c# - 为什么我的程序的异步版本比同步版本慢？

Question

我只是对同步与异步进行一些测试，并编写了以下程序来测试。 也许我做错了什么，或者我只是没有正确理解异步，

我看到我的同步版本需要 318 ms而异步需要 18764 ms

static void Main(string[] args)
        {
            int num = 25;
            Task[] tasks = new Task[num];

            Stopwatch sw = new Stopwatch();
            sw.Start();
            for (int i = 0; i < num; i++)
            {
                int c = i;
                tasks[i] = Task.Factory.StartNew(() => { RunAsync(c).Wait(); });
            }
            Task.WaitAll(tasks);
            sw.Stop();
            Console.WriteLine($"FINISHED (Async) in {sw.ElapsedMilliseconds} ms");

            sw.Start();
            for (int i = 0; i < num; i++)
            {
                RunSync(i + 100);
            }
            sw.Stop();
            Console.WriteLine($"FINISHED (Sync) in {sw.ElapsedMilliseconds} ms");

            Console.ReadLine();
        }

        private static void RunSync(int index)
        {
            FileStream stream = new FileStream(@"c:\\test\ff\tests.txt", FileMode.Open, FileAccess.Read, FileShare.Read);
            string pp = Path.Combine(@"c:\\test\ff", "threadS-" + index + ".txt");
            FileStream sw = File.Create(pp);
            byte[] buffer = new byte[1024];
            long bytesRead = 0;
            long bytestoRead = stream.Length;
            try
            {
                while (bytesRead < bytestoRead)
                {
                    int count = stream.Read(buffer, 0, buffer.Length);
                    bytesRead += count;
                    sw.Write(buffer, 0, count);
                }
            }
            finally
            {
                sw.Close();
                stream.Close();
            }
        }

        private async static Task RunAsync(int index)
        {
            FileStream stream = new FileStream(@"c:\\test\ff\tests.txt", FileMode.Open, FileAccess.Read, FileShare.Read);
            int tId = Thread.CurrentThread.ManagedThreadId;

            string pp = Path.Combine(@"c:\\test\ff", "thread-" + index + ".txt");
            FileStream sw = File.Create(pp);
            byte[] buffer = new byte[1024];
            long bytesRead = 0;
            long bytestoRead = stream.Length;
            try
            {
                while (bytesRead < bytestoRead)
                {
                    int count = await stream.ReadAsync(buffer, 0, buffer.Length);
                    bytesRead += count;
                    await sw.WriteAsync(buffer, 0, count);
                }
            }
            finally
            {
                sw.Close();
                stream.Close();
            }
        }

Answer 1

您做错了几件事，而且您一开始的前提是错误的。那么让我从错误的前提开始:

async / await are designed to keep an app responsive or distribute work among many cores--they don't necessarily improve run-time performance.

换句话说，当您查看总体吞吐量时，您可能会处理比串行处理工作单元更多的工作。但是，该阈值将根据任何给定时间完成的工作量而变化。

正确处理async/await意味着不要将旧的Task函数与新的支持混合在一起。这失去了异步函数的所有优点并增加了同步开销。 当您想要等待后台工作完成时，切勿调用 Task.Wait() 或 Task.WaitAll(tasks)。一个线程完全暂停且无响应，直到后台工作完成。

您想要进行以下调整:

        for (int i = 0; i < num; i++)
        {
            int c = i;
            tasks[i] = RunAsync(c);
        }
        await Task.WhenAll(tasks);

由于您无法使 Main 函数成为 async，因此您可能必须将该调用移至另一个函数，以便执行 async/await 协议(protocol)。