c# - 执行 Parallel.For 从 Array 计算数据的正确方法

Question

想要:对 x 和 x*x 求和。其中 x = 行 [i]。因为不止一个线程想要读取/写入“sumAll”和“sumAllQ”，所以我需要锁定它的访问权限。问题是锁类关闭在这里序列化了东西。我需要将此操作拆分为 #"Environment.ProcessorCount"循环，每个循环对数组的一部分求和，最后对它们的结果求和。但是我怎样才能以编程方式实现呢？

示例代码:

//line is a float[]
Parallel.For(0, line.Length,
new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount },
i =>
{
    x = (double)line[i];
    lock (sumLocker)
    {
        sumAll += x;
        sumAllQ += x * x;
    }
});

编辑 1:Matthew Watson 回答基准测试结果

在家里。 CPU Core 2 四核 Q9550 @ 2.83 GHz:

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: 00:00:02.6983044
Via Loop took: 00:00:00.4811901
Via Partition took: 00:00:00.1595113

在工作。中央处理器 i7 930 2.8 GHz:

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: 00:00:01.5728736
Via Loop took: 00:00:00.3436929
Via Partition took: 00:00:00.0934209

Answer 1

vcjones 想知道您是否真的会看到任何加速。好吧，答案是:这可能取决于您拥有多少个内核。在我的家用 PC(四核)上，PLinq 比普通循环慢。

我想出了一个替代方法，它使用 Partitioner将数字列表分成几个部分，这样您就可以将每个部分分别相加。还有一些 information about using a Partitioner here .

使用 Partitioner 方法似乎更快一些，至少在我的家用 PC 上是这样。

这是我的测试程序。请注意，您必须在任何调试器之外运行此发布版本，以获得正确的时间。

这段代码中的重要方法是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;
}

我运行完整测试程序时的结果(显示在这些结果下方)是:

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

(请注意由于舍入误差造成的细微差异。)

如果能看到其他系统的结果会很有趣。

完整的测试程序如下:

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();
        }
    }
}