编程之美——差之毫厘

先看这一篇文章http://yishan.cc/blogs/gpww/archive/2009/11/03/locality-and-false-sharing.aspx

在尝试运行下面代码

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Diagnostics;

using System.Threading.Tasks;



namespace TestSum

{

    class Program

    {

        static void Main(string[] args)

        {

            int n = 1 << 10;

            int[,] array = new int[n, n];



            for (int x = 0; x < n; x++)

                for (int y = 0; y < n; y++)

                    array[x, y] = 1;

            Test(SumA, array, n);

            Test(SumB, array, n);

            Test(SumC, array, n);

            Test(SumD, array, n);

            Console.ReadKey();

        }



        static void Test(SumDele Sum, int[,] array, int n)

        {

            int sum = 0;

            Stopwatch watch = new Stopwatch();

            watch.Start();

            for (int i = 0; i < n; i++) sum += Sum(array, n);

            watch.Stop();

            Console.WriteLine("{0}: {1}, {2}s",

            Sum.Method.Name, sum, watch.Elapsed.TotalSeconds);

        }



        delegate int SumDele(int[,] array, int n); 



        static int SumA(int[,] array, int n)

        {

            int sum = 0;

            for (int x = 0; x < n; x++)

                for (int y = 0; y < n; y++)

                {

                    sum += array[y, x];

                }

            return sum;

        }



        static int SumB(int[,] array, int n)

        {

            int sum = 0;

            for (int x = 0; x < n; x++)

                for (int y = 0; y < n; y++)

                {

                    sum += array[x, y];

                }

            return sum;

        }



        static int SumC(int[,] array, int n)

        {

            int m = Environment.ProcessorCount;

            int[] result = new int[m];



            int step = n / m;

            int leftover = n % m;



            Parallel.For(0, m, (i) =>

            {

                int from = i * step;

                int to = from + step + (i < m - 1 ? 0 : leftover);



                for (int x = from; x < to; x++)

                    for (int y = 0; y < n; y++)

                    {

                        result[i] += array[x, y];

                    }

            });



            return result.Sum();

        }



        static int SumD(int[,] array, int n)

        {

            int m = Environment.ProcessorCount;

            int[] result = new int[m];



            int step = n / m;

            int leftover = n % m;



            Parallel.For(0, m, (i) =>

            {

                int from = i * step;

                int to = from + step + (i < m - 1 ? 0 : leftover);

                int temp = 0;

                for (int x = from; x < to; x++)

                    for (int y = 0; y < n; y++)

                    {

                        temp += array[x, y];

                    }

                result[ i ] = temp;

            });



            return result.Sum();

        }

    }

}