c# – 为什么在CPU负载很高时阻止Thread.Start方法？

出于测试目的,我编写cpu压力程序：它只在M个线程中执行N个for循环.
我运行这个程序有大量的线程,比如说200.
但是在任务管理器中,我看到线程计数器不会超过一些小值,例如9,并且Thread.Start方法等待完成先前运行的线程.

这种行为看起来像一个ThreadPool行为,但我希望无需等待某些原因,无论如何都必须启动常规的System.Threading.Thread.

下面的代码将重现此问题并提供解决方法选项：

using System;
using System.Diagnostics;
using System.Threading;

namespace HeavyLoad
{
    class Program
    {
        static long s_loopsPerThread;
        static ManualResetEvent s_startFlag;

        static void Main(string[] args)
        {
            long totalLoops = (long)5e10;
            int threadsCount = 200; 

            s_loopsPerThread = totalLoops / threadsCount;

            Thread[] threads = new Thread[threadsCount];

            var watch = Stopwatch.StartNew();
            for (int i = 0; i < threadsCount; i++)
            {
                Thread t = new Thread(IntensiveWork);
                t.IsBackground = true;
                threads[i] = t;
            }
            watch.Stop();
            Console.WriteLine("Creating took {0} ms",watch.ElapsedMilliseconds);

            // *** Comment out s_startFlag creation to change the behavior ***
            // s_startFlag = new ManualResetEvent(false);

            watch = Stopwatch.StartNew();
            foreach (var thread in threads)
            {
                thread.Start();
            }
            watch.Stop();
            Console.WriteLine("Starting took {0} ms",watch.ElapsedMilliseconds);

            if (s_startFlag != null)
                s_startFlag.Set();

            watch = Stopwatch.StartNew();
            foreach (var thread in threads)
            {
                thread.Join();
            }
            watch.Stop();
            Console.WriteLine("Waiting took {0} ms",watch.ElapsedMilliseconds);

            Console.ReadLine();
        }

        private static void IntensiveWork()
        {
            if (s_startFlag != null)
                s_startFlag.WaitOne();

            for (long i = 0; i < s_loopsPerThread; i++)
            {
                // hot point
            }
        }
    }
}

情况1：如果注释了s_startFlag创建,则启动线程会立即开始高密集的cpu工作.在这种情况下,我有一个小的并发(大约9个线程)和我持有线程启动代码的所有时间：

Creating took 0 ms
Starting took 4891 ms
Waiting took 63 ms

情况2：但是如果我创建了s_startFlag,那么所有新线程都会等到它被设置.在这种情况下,我成功地同时启动所有200个线程并获得预期值：启动时间很短,工作时间很长,任务管理器中的线程数为200：

Creating took 0 ms
Starting took 27 ms
Waiting took 4733 ms

为什么线程拒绝在第一种情况下开始？我超过了什么样的限制？

系统：

>操作系统：Windows 7专业版
>框架：NET 4.6
> cpu：Intel Core2 Quad Q9550 @ 2.83GHz
> RAM：8 Gb

解决方法

我做了一些研究,现在我发现高cpu负载确实对线程启动时间有很大的影响.

第一：我将totalLoops设置为100倍的值,以便有更多的观察时间.我看到线程不受限制,但创建速度非常慢. 1个线程在1-2秒内启动！

第二：我使用SetThreadAffinityMask函数(https://sites.google.com/site/dotburger/threading/setthreadaffinitymask-1)将主线程显式绑定到cpu核心#0和工作线程到核心#1,#2,#3.

Stopwatch watch;
using (ProcessorAffinity.BeginAffinity(0))
{
    watch = Stopwatch.StartNew();
    for (int i = 0; i < threadsCount; i++)
    {
        Thread t = new Thread(IntensiveWork);
        t.IsBackground = true;
        threads[i] = t;
    }
    watch.Stop();
    Console.WriteLine("Creating took {0} ms",watch.ElapsedMilliseconds);
}

和

using (ProcessorAffinity.BeginAffinity(1,2,3))
{
    for (long i = 0; i < s_loopsPerThread; i++)
    {
    }
}

现在主线程有自己的专用cpu内核(在进程边界中)和工作线程在~10毫秒后开始(totalLoops = 5e10).

Creating took 0 ms
Starting took 2282 ms
Waiting took 3681 ms

另外,我在MSDN中找到了这句话：

When you call the Thread.Start method on a thread,that thread might
or might not start executing immediately,depending on the number of
processors and the number of threads currently waiting to execute.

https://msdn.microsoft.com/en-us/library/1c9txz50(v=vs.110).aspx

结论：Thread.Start方法对活跃工作线程的数量非常敏感.这可能是一个非常强大的性能影响 – 数百次减速.