Grand Central Dispatch: First Look
September 6, 2009 | Filed Under Apple | No Comments
In the last years I’ve always used a “parallel task” approach foreach loops that I’ve in the code, not always to speedup but even to clean-up the code. How to do it? Wrapping threads and Thread Pool like in this C# Parallel Forech Code.
Snow Leopard has introduced a new BSD-level infrastructure, with simple and efficient API to do this job. Here a little usage preview.
Block objects are a C-based language feature that you can use in your C, Objective-C, and C++ code. Blocks make it easy to define a self-contained unit of work. Blocks are something like Actions (delegate {}) in C#. Very useful to embed function in loops.
Blocks looks like a “private” function pointer, but you can access to the “parent” vars. (If you’re a Python coder, you’ve exactly the same thing).
/* Blocks in Python...
* def main():
* a = 10
* def test(k):
* print a, k
* test(128)
*/
int main (int argc, const char *argv[]) {
int a = 12;
void (^test_block) (int) = ^(int k) {
printf("A Block: PARENT(%d) ARG(%d)\n", a, k);
};
test_block(128);
return 0;
}
The GCD queue API provides dispatch queues from which threads take tasks to be executed. Because the threads are managed by GCD, Mac OS X can optimize the number of threads based upon available memory, number of currently active CPU cores, and so on. This shifts a great deal of the burden of power and resource management to the operating system itself, freeing your application to focus on the actual work to be accomplished.
#include <dispatch/dispatch.h>
#include <stdlib.h>
#include <stdio.h>
#define ITEM_VMIN (1)
#define ITEM_VMAX (200)
#define NR_ITEMS (100)
static void __fill_item (void *items, size_t n) {
int *i_items = (int *)items;
i_items[n] = (ITEM_VMIN + (int)(ITEM_VMAX * ((double)rand() / RAND_MAX)));
}
static void __work_on_item (void *items, size_t n) {
int *i_items = (int *)items;
i_items[n] *= 100; /* Do some Computation on this Item */
}
int main (int argc, const char *argv[]) {
dispatch_queue_t queue;
int data[NR_ITEMS];
/* Get Global Dispatch Queue */
queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
/* Initialize data Elements, and run computation on each element */
dispatch_apply_f(NR_ITEMS, queue, data, __fill_item);
dispatch_apply_f(NR_ITEMS, queue, data, __work_on_item);
/* Brief review of the items */
dispatch_apply(NR_ITEMS, queue, ^(size_t n) {
printf("Results: Item %lu = %d\n", n, data[n]);
});
return 0;
}