如何：使用屏障同步處理並行作業

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下列範例示範如何使用 Barrier 同步處理並行工作。

範例

下列程式的目的是計算兩個執行緒需要多少反覆項目 (或階段)，才能讓兩者透過使用隨機演算法重新排列文字，來找到其另外一半的解決方案。 在各執行緒打散其文字之後，屏障的後續階段作業會比較兩個結果，以查看完整的句子是否已以正確的文字順序顯示。

//#define TRACE
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;

namespace BarrierSimple
{
    class Program
    {
        static string[] words1 = new string[] { "brown",  "jumps", "the", "fox", "quick"};
        static string[] words2 = new string[] { "dog", "lazy","the","over"};
        static string solution = "the quick brown fox jumps over the lazy dog.";

        static bool success = false;
        static Barrier barrier = new Barrier(2, (b) =>
        {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < words1.Length; i++)
            {
                sb.Append(words1[i]);
                sb.Append(" ");
            }
            for (int i = 0; i < words2.Length; i++)
            {
                sb.Append(words2[i]);

                if(i < words2.Length - 1)
                    sb.Append(" ");
            }
            sb.Append(".");
#if TRACE
            System.Diagnostics.Trace.WriteLine(sb.ToString());
#endif
            Console.CursorLeft = 0;
            Console.Write("Current phase: {0}", barrier.CurrentPhaseNumber);
            if (String.CompareOrdinal(solution, sb.ToString()) == 0)
            {
                success = true;
                Console.WriteLine("\r\nThe solution was found in {0} attempts", barrier.CurrentPhaseNumber);
            }
        });

        static void Main(string[] args)
        {

            Thread t1 = new Thread(() => Solve(words1));
            Thread t2 = new Thread(() => Solve(words2));
            t1.Start();
            t2.Start();

            // Keep the console window open.
            Console.ReadLine();
        }

        // Use Knuth-Fisher-Yates shuffle to randomly reorder each array.
        // For simplicity, we require that both wordArrays be solved in the same phase.
        // Success of right or left side only is not stored and does not count.
        static void Solve(string[] wordArray)
        {
            while(success == false)
            {
                Random random = new Random();
                for (int i = wordArray.Length - 1; i > 0; i--)
                {
                    int swapIndex = random.Next(i + 1);
                    string temp = wordArray[i];
                    wordArray[i] = wordArray[swapIndex];
                    wordArray[swapIndex] = temp;
                }

                // We need to stop here to examine results
                // of all thread activity. This is done in the post-phase
                // delegate that is defined in the Barrier constructor.
                barrier.SignalAndWait();
            }
        }
    }
}

Imports System.Collections.Generic
Imports System.Linq
Imports System.Text
Imports System.Threading
Imports System.Threading.Tasks


Class Program
    Shared words1() = New String() {"brown", "jumps", "the", "fox", "quick"}
    Shared words2() = New String() {"dog", "lazy", "the", "over"}
    Shared solution = "the quick brown fox jumps over the lazy dog."

    Shared success = False
    Shared barrier = New Barrier(2, Sub(b)
                                        Dim sb = New StringBuilder()
                                        For i As Integer = 0 To words1.Length - 1
                                            sb.Append(words1(i))
                                            sb.Append(" ")
                                        Next
                                        For i As Integer = 0 To words2.Length - 1

                                            sb.Append(words2(i))

                                            If (i < words2.Length - 1) Then
                                                sb.Append(" ")
                                            End If
                                        Next
                                        sb.Append(".")
                                        System.Diagnostics.Trace.WriteLine(sb.ToString())

                                        Console.CursorLeft = 0
                                        Console.Write("Current phase: {0}", barrier.CurrentPhaseNumber)
                                        If (String.CompareOrdinal(solution, sb.ToString()) = 0) Then
                                            success = True
                                            Console.WriteLine()
                                            Console.WriteLine("The solution was found in {0} attempts", barrier.CurrentPhaseNumber)
                                        End If
                                    End Sub)

    Shared Sub Main()
        Dim t1 = New Thread(Sub() Solve(words1))
        Dim t2 = New Thread(Sub() Solve(words2))
        t1.Start()
        t2.Start()

        ' Keep the console window open.
        Console.ReadLine()
    End Sub

    ' Use Knuth-Fisher-Yates shuffle to randomly reorder each array.
    ' For simplicity, we require that both wordArrays be solved in the same phase.
    ' Success of right or left side only is not stored and does not count.       
    Shared Sub Solve(ByVal wordArray As String())
        While success = False
            Dim rand = New Random()
            For i As Integer = 0 To wordArray.Length - 1
                Dim swapIndex As Integer = rand.Next(i + 1)
                Dim temp As String = wordArray(i)
                wordArray(i) = wordArray(swapIndex)
                wordArray(swapIndex) = temp
            Next

            ' We need to stop here to examine results
            ' of all thread activity. This is done in the post-phase
            ' delegate that is defined in the Barrier constructor.
            barrier.SignalAndWait()
        End While
    End Sub
End Class

Barrier 是一種物件，可防止平行作業中的個別工作繼續執行，直到所有的工作都到達屏障為止。 當階段中發生平行作業，且各階段要求在工作之間進行同步處理時，它會很有用。 在此範例中，作業有兩個階段。 在第一個階段中，每個工作都會使用資料填入其緩衝區區段。 當每個工作完成填入其區段時，工作會向屏障發出訊號，告知它已準備好繼續進行，然後等候。 當所有的工作都已向屏障發出訊號，就會將工作解除封鎖，並開始第二階段。 屏障是必要的，因為第二階段要求每個工作都擁有此點已產生之所有資料的存取權。 若沒有屏障，第一個要完成的工作可能會嘗試從其他工作尚未填入的緩衝區讀取。 您可以此種方式同步處理任何數目的階段。

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