How exactly does AsParallel work?

Question

It doesn't seem to do squat for the following test program. Is this because I'm testing with a small list?

static void Main(string[] args)
{
    List<int> list = 0.UpTo(4);

    Test(list.AsParallel());
    Test(list);
}

private static void Test(IEnumerable<int> input)
{
    var timer = new Stopwatch();
    timer.Start();
    var size = input.Count();
    if (input.Where(IsOdd).Count() != size / 2)
        throw new Exception("Failed to count the odds");

    timer.Stop();
    Console.WriteLine("Tested " + size + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
}

private static bool IsOdd(int n)
{
    Thread.Sleep(1000);
    return n%2 == 1;
}

Both versions take 4 seconds to run.

Answer 1

Task Parallel Library cares about the static type of the sequence. It should be IParallelEnumerable<T> for the operations to be handled by the TPL. You are casting the collection back to IEnumerable<T> when you call Test. Therefore, the compiler will resolve .Where call on the sequence to System.Linq.Enumerable.Where extension method instead of the parallel version provided by the TPL.

Answer 2

As Parallel works by putting your stuff into the ThreadPool. Also, how many cores do you have? If you're working on a single core machine that will still take about 4s to run.

Answer 3

(Updating for .NET4 since this question ranks pretty high in a Google search for AsParallel())

Just a few changes will allow your example to work as I imagine you expected.

Change List<int> list = 0.UpTo(4); to var list = Enumerable.Range(0, 4);

Your example would work if you added a function overload with a signature that takes a ParallelQuery...

    private static void Test(ParallelQuery<int> input)
    {
        var timer = new Stopwatch();
        timer.Start();

        int size = input.Count();
        if (input.Where(IsOdd).Count() != size / 2)
        {
            throw new Exception("Failed to count the odds");
        }
        timer.Stop();

        Console.WriteLine("Tested " + size + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
    }

Alternatively, you could use LINQ syntax....

    private static void Test(ParallelQuery<int> list)
    {
        var timer = Stopwatch.StartNew();

        if ((from n in list.AsParallel()
             where IsOdd(n)
             select n).Count() != (list.Count() / 2))
        {
            throw new Exception("Failed to count the odds");
        }

        Console.WriteLine("Tested " + list.Count() + " numbers in " + timer.Elapsed.TotalSeconds + " seconds");
    }

Hope this helps someone!