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：
-- ::15.124048 RXVerifyExample[:] 串行异步方法1
-- ::15.124244 RXVerifyExample[:] main thread:0x174076a40
-- ::15.125490 RXVerifyExample[:]  thread: 0x174271800
-- ::15.125574 RXVerifyExample[:]  thread: 0x174271800
-- ::15.125643 RXVerifyExample[:]  thread: 0x174271800
-- ::15.125710 RXVerifyExample[:]  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
-- ::16.478508 RXVerifyExample[:] 串行异步方法2
-- ::16.478611 RXVerifyExample[:] main thread:0x170072380
-- ::16.479034 RXVerifyExample[:]  thread: 0x174264240
-- ::16.479069 RXVerifyExample[:]  thread: 0x174264240
-- ::20.584321 RXVerifyExample[:]  thread: 0x174264240
-- ::23.689611 RXVerifyExample[:]  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
-- ::25.692681 RXVerifyExample[:] 并行异步方法1
-- ::25.692880 RXVerifyExample[:] main thread:0x174073000
-- ::25.693828 RXVerifyExample[:]  thread: 0x174267d00
-- ::25.693916 RXVerifyExample[:]  thread: 0x174267d00
-- ::25.693988 RXVerifyExample[:]  thread: 0x174267d00
-- ::25.694056 RXVerifyExample[:]  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
-- ::27.262009 RXVerifyExample[:] 并行异步方法2
-- ::27.262213 RXVerifyExample[:] main thread:0x17406e080
-- ::27.263710 RXVerifyExample[:]  thread: 0x174262e80
-- ::27.263811 RXVerifyExample[:]  thread: 0x174262e80
-- ::30.369209 RXVerifyExample[:]  thread: 0x1742648c0
-- ::31.369205 RXVerifyExample[:]  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
-- ::59.926988 RXVerifyExample[:] 并行异步方法3
-- ::59.927184 RXVerifyExample[:] main thread:0x17007f8c0
-- ::01.033209 RXVerifyExample[:]  thread: 0x170462ac0
-- ::02.033478 RXVerifyExample[:]  thread: 0x170462f40
-- ::03.037799 RXVerifyExample[:]  thread: 0x170462a40
-- ::04.033691 RXVerifyExample[:]  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。