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Grand Central Dispatch (GCD)
Grand Central Dispatch (GCD) - a technology for Apple’s Mac OS X and iOS operating systems-is a combination of extensions to the C language, an API, and a run-time library that allows application developers to identify sections of code to run in parallel. Like OpenMP, GCD manages most of the details of threading. GCD identifies extensions to the C and C++ languages known as blocks. A block is simply a self-contained unit of work. It is specified by a caret ˆ inserted in front of a pair of braces { }. A simple example of a block is shown below −
{
ˆprintf("This is a block");
}It 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. 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 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. 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. Quite simply, blocks with a higher priority should be placed on the high priority dispatch queue. The following code segment illustrates obtaining the default-priority concurrent queue and submitting a block to the queue using the dispatch async() function −
dispatch_queue_t queue = dispatch_get_global_queue
(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch async(queue, ˆ{ printf("This is a block."); });Internally, GCD’s thread pool is composed of POSIX threads. GCD actively manages the pool, allowing the number of threads to grow and shrink according to application demand and system capacity.
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