c# - Why does PLINQ use only two threads?

Question

Say I have an IO-bound task. I'm using WithDegreeOfParallelism = 10 and WithExecution = ForceParallelism mode, but still the query only uses two threads. Why?

I understand PLINQ will usually choose a degree of parallelism equal to my core count, but why does it ignore my specific request for higher parallelism?

static void Main(string[] args)
{
    TestParallel(0.UpTo(8));
}

private static void TestParallel(IEnumerable<int> input)
{
    var timer = new Stopwatch();
    timer.Start();
    var size = input.Count();

    if (input.AsParallel().
        WithDegreeOfParallelism(10).
        WithExecutionMode(ParallelExecutionMode.ForceParallelism).
        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;
}

Answer 1

PLINQ tries to find the optimal number of threads to perform what you want it to do as quickly as possible, if you only have 2 cores on your cpu, that number is most likely 2. If you had a quad core, you would be more likely to see 4 threads appear, but creating 4 threads on a dual core machine wouldn't really improve performance because only 2 threads could be active at the same time.

Also, with IO-based operations, it is likely that any extra threads would simply block on the first IO operation performed.