1

我在这一点上阅读了很多答案，但我还没有找到解决方案。

我有一个带有计数器属性的类，它的缓存值有问题。甚至 volatile 似乎也不起作用：

public class MyClass {
    private Timer _timer;
    private int _threadsCounter = 0;
    public StreamWriter Tracer { get; set; }

    public MyClass() {
        _timer = new Timer(1000.0 * 10);
        _timer.AutoReset = true;
        _timer.Elapsed += new ElapsedEventHandler(OnTimer);
        _timer.Start();
    }

    private void OnTimer(object sender, ElapsedEventArgs e) {
        HashSet<Task> taskPool = new HashSet<Task>();
        try {
            if (Tracer != null) Tracer.WriteLine("[{0}] onTimer start. Current threads counter is {1}.", DateTime.Now, _threadsCounter);
            if (_threadsCounter >= 10) return;

            // create parallel tasks
            for (int i = 0; i < 8; i++) {
                // limit on the max num of parallel processing but the counter remains unchanged during this timer event!!!
                if (_threadsCounter >= 10) break;

                var timeout = (30 + i * 2);
                var task = Task.Run(() => {
                        var localCounter = System.Threading.Interlocked.Increment(ref _threadsCounter);
                        try {
                            System.Threading.Thread.Sleep(timeout * 1000);
                        }
                        finally {
                            System.Threading.Interlocked.Decrement(ref _threadsCounter);
                        }
                    });
                taskPool.Add(task);
            }

        }
        finally {
            if (Tracer != null) 
                Tracer.WriteLine("[{0}] onTimer end. Created {1} tasks. Current threads counter is {2}.", DateTime.Now, taskPool.Count, _threadsCounter);
        }
    }

好吧，似乎 onTimer 缓存了 _threadsCounter 变量，因为输出是：

[14:10:47] onTimer start. Current threads counter is 0.
[14:10:47] onTimer end. Created 8 tasks. Current threads counter is 0.

[14:10:57] onTimer start. Current threads counter is 8.
[14:10:57] onTimer end. Created 8 tasks. Current threads counter is 8.

[14:11:07] onTimer start. Current threads counter is 16.
[14:11:07] onTimer end. Created 0 tasks. Current threads counter is 16.

[14:11:17] onTimer start. Current threads counter is 15.
[14:11:17] onTimer end. Created 0 tasks. Current threads counter is 15.

[14:11:37] onTimer start. Current threads counter is 4.
[14:11:37] onTimer end. Created 8 tasks. Current threads counter is 4.

[14:11:47] onTimer start. Current threads counter is 8.
[14:11:47] onTimer end. Created 8 tasks. Current threads counter is 8.

[14:11:57] onTimer start. Current threads counter is 16.
[14:11:57] onTimer end. Created 0 tasks. Current threads counter is 16.

为什么我到了 16 岁？我通过更改代码解决了这个问题：

var localCounter = _threadsCounter;
...
if ((localCounter + taskPool.Count) >= 10) break;

但为什么会有这种行为？

4

2 回答 2

4

Task.Run不会立即启动任务。它将任务添加到线程池队列并返回。

在您的情况下，整个for循环在新任务开始运行之前执行，所以没有任何变化_threadsCounter。这就是为什么volatile没有帮助。

于 2015-11-05T13:46:25.447 回答
1

您正在有效地测试实际开始并达到递增计数器的任务数量。这需要一点时间 - 所以基本上你正在创建所有 8 个任务并启动它们，然后它们正在增加计数器......到那时注意到你有超过 10 个已经太晚了。

更好的解决方案是在开始任务之前增加计数器：

// Increment the counter in expectation of starting a task
var localCounter = Interlocked.Increment(ref _threadsCounter);
if (localCounter >= 10)
{
    // Ah, we're not going to start a task after all, so undo
    // the increment
    Interlocked.Decrement(ref _threadsCounter);
    break;
}
else 
{
    // Start a task, which will decrement the counter at the end.
    // (You could add the "decrement" bit as a continuation, even...)
}
于 2015-11-05T13:46:41.093 回答