为什么异步CTP性能不佳?
我真的不明白为什么await
和async
不会像我们想象的那样改善我的代码的性能。
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。
( 我确实认识到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,并在原始线程中回调我的代码 – 不应该吗? )
我是否使用async
并await
不正确? 使用它们的正确方法是什么?
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 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??
克里斯的答案几乎是正确的,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,不要使用WaitAny
/ WaitAll
– async
等价物是WhenAny
和WhenAll
。
我会这样写:
static async Task SumPageSizesAsync(IEnumerable uris) { // Start one Task 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); }
我可能误读了你的代码,但看起来你正在启动一个后台线程来执行异步读取,然后立即阻塞,等待它完成。 关于代码的“异步”部分,实际上并不是异步。 试试这个:
static async Task SumPageSizesAsync(string[] uris) { int total = 0; var wc = new WebClient(); var tasks = new List>(); foreach (var uri in uris) { tasks .Add(wc.DownloadDataTaskAsync(uri).ContinueWith(() => { total += data.Length; })); } Task.WaitAll(tasks); return total; }
并使用它:
{ 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,我不是100%熟悉它 – 但同步版本的类似时间似乎让我失望。