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POMaxDegreeOfParallelism = Environment.ProcessorCount slows down execution time on my CPU
 

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  1. POMaxDegreeOfParallelism = Environment.ProcessorCount slows down execution time on my CPU
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    <p>I have the following program (that I got from <a href="http://blogs.msdn.com/b/csharpfaq/archive/2010/06/01/parallel-programming-in-net-framework-4-getting-started.aspx" rel="nofollow">http://blogs.msdn.com/b/csharpfaq/archive/2010/06/01/parallel-programming-in-net-framework-4-getting-started.aspx</a>) that splits a task up using <strong>Parallel.For</strong> loop</p> <pre><code>class Program { static void Main(string[] args) { var watch = Stopwatch.StartNew(); Parallel.For(2, 20, (i) =&gt; { var result = SumRootN(i); Console.WriteLine("root {0} : {1} ", i, result); }); Console.WriteLine(watch.ElapsedMilliseconds); Console.ReadLine(); } public static double SumRootN(int root) { double result = 0; for (int i = 1; i &lt; 10000000; i++) { result += Math.Exp(Math.Log(i) / root); } return result; } } </code></pre> <p>When I run this test several times I get times of: </p> <p>1992, 2140, 1783, 1863 ms etc etc.</p> <p>My first question is, <strong>why are the times always different??</strong> I am doing the exact same calculations each time yet the times vary each time.</p> <p>Now when I add in the following code to make use of all the available processors on my CPU:</p> <pre><code> var parallelOptions = new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount (On my CPU this is 8) }; Parallel.For(2, 20, parallelOptions, (i) =&gt; { var result = SumRootN(i); Console.WriteLine("root {0} : {1} ", i, result); }); </code></pre> <p>I notice that the execution times actually increase!! The times are now:</p> <p>2192, 3192, 2603, 2245 ms etc etc.</p> <p><strong>Why does this cause the times to increase? Am I using this wrong?</strong> </p>
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    1. USHarry Boy
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      1. VTUpMod
    1. COWhat else is running on your system when you run the tests? Are you compiling in Release mode? Is a debugger attached? Is a profiler attached?
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      1. POMaxDegreeOfParallelism = Environment.ProcessorCount slows down execution time on my CPU
      1. USLumpN
    2. COTo get really comparable results, avoid outputs to the console from those threads.
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      1. POMaxDegreeOfParallelism = Environment.ProcessorCount slows down execution time on my CPU
      1. USOndrej Tucny
    3. COI have Outlook open, EverNote open and Chrome webbrowser.
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      1. POMaxDegreeOfParallelism = Environment.ProcessorCount slows down execution time on my CPU
      1. USHarry Boy
 

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