SemaphoreSlim.WaitAnc延续代码
本文关键字:代码 延续 WaitAnc SemaphoreSlim | 更新日期: 2023-09-27 18:20:43
我对await
关键字的理解是,await
限定语句后面的代码在该语句完成后将作为该语句的延续运行。
因此,以下两个版本应该产生相同的输出:
public static Task Run(SemaphoreSlim sem)
{
TraceThreadCount();
return sem.WaitAsync().ContinueWith(t =>
{
TraceThreadCount();
sem.Release();
});
}
public static async Task RunAsync(SemaphoreSlim sem)
{
TraceThreadCount();
await sem.WaitAsync();
TraceThreadCount();
sem.Release();
}
但他们没有!
以下是完整的程序:
using System;
using System.Threading;
using System.Threading.Tasks;
namespace CDE
{
class Program
{
static void Main(string[] args)
{
try
{
var sem = new SemaphoreSlim(10);
var task = Run(sem);
Trace("About to wait for Run.");
task.Wait();
Trace("--------------------------------------------------");
task = RunAsync(sem);
Trace("About to wait for RunAsync.");
task.Wait();
}
catch (Exception exc)
{
Console.WriteLine(exc.Message);
}
Trace("Press any key ...");
Console.ReadKey();
}
public static Task Run(SemaphoreSlim sem)
{
TraceThreadCount();
return sem.WaitAsync().ContinueWith(t =>
{
TraceThreadCount();
sem.Release();
});
}
public static async Task RunAsync(SemaphoreSlim sem)
{
TraceThreadCount();
await sem.WaitAsync();
TraceThreadCount();
sem.Release();
}
private static void Trace(string fmt, params object[] args)
{
var str = string.Format(fmt, args);
Console.WriteLine("[{0}] {1}", Thread.CurrentThread.ManagedThreadId, str);
}
private static void TraceThreadCount()
{
int workerThreads;
int completionPortThreads;
ThreadPool.GetAvailableThreads(out workerThreads, out completionPortThreads);
Trace("Available thread count: worker = {0}, completion port = {1}", workerThreads, completionPortThreads);
}
}
}
这是输出:
[9] Available thread count: worker = 1023, completion port = 1000
[9] About to wait for Run.
[6] Available thread count: worker = 1021, completion port = 1000
[9] --------------------------------------------------
[9] Available thread count: worker = 1023, completion port = 1000
[9] Available thread count: worker = 1023, completion port = 1000
[9] About to wait for RunAsync.
[9] Press any key ...
我错过了什么?
async-await
在等待的任务已经完成时进行优化(当信号量设置为10,只有1个线程使用时就是这种情况)。在这种情况下,线程只是同步进行。
您可以通过向RunAsync
添加一个实际的异步操作来了解这一点,并了解它如何更改正在使用的线程池线程(这将是当信号量为空并且调用者实际上需要异步等待时的行为):
public static async Task RunAsync(SemaphoreSlim sem)
{
TraceThreadCount();
await Task.Delay(1000);
await sem.WaitAsync();
TraceThreadCount();
sem.Release();
}
您也可以对Run
进行此更改,并让它同步执行延续,并获得与RunAsync
相同的结果(线程计数):
public static Task Run(SemaphoreSlim sem)
{
TraceThreadCount();
return sem.WaitAsync().ContinueWith(t =>
{
TraceThreadCount();
sem.Release();
}, TaskContinuationOptions.ExecuteSynchronously);
}
输出:
[1] Available thread count: worker = 1023, completion port = 1000
[1] Available thread count: worker = 1023, completion port = 1000
[1] About to wait for Run.
[1] --------------------------------------------------
[1] Available thread count: worker = 1023, completion port = 1000
[1] Available thread count: worker = 1023, completion port = 1000
[1] About to wait for RunAsync.
[1] Press any key ...
重要提示:当说async-await
是一个延续时,它更像是一个类比。这些概念之间有几个关键的区别,尤其是关于SynchronizationContext
s。async-await
自动保留当前上下文(除非指定ConfigureAwait(false)
),以便您可以在重要的环境(UI、ASP.Net等)中安全地使用它。有关同步上下文的更多信息,请点击此处。
此外,await Task.Delay(1000);
可以用await Task.Yield();
代替,以说明定时是不相关的,并且仅仅是该方法异步等待的事实是重要的。Task.Yield()
在异步代码的单元测试中通常很有用。
它们不会像您调用异步方法时那样,它会立即启动。因此,只要你的信号量没有被锁定,WaitAsync()
甚至不会启动,就不会有上下文切换(这是一种优化,同样适用于被取消的任务),所以你的异步方法将是同步的。
同时,continuation版本实际上会在并行线程上启动continuation。