Right way to do a Parallel.For to compute data from Array

vcjones wondered about whether you would really see any speedup. Well the answer is: it probably depends how many cores you have. The PLinq is slower than a plain loop on my home PC (which is quad core).

I've come up with an alternative approach which uses a Partitioner to chop the list of numbers up into several sections so you can add up each one separately. There's also some more information about using a Partitioner here.

Using the Partitioner approach seems a bit faster, at least on my home PC.

Here's my test program. Note that you must run a release build of this outside any debugger to get the right timings.

The important method in this code is ViaPartition():

Result ViaPartition(double[] numbers)
{
    var result = new Result();

    var rangePartitioner = Partitioner.Create(0, numbers.Length);

    Parallel.ForEach(rangePartitioner, (range, loopState) =>
    {
        var subtotal = new Result();

        for (int i = range.Item1; i < range.Item2; i++)
        {
            double n = numbers[i];
            subtotal.SumAll  += n;
            subtotal.SumAllQ += n*n;
        }

        lock (result)
        {
            result.SumAll  += subtotal.SumAll;
            result.SumAllQ += subtotal.SumAllQ;
        }
    });

    return result;
}

My results when I run the full test program (shown below these results) are:

Result via Linq:      SumAll=49999950000, SumAllQ=3.33332833333439E+15
Result via loop:      SumAll=49999950000, SumAllQ=3.33332833333439E+15
Result via partition: SumAll=49999950000, SumAllQ=3.333328333335E+15
Via Linq took: 00:00:01.1994524
Via Loop took: 00:00:00.2357107
Via Partition took: 00:00:00.0756707

(Note the slight differences due to rounding errors.)

It'd be interesting to see the results from other systems.

Here's the full test program:

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

namespace Demo
{
    public class Result
    {
        public double SumAll;
        public double SumAllQ;

        public override string ToString()
        {
            return string.Format("SumAll={0}, SumAllQ={1}", SumAll, SumAllQ);
        }
    }

    class Program
    {
        void run()
        {
            var numbers = Enumerable.Range(0, 1000000).Select(n => n/10.0).ToArray();

            // Prove that the calculation is correct.
            Console.WriteLine("Result via Linq:      " + ViaLinq(numbers));
            Console.WriteLine("Result via loop:      " + ViaLoop(numbers));
            Console.WriteLine("Result via partition: " + ViaPartition(numbers));

            int count = 100;

            TimeViaLinq(numbers, count);
            TimeViaLoop(numbers, count);
            TimeViaPartition(numbers, count);
        }

        void TimeViaLinq(double[] numbers, int count)
        {
            var sw = Stopwatch.StartNew();

            for (int i = 0; i < count; ++i)
                ViaLinq(numbers);

            Console.WriteLine("Via Linq took: " + sw.Elapsed);
        }

        void TimeViaLoop(double[] numbers, int count)
        {
            var sw = Stopwatch.StartNew();

            for (int i = 0; i < count; ++i)
                ViaLoop(numbers);

            Console.WriteLine("Via Loop took: " + sw.Elapsed);
        }

        void TimeViaPartition(double[] numbers, int count)
        {
            var sw = Stopwatch.StartNew();

            for (int i = 0; i < count; ++i)
                ViaPartition(numbers);

            Console.WriteLine("Via Partition took: " + sw.Elapsed);
        }

        Result ViaLinq(double[] numbers)
        {
            return numbers.AsParallel().Aggregate(new Result(), (input, value) => new Result
            {
                SumAll  = input.SumAll+value,
                SumAllQ = input.SumAllQ+value*value
            });
        }

        Result ViaLoop(double[] numbers)
        {
            var result = new Result();

            for (int i = 0; i < numbers.Length; ++i)
            {
                double n = numbers[i];
                result.SumAll  += n;
                result.SumAllQ += n*n;
            }

            return result;
        }

        Result ViaPartition(double[] numbers)
        {
            var result = new Result();

            var rangePartitioner = Partitioner.Create(0, numbers.Length);

            Parallel.ForEach(rangePartitioner, (range, loopState) =>
            {
                var subtotal = new Result();

                for (int i = range.Item1; i < range.Item2; i++)
                {
                    double n = numbers[i];
                    subtotal.SumAll  += n;
                    subtotal.SumAllQ += n*n;
                }

                lock (result)
                {
                    result.SumAll  += subtotal.SumAll;
                    result.SumAllQ += subtotal.SumAllQ;
                }
            });

            return result;
        }

        static void Main()
        {
            new Program().run();
        }
    }
}