.NET 4.5文件读取性能同步与异步 [英] .NET 4.5 file read performance sync vs async

问题描述

我们正在尝试评估使用同步方法与异步方法读取一系列文件之间的性能。原来期望两者之间有相同的时间，但事实证明使用异步的速度要慢5.5倍。

We're trying to measure the performance between reading a series of files using sync methods vs async. Was expecting to have about the same time between the two but turns out using async is about 5.5x slower.

这可能是由于管理线程的开销所致，但仅仅是想知道你的意见。

This might be due to the overhead of managing the threads but just wanted to know your opinion. Maybe we're just measuring the timings wrong.

这些是正在测试的方法：

These are the methods being tested:

    static void ReadAllFile(string filename)
    {
        var content = File.ReadAllBytes(filename);
    }

    static async Task ReadAllFileAsync(string filename)
    {
        using (var file = File.OpenRead(filename))
        {
            using (var ms = new MemoryStream())
            {
                byte[] buff = new byte[file.Length];
                await file.ReadAsync(buff, 0, (int)file.Length);
            }
        }
    }

这是运行它们并启动秒表：

And this is the method that runs them and starts the stopwatch:

    static void Test(string name, Func<string, Task> gettask, int count)
    {
        Stopwatch sw = new Stopwatch();

        Task[] tasks = new Task[count];
        sw.Start();
        for (int i = 0; i < count; i++)
        {
            string filename = "file" + i + ".bin";
            tasks[i] = gettask(filename);
        }
        Task.WaitAll(tasks);
        sw.Stop();
        Console.WriteLine(name + " {0} ms", sw.ElapsedMilliseconds);

    }

都是从这里运行的：

    static void Main(string[] args)
    {
        int count = 10000;

        for (int i = 0; i < count; i++)
        {
            Write("file" + i + ".bin");
        }

        Console.WriteLine("Testing read...!");            

        Test("Read Contents", (filename) => Task.Run(() => ReadAllFile(filename)), count);
        Test("Read Contents Async", (filename) => ReadAllFileAsync(filename), count);

        Console.ReadKey();
    }

然后编写辅助方法：

    static void Write(string filename)
    {
        Data obj = new Data()
        {
            Header = "random string size here"
        };
        int size = 1024 * 20; // 1024 * 256;

        obj.Body = new byte[size];

        for (var i = 0; i < size; i++)
        {
            obj.Body[i] = (byte)(i % 256);
        }

        Stopwatch sw = new Stopwatch();
        sw.Start();

        MemoryStream ms = new MemoryStream();
        Serializer.Serialize(ms, obj);
        ms.Position = 0;

        using (var file = File.Create(filename))
        {
            ms.CopyToAsync(file).Wait();
        }

        sw.Stop();
        //Console.WriteLine("Writing file {0}", sw.ElapsedMilliseconds); 
    }

结果：

-Read Contents 574 ms
-Read Contents Async 3160 ms

如果在我们搜索堆栈和网络时确实能找到一些启示，但却找不到真正的解释，将非常感谢。

Will really appreciate if anyone can shed some light on this as we searched the stack and the web but can't really find a proper explanation.

推荐答案

测试代码有很多错误。最值得注意的是，您的异步测试未使用异步I / O；对于文件流，您必须显式地将它们以异步方式打开，否则，您只是在后台线程上进行同步操作。另外，您的文件非常小，可以轻松缓存。

There are lots of things wrong with the testing code. Most notably, your "async" test does not use async I/O; with file streams, you have to explicitly open them as asynchronous or else you're just doing synchronous operations on a background thread. Also, your file sizes are very small and can be easily cached.

我修改了测试代码，以写出更大的文件，具有可比的同步与异步代码，并且使异步代码异步：

I modified the test code to write out much larger files, to have comparable sync vs async code, and to make the async code asynchronous:

static void Main(string[] args)
{
    Write("0.bin");
    Write("1.bin");
    Write("2.bin");

    ReadAllFile("2.bin"); // warmup

    var sw = new Stopwatch();
    sw.Start();
    ReadAllFile("0.bin");
    ReadAllFile("1.bin");
    ReadAllFile("2.bin");
    sw.Stop();

    Console.WriteLine("Sync: " + sw.Elapsed);

    ReadAllFileAsync("2.bin").Wait(); // warmup

    sw.Restart();
    ReadAllFileAsync("0.bin").Wait();
    ReadAllFileAsync("1.bin").Wait();
    ReadAllFileAsync("2.bin").Wait();
    sw.Stop();

    Console.WriteLine("Async: " + sw.Elapsed);

    Console.ReadKey();
}

static void ReadAllFile(string filename)
{
    using (var file = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, false))
    {
        byte[] buff = new byte[file.Length];
        file.Read(buff, 0, (int)file.Length);
    }
}

static async Task ReadAllFileAsync(string filename)
{
    using (var file = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read, 4096, true))
    {
        byte[] buff = new byte[file.Length];
        await file.ReadAsync(buff, 0, (int)file.Length);
    }
}

static void Write(string filename)
{
    int size = 1024 * 1024 * 256;
    var data = new byte[size];
    var random = new Random();
    random.NextBytes(data);
    File.WriteAllBytes(filename, data);
}

在我的计算机上，此测试（内置于Release中，在调试器外部运行）产生这些数字：

On my machine, this test (built in Release, run outside the debugger) yields these numbers:

Sync: 00:00:00.4461936
Async: 00:00:00.4429566

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