C#多线程编程笔记-线程池ThreadPool与并行度

近来在学习Eugene Agafonov编写的《C#多线程编程实战》（译），做些笔记也顺便分享一下^-^

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

namespace 线程池与并行度
{
    class Program
    {
        static void Main(string[] args)
        {
            const int numberOfOperations = 500;
            var sw = new Stopwatch();
            sw.Start();
            UseThreads(numberOfOperations);
            sw.Stop();
            Console.WriteLine("Execution time using threads: {0}", sw.ElapsedMilliseconds);

            sw.Reset();
            sw.Start();
            UseThreadPool(numberOfOperations);
            sw.Stop();
            Console.WriteLine("Execution time using threads: {0}", sw.ElapsedMilliseconds);

            Console.ReadKey();
        }

        static void UseThreads(int numberOfOperations)
        {
            using(var countdown=new CountdownEvent(numberOfOperations))
            {
                Console.WriteLine("Scheduling work by creating threads");
                for (int i = 0; i < numberOfOperations; i++)
                {
                    var thread = new Thread(() =>
                      {
                          Console.Write("{0},", Thread.CurrentThread.ManagedThreadId);
                          Thread.Sleep(TimeSpan.FromSeconds(0.1));
                          countdown.Signal();
                      });
                    thread.Start();
                }
                countdown.Wait();
                Console.WriteLine();
            }
        }

        static void UseThreadPool(int numberOfOperations)
        {
            using(var countdown=new CountdownEvent(numberOfOperations))
            {
                Console.WriteLine("Starting work on a threadpool");
                for (int i = 0; i < numberOfOperations; i++)
                {
                    ThreadPool.QueueUserWorkItem(_ =>
                    {
                        Console.Write("{0},", Thread.CurrentThread.ManagedThreadId);
                        Thread.Sleep(TimeSpan.FromSeconds(0.1));
                        countdown.Signal();
                    });
                }
                countdown.Wait();
                Console.WriteLine();
            }
        }

    }
}

当主程序启动时，创建了很多不同的线程，每个线程都运行一个操作。该操作打印出线程ID并阻塞线程100毫秒。结果我们创建了500个线程，全部并行运行这些操作。虽然在我的机器上总耗时是300毫秒，但所有的线程消耗了大量的操作系统资源。

然后我们使用了执行同样的任务，只不过将每个任务都加入到线程池中，在线程池中执行这些操作。线程池在快结束时创建了更多的线程，但仍然花费了更多的时间。我们为操作系统节省了内存和线程数，但是为此付出了更长的执行时间！