再次使用联锁

本文关键字： | 更新日期: 2023-09-27 18:20:44

我读了很多关于这一点的答案，但我还没有找到解决方案。

我有一个具有counter属性的类，它的缓存值有问题。即使是波动性似乎也不起作用：

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，看起来onTimer缓存_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.

我为什么要到16岁？我通过更改一点代码来解决问题：

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

但为什么会有这种行为呢？

再次使用联锁

Task.Run不会立即启动任务。它将任务添加到线程池队列中并返回。

在您的情况下，整个for循环在新任务开始运行之前执行，因此_threadsCounter不会发生任何更改。这就是为什么volatile没有帮助。

您有效地测试了实际启动的任务数量，并使计数器递增。这需要一段时间，所以基本上你要创建所有8个任务并启动它们，然后它们会递增计数器。。。到那时，你还没有注意到你已经超过10个了。

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...)
}