If dispatch_sync does not use a different thread, can it be faster?

问题

I'm running this code

dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
      NSLog(@"Main Thread? %d", [NSThread isMainThread]);
      // non-trivial non-UI code here

});

The NSLog indicates that I'm actually on the main thread, but... my impression is that the non-trivial non-UI code runs much faster if I use dispatch_sync (as opposed to nothing).

If it is running on the main thread, is there any way it could be faster?

回答1:

The global concurrent GCD queues have no ordering, passing one of them to dispatch_sync() is a noop and dispatch_sync() will just execute the specified block directly in this case, as if you had written

^{...}();

回答2:

By limiting the number of threads running, technically sure you may improve performance, however this isn't recommended. If you have a task that will take a substantial amount of time (substantial meaning, will the user experience be affected by an unresponsive UI) then just use a new thread using dispatch_async or an inline block (closure). The best option is to optimize your algorithm to improve the length of your task.

You won't notice a significant enough difference in speed by running on the main thread.

来源：https://stackoverflow.com/questions/14742184/if-dispatch-sync-does-not-use-a-different-thread-can-it-be-faster