为什么PLINQ只使用两个线程？ [英] Why does PLINQ use only two threads?

问题描述

说我有一个IO绑定的任务。我使用WithDegreeOfParallelism = 10，WithExecution = ForceParallelism模式，但仍是查询只使用两个线程。为什么呢？

我理解PLINQ通常会选择并行度等于我的核心数量，但为什么它忽略我的更高的并行具体要求？

 静态无效的主要（字串[] args）
{
    TestParallel（0.UpTo（8））;
}

私有静态无效TestParallel（IEnumerable的＆LT; INT＆GT;输入）
{
    VAR定时器=新的秒表（）;
    timer.Start（）;
    变种大小= input.Count（）;

    如果（input.AsParallel（）。
        WithDegreeOfParallelism（10）。
        WithExecutionMode（ParallelExecutionMode.ForceParallelism）。
        凡（ISODD）.Count之间的（）！=尺寸/ 2）
        抛出新的异常（无法计数的可能性）;

    timer.Stop（）;
    Console.WriteLine（测试+规模+号码+ timer.Elapsed.TotalSeconds +秒）;
}

私有静态布尔ISODD（INT N）
{
    Thread.sleep代码（1000）;
    回报N％2 == 1;
}
 

解决方案

PLINQ试图找到最佳线程数来执行，你想要它做的尽可能快的，如果你只对你的CPU的2个核心是什么，那数字是最有可能的2。如果你有一个四核，你会更有可能看到4个线程出现，但双核的机器上创建4个线程不会真的提高性能，因为只有2个线程可以在同一时间被激活

另外，对于基于IO的操作，它很可能是任何额外的线程将简单地阻止执行的第一个IO操作

Say I have an IO-bound task. I'm using WithDegreeOfParallelism = 10 and WithExecution = ForceParallelism mode, but still the query only uses two threads. Why?

I understand PLINQ will usually choose a degree of parallelism equal to my core count, but why does it ignore my specific request for higher parallelism?

static void Main(string[] args)
{
    TestParallel(0.UpTo(8));
}

private static void TestParallel(IEnumerable<int> input)
{
    var timer = new Stopwatch();
    timer.Start();
    var size = input.Count();

    if (input.AsParallel().
        WithDegreeOfParallelism(10).
        WithExecutionMode(ParallelExecutionMode.ForceParallelism).
        Where(IsOdd).Count() != size / 2)
        throw new Exception("Failed to count the odds");

    timer.Stop();
    Console.WriteLine("Tested " + size + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
}

private static bool IsOdd(int n)
{
    Thread.Sleep(1000);
    return n%2 == 1;
}

解决方案

PLINQ tries to find the optimal number of threads to perform what you want it to do as quickly as possible, if you only have 2 cores on your cpu, that number is most likely 2. If you had a quad core, you would be more likely to see 4 threads appear, but creating 4 threads on a dual core machine wouldn't really improve performance because only 2 threads could be active at the same time.

Also, with IO-based operations, it is likely that any extra threads would simply block on the first IO operation performed.

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