为什么异步CTP表现不佳？
我不明白为什么await和async不会像他们应该那样改善我的代码的性能。
尽管持怀疑态度，但我认为编译器应该重写我的方法，以便下载可以并行进行......但似乎并未实际发生。
（我不知道， await和async不创建单独的线程;但是，操作系统应该做在parallal的下载，并在原来的线程中调用回我的代码-不应该吗？）
我是否使用async并await不当？ 什么是使用它们的正确方法？
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??
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。 此外，不要使用WaitAny / WaitAll - async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);
}
我可能会误读你的代码，但看起来你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。 你的代码的'异步'部分实际上是异步的。 尝试这个：
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;
}
并如此使用它：
{
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机似乎支持我。
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