple blocks to execute in parallel.
• There are three system-wide concurrent dispatch queues, and they are distinguished according to priority: low, default, and high.
• Priorities represent an approximation of the relative importance of blocks.