dispatch_apply_f3

Usage

Overview

The dispatch_apply() function provides data-level concurrency through a "for (;;)" loop like primitive:

dispatch_queue_t the_queue = dispatch_get_concurrent_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT);
size_t iterations = 10;
// 'idx' is zero indexed, just like:
// for (idx = 0; idx < iterations; idx++)
dispatch_apply(iterations, the_queue, ^(size_t idx) {
	printf("%zu\n", idx);
});

Like a "for (;;)" loop, the dispatch_apply() function is synchronous. If asynchronous behavior is desired, please wrap the call to dispatch_apply() with a call to dispatch_async() against another queue.

Sometimes, when the block passed to dispatch_apply() is simple, the use of striding can tune performance. Calculating the optimal stride is best left to experimentation. Start with a stride of one and work upwards until the desired performance is achieved (perhaps using a power of two search):

#define	STRIDE	3
dispatch_apply(count / STRIDE, queue, ^(size_t idx) {
	size_t j = idx * STRIDE;
	size_t j_stop = j + STRIDE;
	do {
		printf("%zu\n", j++);
	} while (j < j_stop);
});
size_t i;
for (i = count - (count % STRIDE); i < count; i++) {
	printf("%zu\n", i);
}

Fundamentals

Conceptually, dispatch_apply() is a convenient wrapper around dispatch_async() and a semaphore to wait for completion. In practice, the dispatch library optimizes this function.

The dispatch_apply() function is a wrapper around dispatch_apply_f().