HOW TO：撰寫簡單的 Parallel.For 迴圈
此範例說明如何使用 Parallel.For 方法最簡單的多載，計算兩個矩陣的乘積。 另外也會說明如何使用 System.Diagnostics.Stopwatch 類別比較平行迴圈與非平行迴圈的效能。
.gif "注意事項") 注意事項 |
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本文件使用 Lambda 運算式來定義 TPL 中的委派。如果您不太熟悉 C# 或 Visual Basic 中的 Lambda 運算式，請參閱 PLINQ 和 TPL 中的 Lambda 運算式。 |
範例
' How to: Write a Simple Parallel.For Loop
Imports System.Threading.Tasks
Module MultiplyMatrices
#Region "Sequential_Loop"
Sub MultiplyMatricesSequential(ByVal matA As Double(,), ByVal matB As Double(,), ByVal result As Double(,))
Dim matACols As Integer = matA.GetLength(1)
Dim matBCols As Integer = matB.GetLength(1)
Dim matARows As Integer = matA.GetLength(0)
For i As Integer = 0 To matARows - 1
For j As Integer = 0 To matBCols - 1
For k As Integer = 0 To matACols - 1
result(i, j) += matA(i, k) * matB(k, j)
Next
Next
Next
End Sub
#End Region
#Region "Parallel_Loop"
Private Sub MultiplyMatricesParallel(ByVal matA As Double(,), ByVal matB As Double(,), ByVal result As Double(,))
Dim matACols As Integer = matA.GetLength(1)
Dim matBCols As Integer = matB.GetLength(1)
Dim matARows As Integer = matA.GetLength(0)
' A basic matrix multiplication.
' Parallelize the outer loop to partition the source array by rows.
Parallel.For(0, matARows, Sub(i)
For j As Integer = 0 To matBCols - 1
' Use a temporary to improve parallel performance.
Dim temp As Double = 0
For k As Integer = 0 To matACols - 1
temp += matA(i, k) * matB(k, j)
Next
result(i, j) += temp
Next
End Sub)
End Sub
#End Region
#Region "Main"
Sub Main(ByVal args As String())
' Set up matrices. Use small values to better view
' result matrix. Increase the counts to see greater
' speedup in the parallel loop vs. the sequential loop.
Dim colCount As Integer = 180
Dim rowCount As Integer = 2000
Dim colCount2 As Integer = 270
Dim m1 As Double(,) = InitializeMatrix(rowCount, colCount)
Dim m2 As Double(,) = InitializeMatrix(colCount, colCount2)
Dim result As Double(,) = New Double(rowCount - 1, colCount2 - 1) {}
' First do the sequential version.
Console.WriteLine("Executing sequential loop...")
Dim stopwatch As New Stopwatch()
stopwatch.Start()
MultiplyMatricesSequential(m1, m2, result)
stopwatch.[Stop]()
Console.WriteLine("Sequential loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds)
' For the skeptics.
OfferToPrint(rowCount, colCount2, result)
' Reset timer and results matrix.
stopwatch.Reset()
result = New Double(rowCount - 1, colCount2 - 1) {}
' Do the parallel loop.
Console.WriteLine("Executing parallel loop...")
stopwatch.Start()
MultiplyMatricesParallel(m1, m2, result)
stopwatch.[Stop]()
Console.WriteLine("Parallel loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds)
OfferToPrint(rowCount, colCount2, result)
' Keep the console window open in debug mode.
Console.WriteLine("Press any key to exit.")
Console.ReadKey()
End Sub
#End Region
#Region "Helper_Methods"
Function InitializeMatrix(ByVal rows As Integer, ByVal cols As Integer) As Double(,)
Dim matrix As Double(,) = New Double(rows - 1, cols - 1) {}
Dim r As New Random()
For i As Integer = 0 To rows - 1
For j As Integer = 0 To cols - 1
matrix(i, j) = r.[Next](100)
Next
Next
Return matrix
End Function
Sub OfferToPrint(ByVal rowCount As Integer, ByVal colCount As Integer, ByVal matrix As Double(,))
Console.WriteLine("Computation complete. Print results? y/n")
Dim c As Char = Console.ReadKey().KeyChar
If c = "y"c OrElse c = "Y"c Then
Console.WindowWidth = 168
Console.WriteLine()
For x As Integer = 0 To rowCount - 1
Console.WriteLine("ROW {0}: ", x)
For y As Integer = 0 To colCount - 1
Console.Write("{0:#.##} ", matrix(x, y))
Next
Console.WriteLine()
Next
End If
End Sub
#End Region
End Module
namespace MultiplyMatrices
{
using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;
class Program
{
#region Sequential_Loop
static void MultiplyMatricesSequential(double[,] matA, double[,] matB,
double[,] result)
{
int matACols = matA.GetLength(1);
int matBCols = matB.GetLength(1);
int matARows = matA.GetLength(0);
for (int i = 0; i < matARows; i++)
{
for (int j = 0; j < matBCols; j++)
{
for (int k = 0; k < matACols; k++)
{
result[i, j] += matA[i, k] * matB[k, j];
}
}
}
}
#endregion
#region Parallel_Loop
static void MultiplyMatricesParallel(double[,] matA, double[,] matB, double[,] result)
{
int matACols = matA.GetLength(1);
int matBCols = matB.GetLength(1);
int matARows = matA.GetLength(0);
// A basic matrix multiplication.
// Parallelize the outer loop to partition the source array by rows.
Parallel.For(0, matARows, i =>
{
for (int j = 0; j < matBCols; j++)
{
// Use a temporary to improve parallel performance.
double temp = 0;
for (int k = 0; k < matACols; k++)
{
temp += matA[i, k] * matB[k, j];
}
result[i, j] = temp;
}
}); // Parallel.For
}
#endregion
#region Main
static void Main(string[] args)
{
// Set up matrices. Use small values to better view
// result matrix. Increase the counts to see greater
// speedup in the parallel loop vs. the sequential loop.
int colCount = 180;
int rowCount = 2000;
int colCount2 = 270;
double[,] m1 = InitializeMatrix(rowCount, colCount);
double[,] m2 = InitializeMatrix(colCount, colCount2);
double[,] result = new double[rowCount, colCount2];
// First do the sequential version.
Console.WriteLine("Executing sequential loop...");
Stopwatch stopwatch = new Stopwatch();
stopwatch.Start();
MultiplyMatricesSequential(m1, m2, result);
stopwatch.Stop();
Console.WriteLine("Sequential loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds);
// For the skeptics.
OfferToPrint(rowCount, colCount2, result);
// Reset timer and results matrix.
stopwatch.Reset();
result = new double[rowCount, colCount2];
// Do the parallel loop.
Console.WriteLine("Executing parallel loop...");
stopwatch.Start();
MultiplyMatricesParallel(m1, m2, result);
stopwatch.Stop();
Console.WriteLine("Parallel loop time in milliseconds: {0}", stopwatch.ElapsedMilliseconds);
OfferToPrint(rowCount, colCount2, result);
// Keep the console window open in debug mode.
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
#endregion
#region Helper_Methods
static double[,] InitializeMatrix(int rows, int cols)
{
double[,] matrix = new double[rows, cols];
Random r = new Random();
for (int i = 0; i < rows; i++)
{
for (int j = 0; j < cols; j++)
{
matrix[i, j] = r.Next(100);
}
}
return matrix;
}
private static void OfferToPrint(int rowCount, int colCount, double[,] matrix)
{
Console.WriteLine("Computation complete. Print results? y/n");
char c = Console.ReadKey().KeyChar;
if (c == 'y' || c == 'Y')
{
Console.WindowWidth = 180;
Console.WriteLine();
for (int x = 0; x < rowCount; x++)
{
Console.WriteLine("ROW {0}: ", x);
for (int y = 0; y < colCount; y++)
{
Console.Write("{0:#.##} ", matrix[x, y]);
}
Console.WriteLine();
}
}
}
#endregion
}
}
當您無須取消或中斷反覆運算，或無須維護任何執行緒區域狀態時，您可以使用 For 方法最基本的多載。
平行處理任何程式碼時 (包括迴圈) 的其中一個重要目標是盡可能使用處理器，同時使平行處理的負荷程度不致超過而對效能產生負面影響。 在此特定範例中只會平行處理外部迴圈，因為在內部迴圈中執行的工作並不多。 如果工作量很少，而快取效果又不理想，將可能導致巢狀平行迴圈的效能降低。 因此，在大部分的系統上，僅平行處理外部迴圈是充分發揮並行優勢的最佳途徑。
委派
在 C# 中，For 的這個多載的第三個參數為型別 Action<int> 的委派 (在 Visual Basic 中為 Action(Of Integer))。 Action 委派無論具有零個、一個還是十六個型別參數，一律會傳回 void。 在 Visual Basic 中，Action 的行為是以 Sub 定義的。 範例使用 Lambda 運算式建立委派，但您可以用其他方式建立委派。 如需詳細資訊，請參閱PLINQ 和 TPL 中的 Lambda 運算式。
反覆項目值
委派會採用具有目前反覆項目值的單一輸入參數。 此反覆項目值由執行階段提供，其起始值為目前執行緒上正在處理之來源區段 (磁碟分割) 上第一個項目的索引。
如果您需要進一步控制並行層級，請使用其中一個採用 System.Threading.Tasks.ParallelOptions 輸入參數的多載，例如：Parallel.For(Int32, Int32, ParallelOptions, Action<Int32, ParallelLoopState>)。
傳回值和例外狀況處理
For 會在所有執行緒皆完成時傳回 System.Threading.Tasks.ParallelLoopResult 物件。 此傳回值在您手動停止或中斷迴圈反覆運算時有其用處，因為 ParallelLoopResult 會儲存完成執行的最後一次反覆運算之類的資訊。 如果有其中一個執行緒上發生一個或多個例外狀況，則會擲回 System.AggregateException。
在此範例的程式碼中，不會使用 For 的傳回值。
分析和效能
您可以使用「效能精靈」檢視電腦上的 CPU 使用量。 請試著增加矩陣中的資料行和資料列數目。 矩陣越大，平行和循序計算之間的效能差異就越大。 當矩陣較小時，循序計算的執行速度因為設定平行迴圈有額外負荷所以較快。
同步呼叫主控台或檔案系統之類的共用資源時，會大幅降低平行迴圈的效能。 評估效能時，請避免在迴圈內呼叫 Console.WriteLine 之類的項目。
編譯程式碼
- 將此程式碼剪下並貼到 Visual Studio 2010 專案中。