objective-c - 一个线程休眠时使用GCD的并发队列调度规则

Question

测试代码如下:

dispatch_queue_t queue = dispatch_queue_create("sc", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{
    [NSThread sleepForTimeInterval:10];
    NSLog(@"10s --- %@", [NSThread currentThread]);
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), queue, ^{
    NSLog(@"First: 5s --- %@", [NSThread currentThread]);
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), queue, ^{
    NSLog(@"Second: 5s --- %@", [NSThread currentThread]);
});

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), queue, ^{
    NSLog(@"Third: 5s --- %@", [NSThread currentThread]);
});

这里我创建了一个并发队列，并使用异步方式让一个线程休眠。

但是，输出是:

2016-03-26 12:17:57.164 TestPlayground[28188:551287] 10s --- <NSThread: 0x7fa080511730>{number = 2, name = (null)}
2016-03-26 12:17:57.165 TestPlayground[28188:551287] First: 5s --- <NSThread: 0x7fa080511730>{number = 2, name = (null)}
2016-03-26 12:17:57.166 TestPlayground[28188:551307] Second: 5s --- <NSThread: 0x7fa080704a80>{number = 3, name = (null)}
2016-03-26 12:17:57.166 TestPlayground[28188:551301] Third: 5s --- <NSThread: 0x7fa080511530>{number = 4, name = (null)}

GCD 似乎阻塞了第一个线程 10 秒，然后它开始为并发工作创建新线程。

但是如果我只是将自己创建的队列替换为全局并发队列并且不更改以下作业

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
// ... same as before

输出更合理

2016-03-26 12:23:29.496 TestPlayground[28320:558467] First: 5s --- <NSThread: 0x7ff473c08b80>{number = 4, name = (null)}
2016-03-26 12:23:29.496 TestPlayground[28320:558483] Second: 5s --- <NSThread: 0x7ff473f18c50>{number = 2, name = (null)}
2016-03-26 12:23:29.496 TestPlayground[28320:558471] Third: 5s --- <NSThread: 0x7ff473d0a7c0>{number = 3, name = (null)}
2016-03-26 12:23:34.030 TestPlayground[28320:558456] 10s --- <NSThread: 0x7ff473e218e0>{number = 5, name = (null)}

对于这种工作，GCD 中的调度规则是什么？为什么自建并发队列与全局队列不同？

---

如果我在 sleep 作业之前添加一个新作业

dispatch_queue_t queue = dispatch_queue_create("sc", DISPATCH_QUEUE_CONCURRENT);

dispatch_async(queue, ^{ NSLog(@"start --- %@", [NSThread currentThread]); });

dispatch_async(queue, ^{
    [NSThread sleepForTimeInterval:10];
    NSLog(@"10s --- %@", [NSThread currentThread]);
});
// ... same as before

输出是

2016-03-26 12:44:25.728 TestPlayground[28616:571502] start --- <NSThread: 0x7fb7c1604250>{number = 2, name = (null)}
2016-03-26 12:44:35.733 TestPlayground[28616:571511] 10s --- <NSThread: 0x7fb7c1422e40>{number = 3, name = (null)}
2016-03-26 12:44:35.734 TestPlayground[28616:571517] First: 5s --- <NSThread: 0x7fb7c16032d0>{number = 4, name = (null)}
2016-03-26 12:44:35.734 TestPlayground[28616:571511] Second: 5s --- <NSThread: 0x7fb7c1422e40>{number = 3, name = (null)}
2016-03-26 12:44:35.734 TestPlayground[28616:571552] Third: 5s --- <NSThread: 0x7fb7c1608950>{number = 5, name = (null)}

Answer 1

考虑以下输出:

00:53:38.852 asdfasdfasdf[11650:2114104] start
00:53:38.854 asdfasdfasdf[11650:2114104] main <NSThread: 0x100206110>{number = 1, name = main}
00:53:38.854 asdfasdfasdf[11650:2114126] Enter sleep block
00:53:38.854 asdfasdfasdf[11650:2114127] bare async
00:53:48.858 asdfasdfasdf[11650:2114126] Sleep block done (10 seconds).
00:53:48.859 asdfasdfasdf[11650:2114126] dispatch_after() 5 --- <NSThread: 0x10020aa00>{number = 2, name = (null)}

因此， sleep 线程不会阻止异步排队 block 的调用(即使线程 sleep 被抛出)，但它会阻止 dispatch_after() 的执行。

这让我很困惑。 rdar://25373048

    dispatch_queue_t queue = dispatch_queue_create("sc", DISPATCH_QUEUE_CONCURRENT);

    NSLog(@"start");
    dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        NSLog(@"main %@", [NSThread currentThread]);

        dispatch_async(queue, ^{
            NSLog(@"Enter sleep block");
            [NSThread sleepForTimeInterval:10];
            NSLog(@"Sleep block done (10 seconds).");
        });

        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(5 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"dispatch_after() 5 --- %@", [NSThread currentThread]);
        });

        dispatch_async(queue, ^{
            NSLog(@"bare async");
        });
    });

    [[NSRunLoop currentRunLoop] run];