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

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?

1 Answers

Answers 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.

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