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How to: Write a Simple Parallel.For Loop

This example shows how to use the simplest overload of the Parallel.For method to compute the product of two matrices. It also shows how to use the System.Diagnostics.Stopwatch class to compare the performance of a parallel loop with a non-parallel loop.

Note Note

This documentation uses lambda expressions to define delegates in TPL. If you are not familiar with lambda expressions in C# or Visual Basic, see Lambda Expressions in PLINQ and TPL.

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
}

You can use the most basic overload of the For method when you do not need to cancel or break out of the iterations or maintain any thread-local state.

When parallelizing any code, including loops, one important goal is to utilize the processors as much as possible without over parallelizing to the point where the overhead for parallel processing negates any performance benefits. In this particular example, only the outer loop is parallelized because there is not very much work performed in the inner loop. The combination of a small amount of work and undesirable cache effects can result in performance degradation in nested parallel loops. Therefore, parallelizing the outer loop only is the best way to maximize the benefits of concurrency on most systems.

The Delegate

The third parameter of this overload of For is a delegate of type Action<int> in C# or Action(Of Integer) in Visual Basic. An Action delegate, whether it has zero, one or sixteen type parameters, always returns void. In Visual Basic, the behavior of an Action is defined with a Sub. The example uses a lambda expression to create the delegate, but you can create the delegate in other ways as well. For more information, see Lambda Expressions in PLINQ and TPL.

The Iteration Value

The delegate takes a single input parameter whose value is the current iteration. This iteration value is supplied by the runtime and its starting value is the index of the first element on the segment (partition) of the source that is being processed on the current thread.

If you require more control over the concurrency level, use one of the overloads that takes a System.Threading.Tasks.ParallelOptions input parameter, such as: Parallel.For(Int32, Int32, ParallelOptions, Action<Int32, ParallelLoopState>).

Return Value and Exception Handling

For returns a System.Threading.Tasks.ParallelLoopResult object when all threads have completed. This return value is useful when you are stopping or breaking loop iteration manually, because the ParallelLoopResult stores information such as the last iteration that ran to completion. If one or more exceptions occur on one of the threads, a System.AggregateException will be thrown.

In the code in this example, the return value of For is not used.

Analysis and Performance

You can use the Performance Wizard to view CPU usage on your computer. As an experiment, increase the number of columns and rows in the matrices. The larger the matrices, the greater the performance difference between the parallel and sequential versions of the computation. When the matrix is small, the sequential version will run faster because of the overhead in setting up the parallel loop.

Synchronous calls to shared resources, like the Console or the File System, will significantly degrade the performance of a parallel loop. When measuring performance, try to avoid calls such as Console.WriteLine within the loop.

  • Cut and paste this code into a Visual Studio 2010 project.

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