如果我使用异步，不应该使用更少的线程数吗？

Question

我的理解是，如果我使用异步，线程会发出 Web 请求并继续前进。当响应返回时，另一个线程会从那里获取它。因此，闲置的绑定线程数量较少。 这不是意味着最大活动线程数会下降吗？但在下面的示例中，不使用异步的代码最终会使用较少数量的线程。谁能解释一下原因？

没有异步的代码（使用较少的线程）：

using System;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Threading;

namespace NoAsync
{
    internal class Program
    {
        private const int totalCalls = 100;

        private static void Main(string[] args)
        {
            for (int i = 1; i <= totalCalls; i++)
            {
                ThreadPool.QueueUserWorkItem(GoogleSearch, i);
            }

            Thread.Sleep(100000);
        }

        private static void GoogleSearch(object searchTerm)
        {
            Thread.CurrentThread.IsBackground = false;
            string url = @"https://www.google.com/search?q=" + searchTerm;
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
            WebRequest wr = WebRequest.Create(url);
            var httpWebResponse = (HttpWebResponse) wr.GetResponse();
            var reader = new StreamReader(httpWebResponse.GetResponseStream());
            string responseFromServer = reader.ReadToEnd();
            //Console.WriteLine(responseFromServer); // Display the content.
            httpWebResponse.Close();
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
        }
    }
}

异步代码（使用更多线程）

using System;
using System.Diagnostics;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using System.Collections.Generic;

namespace AsyncAwait
{
    internal class Program
    {
        private const int totalCalls = 100;
        private static DateTime start = System.DateTime.Now;

    private static void Main(string[] args)
    {
        var tasks = new List<Task>();

        for (int i = 1; i <= totalCalls; i++)
        {
            var searchTerm = i;
            var t = GoogleSearch(searchTerm);
            tasks.Add(t);
        }

        Task.WaitAll(tasks.ToArray());
        Console.WriteLine("Hit Enter to exit");
        Console.ReadLine();
    }

    private static async Task GoogleSearch(object searchTerm)
    {
        Thread.CurrentThread.IsBackground = false;
        string url = @"https://www.google.com/search?q=" + searchTerm;
        Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
        using (var client = new HttpClient())
        {
            using (HttpResponseMessage response = await client.GetAsync(url))
            {
                HttpContent content = response.Content;
                content.Dispose();
                Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
                Console.WriteLine("TimeSpan consumed {0}", System.DateTime.Now.Subtract(start));
            }
        }
    }
}

}

我明白我的结果包括非托管线程。但是总线程数不应该还少吗？

更新：我用 Noseratio 提供的代码更新了异步调用

Answer 1

ThreadPool 实际上维护了两个子池，一个用于工作线程，另一个用于 IOCP 线程。当GetAsync 的结果可用时，会从池中分配一个随机 IOCP 线程（I/O 完成端口）来处理异步 HTTP 请求的完成。这就是await 之后的代码被执行的地方。您可以使用ThreadPool.SetMinThreads/SetMaxThreads 控制每个子池的大小，下面将对此进行详细介绍。

事实上，您的非异步代码很难与异步版本相提并论。为了更公平的比较，你应该坚持WebRequest这两种情况，例如：

非异步：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

namespace NoAsync
{
    internal class Program
    {
        private const int totalCalls = 100;

        private static void Main(string[] args)
        {
            int maxWorkers, maxIOCPs;
            ThreadPool.GetMaxThreads(out maxWorkers, out maxIOCPs);
            int minWorkers, minIOCPs;
            ThreadPool.GetMinThreads(out minWorkers, out minIOCPs);
            Console.WriteLine(new { maxWorkers, maxIOCPs, minWorkers, minIOCPs });

            ThreadPool.SetMinThreads(100, 100);

            var tasks = new List<Task>();

            for (int i = 1; i <= totalCalls; i++)
                tasks.Add(Task.Run(() => GoogleSearch(i)));

            Task.WaitAll(tasks.ToArray());
        }

        private static void GoogleSearch(object searchTerm)
        {
            string url = @"https://www.google.com/search?q=" + searchTerm;
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
            WebRequest wr = WebRequest.Create(url);
            var httpWebResponse = (HttpWebResponse)wr.GetResponse();
            var reader = new StreamReader(httpWebResponse.GetResponseStream());
            string responseFromServer = reader.ReadToEnd();
            //Console.WriteLine(responseFromServer); // Display the content.
            reader.Close();
            httpWebResponse.Close();
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
        }
    }
}

异步：

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

namespace Async
{
    internal class Program
    {
        private const int totalCalls = 100;

        private static void Main(string[] args)
        {
            int maxWorkers, maxIOCPs;
            ThreadPool.GetMaxThreads(out maxWorkers, out maxIOCPs);
            int minWorkers, minIOCPs;
            ThreadPool.GetMinThreads(out minWorkers, out minIOCPs);
            Console.WriteLine(new { maxWorkers, maxIOCPs, minWorkers, minIOCPs });

            ThreadPool.SetMinThreads(100, 100);

            var tasks = new List<Task>();

            for (int i = 1; i <= totalCalls; i++)
                tasks.Add(GoogleSearch(i));

            Task.WaitAll(tasks.ToArray());
        }

        private static async Task GoogleSearch(object searchTerm)
        {
            string url = @"https://www.google.com/search?q=" + searchTerm;
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
            WebRequest wr = WebRequest.Create(url);
            var httpWebResponse = (HttpWebResponse) await wr.GetResponseAsync();
            var reader = new StreamReader(httpWebResponse.GetResponseStream());
            string responseFromServer = await reader.ReadToEndAsync();
            //Console.WriteLine(responseFromServer); // Display the content.
            reader.Close();
            httpWebResponse.Close();
            Console.WriteLine("Total number of threads in use={0}", Process.GetCurrentProcess().Threads.Count);
        }
    }
}

默认情况下，我在系统上看到的线程池如下图：

{ maxWorkers = 32767, maxIOCPs = 1000, minWorkers = 4, minIOCPs = 4 }

线程池在增长线程时惰性。新线程的创建最多可以延迟 500 毫秒（有关更多详细信息，请查看 Joe Duffy 的"CLR thread pool injection, stuttering problems"）。

要解决此问题，请使用 ThreadPool.SetMinThreads。对于SetMinThreads(100, 100)，我看到 sync 版本的峰值 ~111 个线程，async 版本的峰值约为 20 个线程（发布构建，在没有调试器的情况下运行）。代表异步版本，这是一个相当大的指示性差异。