What is the overhead of running an empty block on a queue

Question

I realized that I was queuing a lot of blocks calling to empty methods. In the debugger it looks like a lot is happening when really all the blocks are empty.

Is there any real performance impact from having empty blocks?

Answer 1

The overhead should be negligible: You may check this with Instruments and a simple program like:

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        dispatch_queue_t q = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
        void (^b)(void) = ^{ };
        double d = 2.0;

        for(int i = 0; i < 10000000; ++i) {
            dispatch_sync(q, b);
            d = d * 1.5 - 1.0;
        }
        NSLog(@"d = %.3f", d);
    }
    return 0;
}

As you can see in the Instruments stack trace the calls require 40ms for 10 millions synchronous invocations of an empty block. That's not much overhead.