Exploration of thread priorities vs queue priorities

gistfile1.m

  // Main thread priorty = 0.758065
  NSLog(@"main:%@:%f", [NSThread currentThread], [NSThread threadPriority]);

  // High priority = 0.532258
  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^{
    NSLog(@"high:%@:%f", [NSThread currentThread], [NSThread threadPriority]);
    // run forever! Note that other queues still happily process, even though our high-priority block never ends
    while(1) {}
  });

  // Default priorty = 0.5
  dispatch_after(dispatch_time(0, NSEC_PER_SEC),
                 dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
    NSLog(@"default:%@:%f", [NSThread currentThread], [NSThread threadPriority]);
  });

  // Background priorty = 0.0
  dispatch_after(dispatch_time(0, NSEC_PER_SEC),
                 dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    NSLog(@"background:%@:%f", [NSThread currentThread], [NSThread threadPriority]);
  });

  // Runs on the main thread (unsurprisingly)
  // means it gets high priorty thread (since main is waiting for it)
  dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_BACKGROUND, 0), ^{
    NSLog(@"default(sync):%@:%f", [NSThread currentThread], [NSThread threadPriority]);
  });