Выбор инструмента для расчётов с плавающей точкой — практические советы

using System;
using System.Collections.Concurrent;
using System.Diagnostics;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace TempConsoleApplication {

    public static class Program {
        private const double Result = 1.2502304241756868163500362795713040947699040278200;

        private const int IterCount = 200000000;

        private static void Main(string[] args) {
            Start("Simple loop", n => {
                double sum = 0.0;
                for (double i = 1.0; i <= n; ++i) {
                    sum += Math.Sin(i);
                }
                return sum;
            });

            Start("ParallelEnumerable",
                n => ParallelEnumerable.Range(1, n).Select(i => Math.Sin(i)).Sum());

            Start("Parallel.For", n => {
                var sum = 0.0;
                Parallel.For(
                    1,
                    n + 1,
                    () => 0.0,
                    (i, state, sumL) => Math.Sin(i) + sumL,
                    sumL => Sum(ref sum, sumL));
                return sum;
            });

            Start("Parallel.ForEach", n => {
                var sum = 0.0;
                Parallel.ForEach(
                    Partitioner.Create(1, n + 1),
                    () => 0.0,
                    (range, state, sumL) => {
                        for (int i = range.Item1; i < range.Item2; ++i) {
                            sumL += Math.Sin(i);
                        }
                        return sumL;
                    },
                    sumL => Sum(ref sum, sumL));
                return sum;
            });


            Console.ReadKey();
        }

        private static void Sum(ref double sum, double sumL) {
            double initial, computed;
            do {
                initial = sum;
                computed = initial + sumL;
            }
            while (initial != Interlocked.CompareExchange(ref sum, computed, initial));
        }

        private static void Start(string name, Func<int, double> func) {
            func(100);
            var s = Stopwatch.StartNew();
            var sum = func(IterCount);
            s.Stop();
            Console.WriteLine(name);
            Console.WriteLine("Time: " + s.Elapsed);
            Console.WriteLine("Delta: " + (sum - Result));
            Console.WriteLine();
        }
    }

}

#include <iostream>
#include <math.h>
#include <sys/time.h>

using namespace std;

unsigned long int time_delta(struct timeval *from, struct timeval *to) {
        unsigned long int delta = to->tv_sec * 1000000l + to->tv_usec;
        return delta - (from->tv_sec * 1000000l + from->tv_usec);
}

int main() 
{
    struct timeval start, end;
    double res=0.0;

    gettimeofday(&start, 0);
    for (int i = 1; i <= 200000000; i++)
        res+=sin(i);
    gettimeofday(&end, 0);
    cout.precision(20);
    cout << "Result: " << res << " time: " << time_delta(&start, &end)/1000000.0 << "s" << endl;
}