07-17-2014
Quote:
Originally Posted by
Peasant
But if you do a while true do ls -lrt ... it will be slow first time, and fast every other loop iterations.
Sounds like caching behavior. Once it has it, it keeps it a while for faster recall.
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LEARN ABOUT DEBIAN
dispatch_apply
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