Parallel Sort Algorithm

Question

I'm looking for a simple implementation of a parallelized (multi-threaded) sort algorithm in C# that can operate on List<T> or Arrays, and possibly using Parallel Extensions but that part isn't strictly necessary.

Edit: Frank Krueger provides a good answer, however I wish to convert that example to one that doesn't use LINQ. Also note that Parallel.Do() seems to have been superceded by Parallel.Invoke().

Thanks.

Answer 1

Update I now achieve better than 1.7x speedup on a dual core machine.

I thought I would try writing a parallel sorter that worked in .NET 2.0 (I think, check me on this) and that doesn't use anything other than the ThreadPool.

Here are the results of sorting a 2,000,000 element array:

Time Parallel    Time Sequential
-------------    ---------------
2854 ms          5052 ms
2846 ms          4947 ms
2794 ms          4940 ms
...              ...
2815 ms          4894 ms
2981 ms          4991 ms
2832 ms          5053 ms

Avg: 2818 ms     Avg: 4969 ms
Std: 66 ms       Std: 65 ms
Spd: 1.76x

I got a 1.76x speedup - pretty close to the optimal 2x I was hoping for - in this environment:

1. 2,000,000 random Model objects
2. Sorting objects by a comparison delegate that compares two DateTime properties.
3. Mono JIT compiler version 2.4.2.3
4. Max OS X 10.5.8 on 2.4 GHz Intel Core 2 Duo

This time I used Ben Watson's QuickSort in C#. I changed his QuickSort inner loop from:

QuickSortSequential:
    QuickSortSequential (beg, l - 1);
    QuickSortSequential (l + 1, end);

to:

QuickSortParallel:
    ManualResetEvent fin2 = new ManualResetEvent (false);
    ThreadPool.QueueUserWorkItem (delegate {
        QuickSortParallel (l + 1, end);
        fin2.Set ();
    });
    QuickSortParallel (beg, l - 1);
    fin2.WaitOne (1000000);
    fin2.Close ();

(Actually, in the code I do a little load balancing that does seem to help.)

I've found that running this parallel version only pays off when there are more than 25,000 items in an array (though, a minimum of 50,000 seems to let my processor breath more).

I've made as many improvements as I can think of on my little dual core machine. I would love to try some ideas on 8-way monster. Also, this work was done on a little 13" MacBook running Mono. I'm curious how others fare on a normal .NET 2.0 install.

The source code in all its ugly glory is availble here: http://www.praeclarum.org/so/psort.cs.txt. I can clean it up if there's any interest.