如果条件满足，ParallelFor 不会立即取消所有线程答案

【问题标题】：ParallelFor not cancelling all threads immediately if condition met如果条件满足，ParallelFor 不会立即取消所有线程
【发布时间】：2013-05-02 18:03:16
【问题描述】：

代码总是等到当前运行的任务完成后才会抛出 OperationCancelledException。

我希望程序在条件为真时立即停止。

static void Main()
{
    // want to break out of a Parallel.For immediately when a condition occurs
    var cts = new CancellationTokenSource();
    var po = new ParallelOptions();
    po.CancellationToken = cts.Token;

    long counterTotal = 0;

    try
    {
        // want to have a sum of counts at the end
        Parallel.For<long>(1, 26, po, () => 0, delegate(int i, ParallelLoopState state, long counterSubtotal)
                {
                    po.CancellationToken.ThrowIfCancellationRequested();
                    Console.WriteLine(i.ToString());

                    for (int k = 0; k < 1000000000; k++)
                    {
                        counterSubtotal++;

                        if (i == 4 && k == 900000000)
                        {
                            cts.Cancel();
                            // Would like to break out here immediately
                        }
                    }

                    return counterSubtotal;
                }, (x) => Interlocked.Add(ref counterTotal, x)
            );
    }
    catch (OperationCanceledException e)
    {
        Console.WriteLine("Cancelled");
        Console.WriteLine("Total iterations across all threads {0}", String.Format("{0:n0}", counterTotal));
        Console.ReadLine();
    }
}

我发现在cts.Cancel() 上设置了一个断点，并且在 catch 中演示了正在发生的事情。

也看过state.Stop。

这是其他代码的简化版本。

如果我们想立即突破，也许Parallel.For 不适合在方法中运行很长时间的事情。

更新 2： 代码现在可以按预期工作并给出一个很好的总数

static void Main()
{
    // want to break out of a Parallel.For immediately when a condition occurs
    var cts = new CancellationTokenSource();
    var po = new ParallelOptions();
    po.CancellationToken = cts.Token;

    long counterTotal = 0;

    try
    {
        // want to have a sum of counts at the end
        // using type param here to make counterSubtotal a long
        Parallel.For<long>(1, 26, po, () => 0, delegate(int i, ParallelLoopState state, long counterSubtotal)
                {
                    Console.WriteLine(i.ToString());
                    // 1 billion
                    for (int k = 0; k < 1000000000; k++)
                    {
                        //po.CancellationToken.ThrowIfCancellationRequested();
                        if (po.CancellationToken.IsCancellationRequested)
                        {
                            return counterSubtotal;
                        }
                        counterSubtotal++;

                        if (i == 4 && k == 400000000)
                        {
                            Console.WriteLine("Inner Cancelled");
                            cts.Cancel();
                        }
                    }
                    return counterSubtotal;
                }, (x) => Interlocked.Add(ref counterTotal, x)
            );
    }
    catch (OperationCanceledException e)
    {
        Console.WriteLine("Cancelled");
        Console.WriteLine("Total iterations across all threads {0}", String.Format("{0:n0}", counterTotal));
        Console.ReadLine();
    }
}

【问题讨论】：

标签： c# task-parallel-library parallel.foreach cancellationtokensource

【解决方案1】：

如果您希望它“立即”中断，而不是您需要检查内部的取消令牌。就像现在一样，它会在进入之前检查取消，但之后它不会再查看令牌。

for (int k = 0; k < 1000000000; k++)
{
    po.CancellationToken.ThrowIfCancellationRequested();
    counterSubtotal++;

    if (i == 4 && k == 900000000)
    {
       cts.Cancel();

    }
}

【讨论】：