我真的不明白为什么await和async不会在这里提高我的代码性能
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？)
我是否使用异步并等待不正确？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<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",};
{
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??
解决方法
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 – 异步等价物是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);
}
