屏障 (.NET Framework)

项目
08/12/2011

关卡是一个用户定义的同步基元，它使多个线程（称为“参与者”）可以分阶段同时参与执行一个算法。每个参与者将执行到代码中的关卡点为止。关卡表示一个阶段的工作结束。当某个参与者到达关卡时，它将阻塞，直至所有参与者都已到达同一关卡为止。在所有参与者都已到达关卡之后，您可以选择调用后期阶段操作。在所有其他线程仍被阻塞时，单个线程可以使用此后期阶段操作来执行操作。在执行该操作之后，所有参与者都将被取消阻塞。

下面的代码段演示了基本关卡模式。


' Create the Barrier object, and supply a post-phase delegate 
' to be invoked at the end of each phase.
Dim barrier = New Barrier(2, Sub(bar)
                                 ' Examine results from all threads, determine 
                                 ' whether to continue, create inputs for next phase, etc. 
                                 If (someCondition) Then
                                     success = True
                                 End If
                             End Sub)



' Define the work that each thread will perform. (Threads do not
' have to all execute the same method.)
Sub CrunchNumbers(ByVal partitionNum As Integer)

    ' Up to System.Int64.MaxValue phases are supported. We assume
    ' in this code that the problem will be solved before that.
    While (success = False)

        ' Begin phase:
        ' Process data here on each thread, and optionally
        ' store results, for example:
        results(partitionNum) = ProcessData(myData(partitionNum))

        ' End phase:
        ' After all threads arrive,post-phase delegate
        ' is invoked, then threads are unblocked. Overloads
        ' accept a timeout value and/or CancellationToken.
        barrier.SignalAndWait()
    End While
End Sub

' Perform n tasks to run in in parallel. For simplicity
' all threads execute the same method in this example.
Shared Sub Main()

    Dim app = New BarrierDemo()
    Dim t1 = New Thread(Sub() app.CrunchNumbers(0))
    Dim t2 = New Thread(Sub() app.CrunchNumbers(1))
    t1.Start()
    t2.Start()
End Sub


 // Create the Barrier object, and supply a post-phase delegate 
 // to be invoked at the end of each phase.
 Barrier barrier = new Barrier(2, (bar) => 
     {
         // Examine results from all threads, determine 
         // whether to continue, create inputs for next phase, etc. 
         if (someCondition)
             success = true;
     });       


 // Define the work that each thread will perform. (Threads do not
 // have to all execute the same method.)
 void CrunchNumbers(int partitionNum)
 {
     // Up to System.Int64.MaxValue phases are supported. We assume
     // in this code that the problem will be solved before that.
     while (success == false)
     {
         // Begin phase:
         // Process data here on each thread, and optionally
         // store results, for example:
         results[partitionNum] = ProcessData(data[partitionNum]);

         // End phase:
         // After all threads arrive,post-phase delegate
         // is invoked, then threads are unblocked. Overloads
         // accept a timeout value and/or CancellationToken.
         barrier.SignalAndWait();
     }
 }

 // Perform n tasks to run in in parallel. For simplicity
// all threads execute the same method in this example.
 static void Main()
 {
     var app = new BarrierDemo();
     Thread t1 = new Thread(() => app.CrunchNumbers(0));
     Thread t2 = new Thread(() => app.CrunchNumbers(1));
     t1.Start();
     t2.Start();

 }

有关完整示例，请参见如何：使用屏障来使并发操作保持同步。

添加和移除参与者

创建 Barrier 时，指定参与者的数量。也可在任何时间动态地添加或移除参与者。例如，如果一个参与者解决了自己的问题部分，则可以存储结果，停止执行该线程并调用 RemoveParticipant 以减少关卡中的参与者的数量。当通过调用 AddParticipant 来添加参与者时，返回值会指定当前的阶段编号，这对于初始化新的参与者的工作可能会有用。

出现故障的关卡

如果一个参与者未能到达关卡，则会发生死锁。若要避免这些死锁，请使用 SignalAndWait 方法的重载来指定超时期限和取消标记。这些重载将返回一个布尔值，每个参与者都会在它继续下一阶段之前检查该值。

后期阶段异常

如果后期阶段委托引发异常，则在 BarrierPostPhaseException 对象中包装它，然后将其传播到所有参与者。

关卡与 ContinueWhenAll

当线程循环执行多个阶段时，关卡尤其有用。如果您的代码只需要一个或两个阶段的工作，请考虑是否使用带有各种隐式联接的 System.Threading.Tasks.Task 对象，包括：

有关更多信息，请参见如何：用延续将多个任务链接在一起。