Disappointing performance with Parallel.For

Question

I am trying to speed up my calculation times by using Parallel.For. I have an Intel Core i7 Q840 CPU with 8 cores, but I only manage to get a performance ratio of 4

Answer 1

foreach vs parallel for each an example

    for (int i = 0; i < 10; i++)
    {
        int[] array = new int[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29 };
        Stopwatch watch = new Stopwatch();
        watch.Start();
        //Parallel foreach
        Parallel.ForEach(array, line =>
        {
            for (int x = 0; x < 1000000; x++)
            {

            }

        });

        watch.Stop();
        Console.WriteLine("Parallel.ForEach {0}", watch.Elapsed.Milliseconds);
        watch = new Stopwatch();
        //foreach
        watch.Start();
        foreach (int item in array)
        {
            for (int z = 0; z < 10000000; z++)
            {

            }
        }
        watch.Stop();
        Console.WriteLine("ForEach {0}", watch.Elapsed.Milliseconds);

        Console.WriteLine("####");
    }
    Console.ReadKey();

My CPU

Intel® Core™ i7-620M Processor (4M Cache, 2.66 GHz)