的Parallel.For（）：循环外更新变量 [英] Parallel.For(): Update variable outside of loop

问题描述

我只是在寻找到新的.NET 4.0功能。就这样，我试图用一个简单的计算的Parallel.For 和正常为（X，X，X）循环。

I'm just looking in to the new .NET 4.0 features. With that, I'm attempting a simple calculation using Parallel.For and a normal for(x;x;x) loop.

不过，我得到不同的结果，约50％的时间。

However, I'm getting different results about 50% of the time.

long sum = 0;

Parallel.For(1, 10000, y =>
    {
        sum += y;
    }
);

Console.WriteLine(sum.ToString());

sum = 0;

for (int y = 1; y < 10000; y++)
{
   sum += y;
}
Console.WriteLine(sum.ToString());

我的猜测是，线程试图更新求和在同一时间。
有没有办法解决它的一个明显的方式？

My guess is that the threads are trying to update "sum" at the same time.
Is there an obvious way around it?

推荐答案

您无法做到这一点。 之正在穿过你的并行线程共享。你需要确保的之变量只由一个线程在同一时间访问：

You can't do this. sum is being shared across you parallel threads. You need to make sure that the sum variable is only being accessed by one thread at a time:

// DON'T DO THIS!
Parallel.For(0, data.Count, i =>
{
    Interlocked.Add(ref sum, data[i]);
});

但...这是一个反模式，因为你已经有效地序列化的循环，因为每个线程都在锁定 Interlocked.Add 。

您需要做的就是添加子汇总，并在年底这样的合并它们：

What you need to do is add sub totals and merge them at the end like this:

Parallel.For<int>(0, result.Count, () => 0, (i, loop, subtotal) =>
    {
        subtotal += result[i];
        return subtotal;
    },
    (x) => Interlocked.Add(ref sum, x)
);

您可以找到这个MSDN上进一步讨论：<一href="http://msdn.microsoft.com/en-us/library/dd460703.aspx">http://msdn.microsoft.com/en-us/library/dd460703.aspx

You can find further discussion of this on MSDN: http://msdn.microsoft.com/en-us/library/dd460703.aspx

插头：：您可以找到更多关于这在指南并行编程

PLUG: You can find more on this in Chapter 2 on A Guide to Parallel Programming

下面是也绝对值得一读...

The following is also definitely worth a read...

<一个href="http://www.microsoft.com/downloads/details.aspx?FamilyID=86b3d32b-ad26-4bb8-a3ae-c1637026c3ee&displaylang=en">Patterns并行编程：了解和应用并行模式与.NET Framework 4 - 斯蒂芬Toub

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