PLINQ bad performance

Question

I'm trying to implement PLINQ example but facing following problem My sequential queries are executed faster than parallel queries.

here is the code example:

        Stopwatch sw = new Stopwatch();

        int[] vals = Enumerable.Range(0, Int16.MaxValue).ToArray();

        sw.Start();
        int[] x1 = vals.Where(x => x % 2 == 0).ToArray();
        sw.Stop();
        Console.WriteLine("Sequential Execution {0} milliseconds", sw.ElapsedMilliseconds);


        sw.Restart();
        int[] x2 = vals.AsParallel().Where(x => x % 2 == 0).ToArray();
        sw.Stop();
        Console.WriteLine("Parallel Execution {0} milliseconds", sw.ElapsedMilliseconds);

My machine is Pentium(R) Dual - Core I've also tried on Quad - Core AMD Opteron(tm).

The same result Parallel queries run slower than sequential. Can you please tell me what is my problem?

Thanks.

Answer 1

I guess this has to do with some overhead. The collection you iterate is quite small (32k shorts) and the operation performed on those items is trivial.

In such a scenario the partitioning of a collection, filtering and remerging may be much more expensive than performing the same within a single iteration.

If your comparison is more expensive (e.g. searching strings) and your collection grows, you´ll see the results changing.