并发：线程池异步执行与创建单独的线程执行

线程池与并行度

了解线程池如何工作于大量的异步操作，以及它与创建大量单独的线程的方式的不同之处。

实例代码验证

        static void Main(string[] args)
        {
            const int numberOfOperations = 500;
            var sw = new Stopwatch();
            sw.Start();
            UseThreads(numberOfOperations);
            sw.Stop();
            Console.WriteLine("Thread Execution time using threads: {0}", sw.ElapsedMilliseconds);

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

            Console.ReadKey();
        }

        static void UseThreads(int numberOfOperations)
        {
            using (var countdown = new CountdownEvent(numberOfOperations))//并发执行个数
            {
                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();//注册当前线程
                    });
                    //thread.IsBackground = true;
                    thread.Start();
                }
                countdown.Wait();
                Console.WriteLine();
            }
        }

        static void UseThreadPool(int numberOfOperations)
        {
            using (var countdown = new CountdownEvent(numberOfOperations))//并发执行个数
            {
                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();//注册当前线程
                    });
                }
                countdown.Wait();
                Console.WriteLine();
            }
        }

Thread Execution time using threads: 5211
ThreadPool Execution time using threads: 5948

结论

线程池为操作系统节省了内存和线程数，但是也为此付出了更长的执行时间。