How to: Speed Up Small Loop Bodies
When a Parallel.For loop has a small body, it might perform more slowly than the equivalent sequential loop, such as the for loop in C# and the For loop in Visual Basic. Slower performance is caused by the overhead involved in partitioning the data and the cost of invoking a delegate on each loop iteration. To address such scenarios, the Partitioner class provides the Partitioner.Create method, which enables you to provide a sequential loop for the delegate body, so that the delegate is invoked only once per partition, instead of once per iteration. For more information, see Custom Partitioners for PLINQ and TPL.
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
The approach demonstrated in this example is useful when the loop performs a minimal amount of work. As the work becomes more computationally expensive, you will probably get the same or better performance by using a For or ForEach loop with the default partitioner.
