dispatch_set_target_queue会将其他queue转为和targetqueue一样的优先级和执行时序，看下面的例子

即使queue1，queue2，queue3，都是并行的queue，但是输出结果仍然是串行的结果；

- (void)testTargetQueue{    dispatch_queue_t targetQueue = dispatch_queue_create("test.target.queue", DISPATCH_QUEUE_SERIAL);    dispatch_queue_t queue1 = dispatch_queue_create("test.1", DISPATCH_QUEUE_CONCURRENT);    dispatch_queue_t queue2 = dispatch_queue_create("test.2", DISPATCH_QUEUE_CONCURRENT);    dispatch_queue_t queue3 = dispatch_queue_create("test.3", DISPATCH_QUEUE_CONCURRENT);    dispatch_set_target_queue(queue1, targetQueue);    dispatch_set_target_queue(queue2, targetQueue);    dispatch_set_target_queue(queue3, targetQueue);        dispatch_async(queue1, ^{        NSLog(@"1 in");        [NSThread sleepForTimeInterval:3.f];        NSLog(@"1 out");    });        dispatch_async(queue2, ^{        NSLog(@"2 in");        [NSThread sleepForTimeInterval:2.f];        NSLog(@"2 out");    });    dispatch_async(queue3, ^{        NSLog(@"3 in");        [NSThread sleepForTimeInterval:1.f];        NSLog(@"3 out");    });}

　　

2017-03-07 20:31:11.636 DispatchTargetQueueTest[42985:377250] 1 in

2017-03-07 20:31:14.711 DispatchTargetQueueTest[42985:377250] 1 out

2017-03-07 20:31:14.711 DispatchTargetQueueTest[42985:377250] 2 in

2017-03-07 20:31:16.784 DispatchTargetQueueTest[42985:377250] 2 out

2017-03-07 20:31:16.784 DispatchTargetQueueTest[42985:377250] 3 in

2017-03-07 20:31:17.859 DispatchTargetQueueTest[42985:377250] 3 out