Question

我读过许多文章说IOCP用于BeginXX / EndXX对调用。但是，当我测试它们时，我的结果显示IOCP在BeginExecuteReader调用中不起作用，而它在BeginGetResponse调用中工作正常。

我对这个结果非常困惑。谁能告诉我原因？我的测试代码有什么问题吗？

以下是测试：

使用BeginGetResponse进行测试

代码：

public static void IoThread2()
{
  ThreadPool.SetMinThreads(5, 3);
  ThreadPool.SetMaxThreads(5, 3);
  int w; int io;
  ThreadPool.GetAvailableThreads(out w, out io);
  int cid = Thread.CurrentThread.ManagedThreadId;
  Console.WriteLine("Begin:" + w.ToString() + ";" + io.ToString() + "; id = " + cid.ToString());
  ManualResetEvent waitHandle = new ManualResetEvent(false);
  WebRequest request = HttpWebRequest.Create("http://www.cnblogs.com/");
  AsyncCallback cb = new AsyncCallback(IOThread2CallBack);
  request.BeginGetResponse(cb, request);
  waitHandle.WaitOne();
}

public static void IOThread2CallBack(IAsyncResult ar)
{
 try
 {
     WebRequest request = (WebRequest)ar.AsyncState;
     int w2; int io2;
     ThreadPool.GetAvailableThreads(out w2, out io2);
     int cid2 = Thread.CurrentThread.ManagedThreadId;
     Console.WriteLine("End:" + w2.ToString() + ";" + io2.ToString() + "; id = " + cid2.ToString());
     var response = request.EndGetResponse(ar);
  }catch (Exception ex){ }
}

结果：

Begin:5;3; id = 10
End:5;2; id = 13

使用一个IO线程来执行回调。

使用BeginExecuteReader进行测试代码：

public static void IoThread1()
{
     ThreadPool.SetMinThreads(5, 3);
     ThreadPool.SetMaxThreads(5, 3);
     int w; int io;
     ThreadPool.GetAvailableThreads(out w, out io);
     int cid = Thread.CurrentThread.ManagedThreadId;
     Console.WriteLine("Begin:" + w.ToString() + ";" + io.ToString() + "; id = " + 
                    cid.ToString());
     SqlConnection connection = new SqlConnection(connectionString);
     connection.Open();
     AsyncCallback da = new AsyncCallback(IoThreadCallBack);
     SqlCommand command = new SqlCommand(s_QueryDatabaseListScript, connection);
     IAsyncResult ir = command.BeginExecuteReader(da,                             
                          command,System.Data.CommandBehavior.CloseConnection);
     ManualResetEvent waitHandle = new ManualResetEvent(false);
     waitHandle.WaitOne();
}

public static void IoThreadCallBack(IAsyncResult ar)
{
 int w; int io;
 ThreadPool.GetAvailableThreads(out w, out io);
 int cid = Thread.CurrentThread.ManagedThreadId;
 Console.WriteLine("End:" + w.ToString() + ";" + io.ToString() + "; id = " +  
                   cid.ToString());
 SqlCommand command = (SqlCommand)ar.AsyncState;
 StringBuilder sb = new StringBuilder();
 try
 {
       using (SqlDataReader reader = command.EndExecuteReader(ar))
       {
            while (reader.Read())
            {
                sb.Append(reader.GetString(0)).Append("; ");
            }
       }
  }
  catch (Exception ex){ }
  finally
  {
       command.Connection.Close();  
  }
 }

结果：

Begin:5;3; id = 10
End:4;3; id = 7

另一个工作线程用于执行回调

有什么问题？

Answer 1

你的repro很有创意。这是一个有趣的案例。

尝试使用命令文本“WAITFOR DELAY '02：00'启动100个SqlCommands;”并观察线程计数随时间的变化（启动打印统计信息的计时器）。

我的猜测是，我们会看到消耗的线程少于100个（意味着这部分异步行为正在发挥作用）。由于某种原因，SqlCommand正在使用“错误的”线程池线程，但这可能根本不会降低可伸缩性。

为什么IOCP在BeginExecuteReader中不起作用

1 个答案: