Question

我真的不明白为什么await和async不能提高我的代码效果like they're supposed to。

虽然持怀疑态度，但我认为编译器应该重写我的方法，以便下载并行完成......但似乎并没有真正发生。
（我做意识到await和async不会创建单独的线程;但是，操作系统应该在parallal中进行下载，并回调我的代码原始主题 - 不应该吗？）

我是否未正确使用async和await？使用它们的正确方法是什么？

代码：

using System;
using System.Net;
using System.Threading;
using System.Threading.Tasks;

static class Program
{
    static int SumPageSizesSync(string[] uris)
    {
        int total = 0;
        var wc = new WebClient();
        foreach (var uri in uris)
        {
            total += wc.DownloadData(uri).Length;
            Console.WriteLine("Received synchronized data...");
        }
        return total;
    }

    static async Task<int> SumPageSizesAsync(string[] uris)
    {
        int total = 0;
        var wc = new WebClient();
        foreach (var uri in uris)
        {
            var data = await wc.DownloadDataTaskAsync(uri);
            Console.WriteLine("Received async'd CTP data...");
            total += data.Length;
        }
        return total;
    }

    static int SumPageSizesManual(string[] uris)
    {
        int total = 0;
        int remaining = 0;
        foreach (var uri in uris)
        {
            Interlocked.Increment(ref remaining);
            var wc = new WebClient();
            wc.DownloadDataCompleted += (s, e) =>
            {
                Console.WriteLine("Received manually async data...");
                Interlocked.Add(ref total, e.Result.Length);
                Interlocked.Decrement(ref remaining);
            };
            wc.DownloadDataAsync(new Uri(uri));
        }
        while (remaining > 0) { Thread.Sleep(25); }
        return total;
    }

    static void Main(string[] args)
    {
        var uris = new string[]
        {
            // Just found a slow site, to demonstrate the problem :)
            "http://www.europeanchamber.com.cn/view/home",
            "http://www.europeanchamber.com.cn/view/home",
            "http://www.europeanchamber.com.cn/view/home",
            "http://www.europeanchamber.com.cn/view/home",
            "http://www.europeanchamber.com.cn/view/home",
        };
        {
            var start = Environment.TickCount;
            SumPageSizesSync(uris);
            Console.WriteLine("Synchronous: {0} milliseconds", Environment.TickCount - start);
        }
        {
            var start = Environment.TickCount;
            SumPageSizesManual(uris);
            Console.WriteLine("Manual: {0} milliseconds", Environment.TickCount - start);
        }
        {
            var start = Environment.TickCount;
            SumPageSizesAsync(uris).Wait();
            Console.WriteLine("Async CTP: {0} milliseconds", Environment.TickCount - start);
        }
    }
}

输出：

Received synchronized data...
Received synchronized data...
Received synchronized data...
Received synchronized data...
Received synchronized data...
Synchronous: 14336 milliseconds
Received manually async data...
Received manually async data...
Received manually async data...
Received manually async data...
Received manually async data...
Manual: 8627 milliseconds          // Almost twice as fast...
Received async'd CTP data...
Received async'd CTP data...
Received async'd CTP data...
Received async'd CTP data...
Received async'd CTP data...
Async CTP: 13073 milliseconds      // Why so slow??

Answer 1

Chris的回答几乎是正确的，但引入了竞争条件并同步阻止所有任务。

作为一般规则，如果您有await / async可用，最好不要使用任务延续。另外，请勿使用WaitAny / WaitAll - async等效项为WhenAny和WhenAll。

我会这样写：

static async Task<int> SumPageSizesAsync(IEnumerable<string> uris)
{
  // Start one Task<byte[]> for each download.
  var tasks = uris.Select(uri => new WebClient().DownloadDataTaskAsync(uri));

  // Asynchronously wait for them all to complete.
  var results = await TaskEx.WhenAll(tasks);

  // Calculate the sum.
  return results.Sum(result => result.Length);
}

Answer 2

我可能会误读您的代码，但看起来您正在启动后台线程来执行异步读取，然后立即阻止，等待它完成。什么都不是关于'async＆＃39;部分代码实际上是异步的。试试这个：

static async Task<int> SumPageSizesAsync(string[] uris)
{
    int total = 0;
    var wc = new WebClient();
    var tasks = new List<Task<byte[]>>();
    foreach (var uri in uris)
    {
        tasks
          .Add(wc.DownloadDataTaskAsync(uri).ContinueWith(() => { total += data.Length;
        }));
    }
    Task.WaitAll(tasks);
    return total;
}

然后使用它：

    {
        var start = Environment.TickCount;
        await SumPageSizesAsync(uris);
        Console.WriteLine("Async CTP: {0} milliseconds", Environment.TickCount - start);
    }

我可能是错的 - 异步的东西是新的，我并不是100％熟悉它 - 但同步版本的类似时间似乎让我失望。

为什么异步CTP表现不佳？

2 个答案: