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();
        }
    }
}