語言

如何：加速小型迴圈主體

當 Parallel.For 迴圈的主體較小時，其執行速度可能比等效的序列迴圈慢，例如 C# 中的 for 迴圈和 Visual Basic 中的 For 迴圈。 效能較慢是因為資料分割的開銷以及每次迴圈迭代呼叫代理的成本所致。 為了解決這類情況，該 Partitioner 類別提供了 Partitioner.Create 方法，讓你能為委派主體提供一個序列迴圈，使代理在每個分割中只被呼叫一次，而非每次迭代。 如需詳細資訊，請參閱 PLINQ 和 TPL 的自定義分割器。

Example

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

class Program
{
    static void Main()
    {

        // Source must be array or IList.
        var source = Enumerable.Range(0, 100000).ToArray();

        // Partition the entire source array.
        var rangePartitioner = Partitioner.Create(0, source.Length);

        double[] results = new double[source.Length];

        // Loop over the partitions in parallel.
        Parallel.ForEach(rangePartitioner, (range, loopState) =>
        {
            // Loop over each range element without a delegate invocation.
            for (int i = range.Item1; i < range.Item2; i++)
            {
                results[i] = source[i] * Math.PI;
            }
        });

        Console.WriteLine("Operation complete. Print results? y/n");
        char input = Console.ReadKey().KeyChar;
        if (input == 'y' || input == 'Y')
        {
            foreach(double d in results)
            {
                Console.Write("{0} ", d);
            }
        }
    }
}

Imports System.Threading.Tasks
Imports System.Collections.Concurrent

Module PartitionDemo

    Sub Main()
        ' Source must be array or IList.
        Dim source = Enumerable.Range(0, 100000).ToArray()

        ' Partition the entire source array. 
        ' Let the partitioner size the ranges.
        Dim rangePartitioner = Partitioner.Create(0, source.Length)

        Dim results(source.Length - 1) As Double

        ' Loop over the partitions in parallel. The Sub is invoked
        ' once per partition.
        Parallel.ForEach(rangePartitioner, Sub(range, loopState)

                                               ' Loop over each range element without a delegate invocation.
                                               For i As Integer = range.Item1 To range.Item2 - 1
                                                   results(i) = source(i) * Math.PI
                                               Next
                                           End Sub)
        Console.WriteLine("Operation complete. Print results? y/n")
        Dim input As Char = Console.ReadKey().KeyChar
        If input = "y"c Or input = "Y"c Then
            For Each d As Double In results
                Console.Write("{0} ", d)
            Next
        End If

    End Sub
End Module

此範例所示方法在迴圈工作量極少時非常有用。 隨著運算成本增加，使用搭配預設分割器的 For 或 ForEach 迴圈，通常能獲得相同甚至更好的效能。

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