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POWhy does multithreading give a worse performance here?
 

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  1. POWhy does multithreading give a worse performance here?
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    <p>I enumerate through a bitarray setting every second bit to false.</p> <p>Now I'd like to speed this up by splitting it up into two threads.. for some weird reason though, the time per Thread to do the <strong>HALF</strong> amount of work takes 64% <strong>MORE</strong> time, and I wonder why's that? </p> <p>Could this be due to some kind of CPU caching effect? How do I do this properly?</p> <p>I have tried 8 threads too previously with lambda expressions but it was always around ~1400 ms, however in single threading I constandly got 850 ms. Also when I let a single thread do all the work, it took me 830 ms. I just don't understand, anyone knowing the cause for that here?</p> <p><em>Code:</em></p> <pre><code> class Program { static int count = 0x10000000; static int half = count / 2; static BitArray bitArray = new BitArray(count); static unsafe void Main(string[] args) { Stopwatch sw = Stopwatch.StartNew(); #if SINGLE for (int i = 0; i &lt; bitArray.Count; i += 2) bitArray.Set(i, true); #else Thread thread1 = new Thread(Thread1); Thread thread2 = new Thread(Thread2); thread1.Start(); thread2.Start(); thread1.Join(); thread2.Join(); #endif sw.Stop(); Console.WriteLine(sw.ElapsedMilliseconds); Console.ReadLine(); } static void Thread1() { Stopwatch sw = Stopwatch.StartNew(); for (int i = 0; i &lt; half; i += 2) bitArray.Set(i, true); sw.Stop(); Console.WriteLine("Thread1: {0}", sw.ElapsedMilliseconds); } static void Thread2() { Stopwatch sw = Stopwatch.StartNew(); for (int i = half; i &lt; count; i += 2) bitArray.Set(i, true); sw.Stop(); Console.WriteLine("Thread2: {0}", sw.ElapsedMilliseconds); } } </code></pre>
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    1. USPatrick Desjardins
    1. UShl3mukkel
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      1. POWhy does multithreading give a worse performance here?
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    1. COHow many CPU cores do you have?
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      1. POWhy does multithreading give a worse performance here?
      1. USJesse C. Slicer
    2. COHave you done any sort of profiling yet? Also, why is your code unsafe?
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      1. USTombatron
    3. COI have 4 cores with 8 Threads, Core i7 2600-K @ 4.0 Ghz
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      1. UShl3mukkel
 

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