为什么 Thread.Start 方法在 CPU 负载高时被阻塞？

Question

出于测试目的，我编写了 CPU 压力程序：它只在 M 个线程中执行 N 个 for 循环。我运行这个程序有很多线程，比如 200。但是在任务管理器中我看到线程计数器不超过一些小值，比如 9 和一个 Thread.Start 方法 等待完成前一个运行宁线程 .

此行为似乎是线程池行为，但我希望常规 System.Threading.Thread 无论如何都必须启动，而无需等待某些原因。

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
            }
        }
    }
}

情况一：如果注释s_startFlag创建，则启动线程立即开始高强度CPU工作。在这种情况下，我有一个小的并发（大约 9 个线程）并且我一直坚持线程启动代码：

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

情况2：但是如果我创建s_startFlag，所有新线程都会等到它被设置。在这种情况下，我成功地同时启动了所有 200 个线程并获得了预期值：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

为什么线程在第一种情况下拒绝启动？我超过了什么样的限制？

系统：

OS: Windows 7 专业
框架：NET 4.6
CPU：Intel Core2 四核 Q9550 @ 2.83GHz
内存：8 Gb

Answer 1

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影响。

首先：为了有更多的观察时间，我将 totalLoops 设置为大 100 倍的值。我看到线程不受限制但创建速度非常慢。 1 线程在 1-2 秒内启动！

其次：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 并将工作线程绑定到核心 #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++)
    {
    }
}

现在主线程有自己专用的 CPU 核心（在进程边界）和工作线程在 ~10 毫秒后启动（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 方法对活动线程数非常敏感。这可能会对性能产生非常大的影响 - 减慢数百倍。

为什么 Thread.Start 方法在 CPU 负载高时被阻塞？

Why Thread.Start method is blocked when CPU load is high?

c#

benchmarking

multithreading