Compare Dispatch Concurrent to for loop

compare_dispatch_apply.m

#import <Foundation/Foundation.h>

#define LOGV(V) NSLog(@"%s = %@", #V, V);
#define LOGD(I) NSLog(@"%s = %d", #I, I);

int main(int argc, const char * argv[])
{
	@autoreleasepool
	{
    	dispatch_queue_t dqueue = NULL;
        size_t size = 24772188; // decrease for iOS 
        NSMutableArray *array = [NSMutableArray array];
        NSDate *startDate;
        NSMutableArray *useableArray;

        // build an array
        for (size_t i = 0; i <= size; ++i) {
            [array addObject:@(i)];
        }

        useableArray = [array mutableCopy];
        startDate = [NSDate date];
        for (int i = 0; i < size; ++i)
        {
            useableArray[i] = @([useableArray[i] intValue] - 0.5);
        }
        NSLog(@"%0.3lf to - 0.5 in a for loop", [[NSDate date] timeIntervalSinceDate:startDate]);


        // now try this with a concurrent queue
        dqueue = dispatch_queue_create("Queue ALL THE THINGS!", DISPATCH_QUEUE_CONCURRENT);
        useableArray = [array mutableCopy];
        startDate = [NSDate date];
        dispatch_apply(size, dqueue, ^(size_t i) {
            useableArray[i] = @([useableArray[i] intValue] - 0.5);
        });
        NSLog(@"%0.3lf to - 0.5 in a for dispatch_apply(CONCURRENT)", [[NSDate date] timeIntervalSinceDate:startDate]);
    }
}

Output on my 2.7ghz i7 dual core MBP

5.589 to - 0.5 in a for loop
3.420 to - 0.5 in a for dispatch_apply(CONCURRENT)