What does MaxDegreeOfParallelism do?

For example, is there anyway to do something like, if CPU has two cores, then use 20, if CPU has four cores then 40?

You can do this to make parallelism dependent on the number of CPU cores:

var options = new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount * 10 };
Parallel.ForEach(sourceCollection, options, sourceItem =>
{
    // do something
});

However, newer CPU's tend to use hyper-threading to simulate extra cores. So if you have a quad-core processor, then Environment.ProcessorCount will probably report this as 8 cores. I've found that if you set the parallelism to account for the simulated cores then it actually slows down other threads such as UI threads.

So although the operation will finish a bit faster, an application UI may experience significant lag during this time. Dividing the `Environment.ProcessorCount' by 2 seems to achieve the same processing speeds while still keeping the CPU available for UI threads.

The answer is that it is the upper limit for the entire parallel operation, irrespective of the number of cores.

So even if you don't use the CPU because you are waiting on IO, or a lock, no extra tasks will run in parallel, only the maximum that you specifiy.

To find this out, I wrote this piece of test code. There is an artificial lock in there to stimulate the TPL to use more threads. The same will happen when your code is waiting for IO or database.

class Program
{
    static void Main(string[] args)
    {
        var locker = new Object();
        int count = 0;
        Parallel.For
            (0
             , 1000
             , new ParallelOptions { MaxDegreeOfParallelism = 2 }
             , (i) =>
                   {
                       Interlocked.Increment(ref count);
                       lock (locker)
                       {
                           Console.WriteLine("Number of active threads:" + count);
                           Thread.Sleep(10);
                        }
                        Interlocked.Decrement(ref count);
                    }
            );
    }
}

If I don't specify MaxDegreeOfParallelism, the console logging shows that up to around 8 tasks are running at the same time. Like this:

Number of active threads:6
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:6
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7
Number of active threads:7

It starts lower, increases over time and at the end it is trying to run 8 at the same time.

If I limit it to some arbitrary value (say 2), I get

Number of active threads:2
Number of active threads:1
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2
Number of active threads:2

Oh, and this is on a quadcore machine.