iOS gcd 串行，并行，同步，异步代码研究

参考文章：

http://www.cnblogs.com/mddblog/p/4767559.html

重要结论：

 　　1）串行队列，同步执行-----串行队列意味着顺序执行，同步执行意味着不开启线程（在当前线程执行）

 

 　　2）串行队列,异步执行-----串行队列意味着任务顺序执行，异步执行说明要开线程, （如果开多个线程的话，不能保证串行队列顺序执行，所以只开一个线程）

 

 　　3）并行队列,异步执行-----并行队列意味着执行顺序不确定，异步执行意味着会开启线程，而并行队列又允许不按顺序执行，所以系统为了提高性能会开启多个线程，来队列取任务（队列中任务取出仍然是顺序取出的，只是线程执行无序）。

 

 　　4）并行队列,同步执行-----同步执行意味着不开线程,则肯定是顺序执行

分别进行验证

串行同步

// 串行同步
- (void)test_queue_serial_sync
{
    NSLog(@"串行同步");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.001", DISPATCH_QUEUE_SERIAL);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_sync(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
// 结果

2017-02-10 18:00:21.991383 RXVerifyExample[4927:2465789] 串行同步
2017-02-10 18:00:21.991636 RXVerifyExample[4927:2465789] main thread:0x17406cb00
2017-02-10 18:00:21.992020 RXVerifyExample[4927:2465789] 1 thread: 0x17406cb00
2017-02-10 18:00:21.992097 RXVerifyExample[4927:2465789] 2 thread: 0x17406cb00
2017-02-10 18:00:21.992165 RXVerifyExample[4927:2465789] 3 thread: 0x17406cb00
2017-02-10 18:00:21.992230 RXVerifyExample[4927:2465789] 4 thread: 0x17406cb00

结论：

串行同步，不创建任何新的线程，因为是在主线程中执行这个方法的，所以，同步方法都在主线程。

并行同步

// 并行同步
- (void)test_queue_concurrent_sync
{
    NSLog(@"并行同步");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.002", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_sync(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_sync(queue, ^{
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
// 结果：

2017-02-10 18:04:33.107907 RXVerifyExample[4930:2467403] 并行同步
2017-02-10 18:04:33.108053 RXVerifyExample[4930:2467403] main thread:0x174066600
2017-02-10 18:04:33.108121 RXVerifyExample[4930:2467403] 1 thread: 0x174066600
2017-02-10 18:04:33.108177 RXVerifyExample[4930:2467403] 2 thread: 0x174066600
2017-02-10 18:04:33.108219 RXVerifyExample[4930:2467403] 3 thread: 0x174066600
2017-02-10 18:04:33.108259 RXVerifyExample[4930:2467403] 4 thread: 0x174066600

结论：跟串行同步是一样的。

串行异步1

// 串行异步1
- (void)test_queue_serial_async1
{
    NSLog(@"串行异步方法1");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.0031", DISPATCH_QUEUE_SERIAL);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_async(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
// 结果：
2017-02-10 18:06:15.124048 RXVerifyExample[4932:2468052] 串行异步方法1
2017-02-10 18:06:15.124244 RXVerifyExample[4932:2468052] main thread:0x174076a40
2017-02-10 18:06:15.125490 RXVerifyExample[4932:2468084] 1 thread: 0x174271800
2017-02-10 18:06:15.125574 RXVerifyExample[4932:2468084] 2 thread: 0x174271800
2017-02-10 18:06:15.125643 RXVerifyExample[4932:2468084] 3 thread: 0x174271800
2017-02-10 18:06:15.125710 RXVerifyExample[4932:2468084] 4 thread: 0x174271800

串行异步2

// 串行异步2
- (void)test_queue_serial_async2
{
    NSLog(@"串行异步方法2");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.0032", DISPATCH_QUEUE_SERIAL);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_async(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:4.1];
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:3.1];
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
// 结果
2017-02-10 18:08:16.478508 RXVerifyExample[4935:2468827] 串行异步方法2
2017-02-10 18:08:16.478611 RXVerifyExample[4935:2468827] main thread:0x170072380
2017-02-10 18:08:16.479034 RXVerifyExample[4935:2468869] 1 thread: 0x174264240
2017-02-10 18:08:16.479069 RXVerifyExample[4935:2468869] 2 thread: 0x174264240
2017-02-10 18:08:20.584321 RXVerifyExample[4935:2468869] 3 thread: 0x174264240
2017-02-10 18:08:23.689611 RXVerifyExample[4935:2468869] 4 thread: 0x174264240

结论：

串行异步只创建一个新的线程

并行异步1

// 并行异步1
- (void)test_queue_concurrent_async1
{
    NSLog(@"并行异步方法1");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.0041", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_async(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}

// 结果
2017-02-10 18:09:25.692681 RXVerifyExample[4937:2469287] 并行异步方法1
2017-02-10 18:09:25.692880 RXVerifyExample[4937:2469287] main thread:0x174073000
2017-02-10 18:09:25.693828 RXVerifyExample[4937:2469323] 1 thread: 0x174267d00
2017-02-10 18:09:25.693916 RXVerifyExample[4937:2469323] 2 thread: 0x174267d00
2017-02-10 18:09:25.693988 RXVerifyExample[4937:2469323] 3 thread: 0x174267d00
2017-02-10 18:09:25.694056 RXVerifyExample[4937:2469323] 4 thread: 0x174267d00

并行异步2

// 并行异步2
- (void)test_queue_concurrent_async2
{
    NSLog(@"并行异步方法2");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.0042", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_async(queue, ^{
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:4.1];
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:3.1];
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
// 结果
2017-02-10 18:10:27.262009 RXVerifyExample[4940:2469782] 并行异步方法2
2017-02-10 18:10:27.262213 RXVerifyExample[4940:2469782] main thread:0x17406e080
2017-02-10 18:10:27.263710 RXVerifyExample[4940:2469821] 1 thread: 0x174262e80
2017-02-10 18:10:27.263811 RXVerifyExample[4940:2469821] 2 thread: 0x174262e80
2017-02-10 18:10:30.369209 RXVerifyExample[4940:2469813] 4 thread: 0x1742648c0
2017-02-10 18:10:31.369205 RXVerifyExample[4940:2469821] 3 thread: 0x174262e80

并行异步3

// 并行异步3
- (void)test_queue_concurrent_async3
{
    NSLog(@"并行异步方法3");
    dispatch_queue_t queue = dispatch_queue_create("com.demo.0043", DISPATCH_QUEUE_CONCURRENT);
    NSLog(@"main thread:%p", [NSThread mainThread]);
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:2.1];
        NSLog(@"1 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:1.1];
        NSLog(@"2 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:4.1];
        NSLog(@"3 thread: %p", [NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        [NSThread sleepForTimeInterval:3.1];
        NSLog(@"4 thread: %p", [NSThread currentThread]);
    });
}
//结果
2017-02-10 18:10:59.926988 RXVerifyExample[4942:2470140] 并行异步方法3
2017-02-10 18:10:59.927184 RXVerifyExample[4942:2470140] main thread:0x17007f8c0
2017-02-10 18:11:01.033209 RXVerifyExample[4942:2470183] 2 thread: 0x170462ac0
2017-02-10 18:11:02.033478 RXVerifyExample[4942:2470185] 1 thread: 0x170462f40
2017-02-10 18:11:03.037799 RXVerifyExample[4942:2470176] 4 thread: 0x170462a40
2017-02-10 18:11:04.033691 RXVerifyExample[4942:2470177] 3 thread: 0x170462480

从并行异步的三个例子来看。

创建的新的线程的数目是不确定的，是跟队列中异步方法个数和异步方法执行的时间有关。