Question

简短问题：

为什么.Net Framework添加了很多* Async版本的方法而不是开发人员只使用Task.Run异步运行同步方法？

详细问题：

我理解异步性的概念。
我知道Tasks
我知道async / await关键字。
我知道.Net Framework中的*异步方法。

我不明白的是库中* Async方法的目的。

假设您有两行代码：

F1();
F2();

关于数据/控制流程，只有两种情况：

F2需要在F1完成后执行。
F2无需等待F1完成。

我没有看到任何其他情况。我没有看到任何一般需要知道执行某些功能的具体线程（除了UI）。线程中代码的基本执行模式是同步的。并行性需要多个线程。异步性基于并行性和代码重新排序。但基数仍然是同步的。

F1的工作量很小时，差异无关紧要。但是当A花费大量时间完成时，我们可能需要查看情况，如果F2不需要等待F1完成，我们可以运行F1与F2平行。

很久以前我们使用线程/线程池来做到这一点。现在我们有Tasks。

如果我们想并行运行F1和F2，我们可以写一下：

var task1 = Task.Run(F1);
F2();

任务很酷，我们可以在最终需要完成任务的地方使用await。

到目前为止，我认为没有必要制作F1Async()方法。

现在，我们来看一些特殊情况。我看到的唯一真正特殊情况是UI。 UI线程是特殊的并且停止它会使UI冻结很糟糕。在我看来，Microsoft建议我们标记UI事件处理程序async。标记方法async意味着我们可以使用await关键字基本上在另一个线程上安排繁重的处理并释放UI线程，直到处理完成。

我不能再得到的是为什么我们需要任何* Async方法来等待它们。我们总是可以写await Task.Run(F1);。我们为什么需要F1Async？

你可能会说* Async方法使用一些特殊的魔法（比如处理外部信号），使它们比同步方法更有效。问题是，我不认为这是事实。

让我们看一下Stream.ReadAsync。如果查看源代码，ReadAsync只会浪费数百行铃声和口哨代码来创建一个只调用同步Read方法的任务。那为什么我们需要呢？为什么不将Task.Run与Stream.Read一起使用？

这就是为什么我不理解通过创建同步方法的简单*异步副本来膨胀库的原因。 MS甚至可以添加语法糖，这样我们就可以写await async Stream.Read而不是await Stream.ReadAsync或Task.Run(Stream.Read)。

现在您可能会问“为什么不将* Async方法作为唯一方法并删除同步方法？”。正如我之前所说，基本代码执行模式是同步的。异步运行同步方法很容易，但不是另一种方式。

那么，.Net Framework中* Async方法的目的是什么，因为能够使用Task.Run异步运行任何方法？

P.S。如果非冻结UI非常重要，为什么不默认运行处理程序异步并防止任何冻结的可能性？

“无线程”参数：

回答这个问题的人似乎暗示* Async方法的优势在于它们非常有效，因为它们不会创建新线程。问题是我没有看到这样的行为。并行异步任务的行为与我想的一样 - 为每个并行任务创建（或从线程池中获取）线程（并非所有任务都是并行执行）。

这是我的测试代码：

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

namespace ConsoleApplication32167 {
    class Program {
        static async Task TestAsync() {
            var httpClient = new HttpClient() { Timeout = TimeSpan.FromMinutes(20) };

            var tasks = Enumerable.Range(1, 100).Select((i) =>
                httpClient.GetStringAsync("http://localhost/SlowWebsite/"));

            Console.WriteLine("Threads before completion: " + Process.GetCurrentProcess().Threads.Count);

            await Task.WhenAll(tasks);

            Console.WriteLine("Threads after completion: " + Process.GetCurrentProcess().Threads.Count);
        }

        static void Main(string[] args) {
            Console.WriteLine("Threads at start: " + Process.GetCurrentProcess().Threads.Count);

            var timer = new Stopwatch();
            timer.Start();

            var testTask = TestAsync();

            var distinctThreadIds = new HashSet<int>();
            while (!testTask.IsCompleted) {
                var threadIds = Process.GetCurrentProcess().Threads.OfType<ProcessThread>().Select(thread => thread.Id).ToList();
                distinctThreadIds.UnionWith(threadIds);
                Console.WriteLine("Current thread count: {0}; Cumulative thread count: {1}.", threadIds.Count, distinctThreadIds.Count);
                Thread.Sleep(250);
            }

            testTask.Wait();

            Console.WriteLine(timer.Elapsed);
            Console.ReadLine();
        }
    }
}

此代码尝试运行100 HttpClient.GetStringAsync个任务，向网站发出请求，需要1分钟才能响应。同时，它计算活动线程的数量和进程创建的不同累积数量。正如我所预测的，这个程序会创建许多新线程。输出如下：

Current thread count: 4; Cumulative thread count: 4.
....
Current thread count: 25; Cumulative thread count: 25.
....
Current thread count: 7; Cumulative thread count: 63.
Current thread count: 9; Cumulative thread count: 65.
00:10:01.9981006

这意味着：

在异步任务执行过程中创建了61个新线程。
新活动线程的峰值数为21。
~~执行需要的时间多10倍（10分钟而不是1分钟）。~~ 这是由本地IIS限制引起的。

Answer 1

将方法标记为异步意味着我们可以使用await关键字基本上在另一个线程上安排繁重的处理并释放UI线程，直到处理完成。

这根本不是async的工作方式。查看我的async intro。

你可能会说* Async方法使用一些特殊的魔法（比如处理外部信号），使它们比同步方法更有效。问题是，我不认为这是事实。

在纯异步代码中，there is no thread（正如我在博客中解释的那样）。实际上，在设备驱动程序级别，所有（非平凡）I / O是异步的。它是同步API（在操作系统级别），是自然异步API的抽象层。

让我们看一下Stream.ReadAsync。

Stream是一个不寻常的案例。作为基类，它必须尽可能地防止破坏性更改。因此，当他们添加虚拟ReadAsync方法时，他们必须添加默认实现。此实现必须使用非理想的实现（Task.Run），这是不幸的。在一个理想的世界中，ReadAsync将是（或调用）一个抽象的异步实现，但这会破坏Stream的每个现有实现。

有关更恰当的示例，请比较WebClient和HttpClient之间的差异。

Answer 2

让我们进行真实的测试：自然异步WebRequest.GetResponseAsync与非自然同步WebRequest.GetResponse。

首先，我们扩展ThreadPool：

的标准限制

ThreadPool.SetMaxThreads(MAX_REQS * 2, MAX_REQS * 2);
ThreadPool.SetMinThreads(MAX_REQS, MAX_REQS);

注意我请求相同数量的workerThreads和completionPortThreads。然后，我们将使用每个API向bing.com执行MAX_REQS = 200个并行请求。

代码（独立控制台应用）：

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

namespace Console_21690385
{
    class Program
    {
        const int MAX_REQS = 200;

        // implement GetStringAsync
        static async Task<string> GetStringAsync(string url)
        {
            using (var response = await WebRequest.Create(url).GetResponseAsync())
            using (var stream = response.GetResponseStream())
            using (var reader = new System.IO.StreamReader(stream))
            {
                return await reader.ReadToEndAsync();
            }
        }

        // test using GetStringAsync
        static async Task TestWithGetStringAsync()
        {
            var tasks = Enumerable.Range(1, MAX_REQS).Select((i) =>
                GetStringAsync("http://www.bing.com/search?q=item1=" + i));

            Console.WriteLine("Threads before completion: " + Process.GetCurrentProcess().Threads.Count);

            await Task.WhenAll(tasks);

            Console.WriteLine("Threads after completion: " + Process.GetCurrentProcess().Threads.Count);
        }

        // implement GetStringSync
        static string GetStringSync(string url)
        {
            using (var response = WebRequest.Create(url).GetResponse())
            using (var stream = response.GetResponseStream())
            using (var reader = new System.IO.StreamReader(stream))
            {
                return reader.ReadToEnd();
            }
        }

        // test using GetStringSync
        static async Task TestWithGetStringSync()
        {
            var tasks = Enumerable.Range(1, MAX_REQS).Select((i) =>
                Task.Factory.StartNew(
                    () => GetStringSync("http://www.bing.com/search?q=item1=" + i),
                    CancellationToken.None, TaskCreationOptions.PreferFairness, TaskScheduler.Default));

            Console.WriteLine("Threads before completion: " + Process.GetCurrentProcess().Threads.Count);

            await Task.WhenAll(tasks);

            Console.WriteLine("Threads after completion: " + Process.GetCurrentProcess().Threads.Count);
        }

        // run either of the tests
        static void RunTest(Func<Task> runTest)
        {
            Console.WriteLine("Threads at start: " + Process.GetCurrentProcess().Threads.Count);

            var stopWatch = new Stopwatch();
            stopWatch.Start();

            var testTask = runTest();

            while (!testTask.IsCompleted)
            {
                Console.WriteLine("Currently threads: " + Process.GetCurrentProcess().Threads.Count);
                Thread.Sleep(1000);
            }
            Console.WriteLine("Threads at end: " + Process.GetCurrentProcess().Threads.Count + ", time: " + stopWatch.Elapsed);

            testTask.Wait();
        }

        static void Main(string[] args)
        {
            ThreadPool.SetMaxThreads(MAX_REQS * 2, MAX_REQS * 2);
            ThreadPool.SetMinThreads(MAX_REQS, MAX_REQS);

            Console.WriteLine("Testing using GetStringAsync");
            RunTest(TestWithGetStringAsync);
            Console.ReadLine();

            Console.WriteLine("Testing using GetStringSync");
            RunTest(TestWithGetStringSync);
            Console.ReadLine();
        }
    }
}

<强>输出：

Testing using GetStringAsync
Threads at start: 3
Threads before completion: 3
Currently threads: 25
Currently threads: 84
Currently threads: 83
Currently threads: 83
Currently threads: 83
Currently threads: 83
Currently threads: 83
Currently threads: 84
Currently threads: 83
Currently threads: 83
Currently threads: 84
Currently threads: 84
Currently threads: 84
Currently threads: 83
Currently threads: 83
Currently threads: 84
Currently threads: 83
Currently threads: 82
Currently threads: 82
Currently threads: 82
Currently threads: 83
Currently threads: 25
Currently threads: 25
Currently threads: 26
Currently threads: 25
Currently threads: 25
Currently threads: 25
Currently threads: 23
Currently threads: 23
Currently threads: 24
Currently threads: 20
Currently threads: 20
Currently threads: 19
Currently threads: 19
Currently threads: 19
Currently threads: 19
Currently threads: 18
Currently threads: 19
Currently threads: 19
Currently threads: 19
Currently threads: 18
Currently threads: 18
Currently threads: 18
Currently threads: 19
Currently threads: 19
Currently threads: 18
Currently threads: 19
Currently threads: 19
Currently threads: 18
Currently threads: 18
Currently threads: 17
Threads after completion: 17
Threads at end: 17, time: 00:00:51.2605879

Testing using GetStringSync
Threads at start: 15
Threads before completion: 15
Currently threads: 55
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 213
Currently threads: 212
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 210
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 209
Currently threads: 205
Currently threads: 201
Currently threads: 196
Currently threads: 190
Currently threads: 186
Currently threads: 182
Threads after completion: 178
Threads at end: 173, time: 00:00:47.2603652

结果：

两个测试都需要大约50秒才能完成，但GetStringAsync在83个线程处达到峰值，而GetStringSync在213处达到峰值。MAX_REQS数字越高，浪费的线程就越多阻止WebRequest.GetResponse API。

@Ark-kun，我希望你现在明白这一点。

考虑到使用Task.Run异步运行任何方法的能力，.Net Framework中的* Async方法的目的是什么？

2 个答案: