「面向打野编程」iOS多线程：CGD

2022-02-16 10:16:07

「面向打野编程」iOS多线程：CGD

前言

参考网络其他文章而写，渣水平，抛砖引玉。

虽然Concurrent意思为并发，但由于队列的实际效果，以下称为并行队列。

当前iPhone的CPU核心数远小于GCD的线程池，故不讨论GCD的线程池，没有意义。

GCD = 主队列 + 并行队列 * n

异步串行队列 = 并行队列 * 1

1. 同异步队列

  • 主队列dispatch_get_main_queue()

  • 全局并行队列dispatch_get_global_queue(0, 0)

  • 串行队列dispatch_queue_t sQue = dispatch_queue_create("q1", DISPATCH_QUEUE_SERIAL);

  • 并行队列dispatch_queue_t cQue = dispatch_queue_create("q2", DISPATCH_QUEUE_CONCURRENT);

GCD 串行队列 并行队列
同步执行 抢占执行 抢占执行
异步执行 等待执行 并行执行
  • 同步 + 队列 = 主队列
  • (异步 + 并行) * n = 串行队列 * n
  • 串行（同步 + 自身） = 死锁（无法抢占）

同步执行：抢占当前主线程，不能在主线程中使用，会造成死锁。

异步执行：将任务加入队列末尾，或将任务加入新线程。

主队列与并行队列互不影响，即使并行队列被长耗时任务占满，主队列依然畅通。

经过测试，全局并行队列与自定义并行队列并没有太多明显的区别。当全局并行队列占满核心后，自定义并行队列依然会陷入等待，无法并发。

dispatch_queue_t cQue = dispatch_queue_create("cq", DISPATCH_QUEUE_CONCURRENT);

for(int i = 0; i < 20; ++i) {

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        for(int j = 1; j < INT_MAX; ++j);

        for(int j = 1; j < INT_MAX; ++j);

        printf("_");

    });

}

for(int i = 0; i < 20; ++i) {

    dispatch_async(cQue, ^{

        printf("2");

    });

}
________________22222222222222222222____

CPU的核心数量是有限的，当异步线程的数量主够多的时候，会退化为串行队列，并不会变为并发执行。对于多个长时间运行的并行线程，可能会因为占满处理器而导致其他线程任务的堆积，可以使用[NSThread sleepForTimeInterval:(NSTimeInterval)]或手动分割控制阻塞实现伪并发操作来改善长时间等待。

[NSThread sleepForTimeInterval:(NSTimeInterval)]

dispatch_queue_t cQue = dispatch_queue_create("cq", DISPATCH_QUEUE_CONCURRENT);

for(int i = 0; i < 20; ++i) {

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        for(int j = 1; j < INT_MAX>>1; ++j);

        [NSThread sleepForTimeInterval:2];

        for(int j = 1; j < INT_MAX>>1; ++j);

        [NSThread sleepForTimeInterval:1];

        for(int j = 1; j < INT_MAX>>1; ++j);

        [NSThread sleepForTimeInterval:0.5];

        for(int j = 1; j < INT_MAX>>1; ++j);

        printf("_");

    });

}

for(int i = 0; i < 20; ++i) {

    dispatch_async(cQue, ^{

        printf("2");

    });

}
_____22222222222222222222_______________

手动分割

dispatch_queue_t cQue = dispatch_queue_create("cq", DISPATCH_QUEUE_CONCURRENT);

for(int i = 0; i < 20; ++i) {

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        for(int j = 1; j < INT_MAX>>1; ++j);

        dispatch_async(dispatch_get_global_queue(0, 0), ^{

            for(int j = 1; j < INT_MAX>>1; ++j);

            dispatch_async(dispatch_get_global_queue(0, 0), ^{

                for(int j = 1; j < INT_MAX>>1; ++j);

                dispatch_async(dispatch_get_global_queue(0, 0), ^{

                    for(int j = 1; j < INT_MAX>>1; ++j);

                    printf("_");

                });

            });

        });

    });

}

for(int i = 0; i < 20; ++i) {

    dispatch_async(cQue, ^{

        printf("2");

    });

}
22222222222222222222____________________

2. 栅栏

  • 同步栅栏dispatch_barrier_async

  • 异步栅栏dispatch_barrier_async

栅栏对全局并行队列无效，只能操作自定义并行队列。想要运行此处的代码就需要完成先前的操作，是否同步的区别在于能否阻塞当前线程。千万不要对把同步栅栏运行在同个队列，会造成死锁。

同步栅栏

dispatch_queue_t cQue = dispatch_queue_create("q", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("1");

});

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("2");

});

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("3");

});

dispatch_barrier_sync(cQue, ^{

    NSLog(@"4");

});

puts("-----5-------");
3

2

1

4

-----5-------

异步栅栏

dispatch_queue_t cQue = dispatch_queue_create("q", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("1");

});

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("2");

});

dispatch_async(cQue, ^{

    for(int j = 1; j < INT_MAX; ++j);

    puts("3");

});

dispatch_barrier_async(cQue, ^{

    NSLog(@"4");

});

puts("-----5-------");
-----5-------

1

2

3

4

死锁

dispatch_queue_t cQue2 = dispatch_queue_create("q22", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(cQue2, ^{

    dispatch_async(cQue2, ^{

        puts("1");

    });

    dispatch_barrier_sync(cQue2, ^{

        puts("----------");

    });

    dispatch_async(cQue2, ^{

        puts("2");

    });

});
1

3. 延迟执行

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0.5 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{

    //延迟0.5s执行

});

4. 一次性代码

只执行一次，但是需要注意不要出现相互调用，会出现死锁

static dispatch_once_t onceToken;

dispatch_once(&onceToken, ^{

    //code to be executed once

});

5. 快速遍历

不用手写循环，或同时执行多个相似的代码块，会阻塞当前线程。

dispatch_apply(6, dispatch_get_global_queue(0, 0), ^(size_t i) {

    NSLog(@"%zu", i);

});
2017-12-15 11:34:56.748284+0800 GCD-OC[2266:105786] 1

2017-12-15 11:34:56.748260+0800 GCD-OC[2266:105750] 0

2017-12-15 11:34:56.748294+0800 GCD-OC[2266:105785] 2

2017-12-15 11:34:56.748302+0800 GCD-OC[2266:105788] 3

2017-12-15 11:34:56.749694+0800 GCD-OC[2266:105750] 4

2017-12-15 11:34:56.749703+0800 GCD-OC[2266:105786] 5

6. 信号量

控制最大并行数，或用于同步，然而容易被其他方法取代。

  • 创建信号量dispatch_semaphore_create(long value)

相当于并行数，运行同时执行的线程数。

  • 获取信号量dispatch_semaphore_wait(dispatch_semaphore_t _Nonnull dsema, dispatch_time_t timeout)

如果信号量，返回0表示获取成功，否则阻塞主线程开始等待，超过等待时间后继续执行代码。

  • 增加信号量dispatch_semaphore_signal(dispatch_semaphore_t _Nonnull dsema)

信号量+1。

dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);

long a = dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW);

puts(a ? "1" : "0");

a = dispatch_semaphore_wait(semaphore, DISPATCH_TIME_NOW);

puts(a ? "1" : "0");

dispatch_semaphore_signal(semaphore);

for(int i = 0; i < 6; ++i) {

    dispatch_async(dispatch_get_global_queue(0, 0), ^{

        long a = dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        sleep(3);

        NSLog(a ? @"YES" : @"NO");

        dispatch_semaphore_signal(semaphore);

    });

}

puts("----------");
0

1

----------

2017-12-15 15:55:11.589061+0800 GCD-OC[4209:250818] NO

2017-12-15 15:55:14.593238+0800 GCD-OC[4209:250816] NO

2017-12-15 15:55:17.594262+0800 GCD-OC[4209:250817] NO

2017-12-15 15:55:20.597262+0800 GCD-OC[4209:250815] NO

2017-12-15 15:55:23.599650+0800 GCD-OC[4209:250850] NO

2017-12-15 15:55:26.603327+0800 GCD-OC[4209:250851] NO

7. dispatch_group

先完成前面的任务，再执行最后一个任务。相当于异步栅栏，但可以将任务给予任意队列。

创建groupdispatch_group_t group = dispatch_group_create();

添加任务

dispatch_group_async(dispatch_group_t  _Nonnull group, dispatch_queue_t  _Nonnull queue, ^{

    code

});

最终任务

dispatch_group_notify(dispatch_group_t  _Nonnull group, dispatch_queue_t  _Nonnull queue, ^{

    code

});
dispatch_queue_t cQue1 = dispatch_queue_create("q11", DISPATCH_QUEUE_CONCURRENT);

dispatch_queue_t cQue2 = dispatch_queue_create("q22", DISPATCH_QUEUE_CONCURRENT);

dispatch_group_t group = dispatch_group_create();

dispatch_group_async(group, cQue1, ^(){

    sleep(2);

    puts("1");

});

dispatch_group_async(group, cQue2, ^(){

    sleep(1);

    puts("2");

});

dispatch_group_notify(group, dispatch_get_main_queue(), ^(){

    puts("3");

});

puts("-------");
-------

2

1

3

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