Parallel.For: Example

Question

Hi guys

i just want to ask. is there someone here who has experience using the parallel.for loop? can you give me an example or snippet of you work? and if you are too kind enough explain to me how it works...

Thank you very much
Imo

Vulpes · Answer

Here's a simple example I've devised to illustrate Parallel.For and to compare it with the 'normal' sequential For statement. On my dual core Pentium I3 laptop and on most occasions the time taken to process the task (calculate 10 million square roots) using parallel processing is around 50% of that taken using sequential processing which of course is what you'd expect: Imports System Imports System.Diagnostics Imports System.Threading.Tasks Module Test Sub Main() ' find square roots of first 10 million non-negative integers store in an array ' and find the average to check that the results are the same ' for both sequential and parallel processing Dim roots(10000000) As Double Dim watch As StopWatch ' first do it using a sequential for statement watch = StopWatch.StartNew() For x As Integer = 0 To 9999999 roots(x) = Math.Sqrt(x) Next watch.Stop() Console.WriteLine(Time taken by sequential for = {0} ms watch.ElapsedMilliseconds) Dim Average As Double = AverageRoot(roots) Console.WriteLine(Average using sequential for = {0} Average) ' now do it using a parallel for statement Console.WriteLine() Array.Clear(roots 0 roots.Length) watch = StopWatch.StartNew() Parallel.For(0 10000000 Sub(x) roots(x) = Math.Sqrt(x)) watch.Stop() Console.WriteLine(Time taken by parallel for = {0} ms watch.ElapsedMilliseconds) Average = AverageRoot(roots) Console.WriteLine(Average using sequential for = {0} Average) Console.ReadKey End Sub Function AverageRoot(ByVal roots As Double()) As Double Dim Total As Double = 0 For Each d As Double in roots Total += d Next Return Total / roots.Length End Function End Module The overload of the Parallel.For method I'm using takes 3 arguments: 1. A starting value (inclusive). 2. An ending value (exclusive). 3. A lambda expression which calculates the root and stores it in the array. As you probably know some iterations will be allocated to a thread running on one core and the other iterations to a thread running on the other core. As only one thread can deal with a particular iteration there's no chance of a collision when writing the result to the array and a lock is not therefore needed.