.NET 4.5文件读取性能同步与异步

我们试图衡量使用同步方法和异步方法读取一系列文件之间的性能。原本预计两者之间的时间大致相同，但使用async的速度慢了5.5倍。

这可能是由于管理线程的开销，但只是想知道您的意见。也许我们只是测量时间不对。

以下是正在测试的方法：

    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);
            }
        }
    }

这就是运行它们并启动秒表的方法：

    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);
    }

这一切都是从这里运行的：

    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();
    }

和助手编写方法：

    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

如果有人能在我们搜索堆栈和网络时对此有所了解，但却找不到合适的解释，我们将不胜感激。

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);
}

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