|
|
Victor Palau's Blog: Going Agile: A few iterations under the belt
|
|
dispatch_apply(3) BSD Library Functions Manual dispatch_apply(3) NAME
dispatch_apply -- schedule blocks for iterative execution SYNOPSIS
#include <dispatch/dispatch.h> void dispatch_apply(size_t iterations, dispatch_queue_t queue, void (^block)(size_t)); void dispatch_apply_f(size_t iterations, dispatch_queue_t queue, void *context, void (*function)(void *, size_t)); DESCRIPTION
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 ", 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 ", j++); } while (j < j_stop); }); size_t i; for (i = count - (count % STRIDE); i < count; i++) { printf("%zu ", i); } FUNDAMENTALS
Conceptually, dispatch_apply() is a convenient wrapper around dispatch_async() and a semaphore to wait for completion. In practice, the dis- patch library optimizes this function. The dispatch_apply() function is a wrapper around dispatch_apply_f(). SEE ALSO
dispatch(3), dispatch_async(3), dispatch_queue_create(3), dispatch_semaphore_create(3) Darwin May 1, 2009 Darwin
