TPL 数据并行问题

Question

我有一个情况是并行处理这组数据，最后我想知道总共有多少个数据处理成功了。我按照http://msdn.microsoft.com/en-us/library/dd460703.aspx 和http://reedcopsey.com/2010/01/22/parallelism-in-net-part-4-imperative-data-parallelism-aggregation/ 的示例提供了以下虚拟代码

    public void DoWork2()
    {
        int sum = 0;
        Parallel.For<int>(0, 10,
            () => 0,
            (i, lockState, localState) =>
            {
                DummyEntity entity = DoWork3(i);
                if (entity != null)
                {
                    Console.WriteLine("Processed {0}, sum need to be increased by 1.", i);
                    return 1;
                }
                else
                {
                    Console.WriteLine("Processed {0}, sum need to be increased by 0.", i);
                    return 0;
                }
            },
            localState =>
            {
                lock (syncRoot)
                {
                    Console.WriteLine("Increase sum {0} by {1}", sum, localState);
                    sum += localState;
                }
            }
            );
        Console.WriteLine("Total items {0}", sum);
    }

    private DummyEntity DoWork3(int i)
    {
        if (i % 2 == 0)
        {
            return new DummyEntity();
        }
        else
        {
            return null;
        }
    }

但是每次我运行时结果都会发生变化。我认为代码有问题。但不知道为什么。

Answer 1

您的问题是您对重载的选择。您已存储本地状态信息以尽量减少使用全局状态，但您并未使用本地状态。

如果您从给出的示例中注意到，他们在循环主体中使用了小计（您称之为 localState）：

subtotal += nums[j];
return subtotal;

将此与您的代码进行比较（更简洁一些）：

if (entity != null)
{
    return 1;
}
else
{
    return 0;
}

那里没有提到localState，所以你实际上已经丢弃了一些答案。如果您将其改为阅读：

if (entity != null)
{
    return localState + 1;
}
else
{
    return localState;
}

您会在命令行上找到以下答案（针对这个给定的问题）：

Total items 5

本地状态的这种用法是为了减少对共享状态的访问。

这是使用 0..50 作为范围的 sn-p：

Processed 22, sum need to be increased by 1.
Processed 23, sum need to be increased by 0.
Increase sum 0 by 1
Processed 8, sum need to be increased by 1.
Processed 9, sum need to be increased by 0.
Processed 10, sum need to be increased by 1.
Processed 11, sum need to be increased by 0.
Increase sum 1 by 2
Increase sum 3 by 8
Increase sum 11 by 10
Processed 16, sum need to be increased by 1.
Processed 17, sum need to be increased by 0.
Processed 18, sum need to be increased by 1.
Increase sum 21 by 4
Total items 25