c# - 并行化 pi 数计算

Question

下面的程序计算 pi=3.1415 最初是用 C++ 使用 OpenMP #pragma omp for 和 Reduction(+:sum) 编写的我正在尝试使用 Parallel.For 在 C#(新手)中进行相同的计算出于某种原因我无法让它工作，我如何让 C# 中的每个线程都私有(private) sum ？

using System;
using System.Diagnostics;
using System.Threading.Tasks;


namespace pi
{
    class Program
    {
        static void Main(string[] args)
        {
            long num_steps = 100000000;
            double step;
            double x, pi, sum = 0.0;
            step = 1.0 / num_steps;
            Stopwatch timer = Stopwatch.StartNew();
            Parallel.For(1, num_steps + 1, new ParallelOptions { MaxDegreeOfParallelism = 4 }, i =>
                {
                    x = (i - 0.5) * step;                    
                    sum = sum + 4.0 / (1.0 + x * x);
                });            
            pi = step * sum;
            timer.Stop();
            Console.WriteLine("\n pi with {0} steps is {1} in {2} miliseconds ", num_steps, pi, (timer.ElapsedMilliseconds));
            Console.ReadKey();
        }
    }
}

Answer 1

我不认为通过使用 Parallel.For 并行化这个任务你会得到更好(更快)的结果。但如果您仍然想这样做 - 请查看 ThreadLocal类(class)。它为每个线程提供“私有(private)”存储。

long num_steps = 100000000;
var sum = new ThreadLocal<double>(true);
var step = 1.0 / num_steps;
Stopwatch timer = Stopwatch.StartNew();
Parallel.For(1, num_steps + 1, new ParallelOptions { MaxDegreeOfParallelism = 4 }, i =>
{
    var x = (i - 0.5) * step;
    sum.Value = sum.Value + 4.0 / (1.0 + x * x);
});
var pi = step * sum.Values.Sum();
timer.Stop();
sum.Dispose();
Console.WriteLine("\n pi with {0} steps is {1} in {2} miliseconds ", num_steps, pi, (timer.ElapsedMilliseconds));
Console.ReadKey();