Question

For school we got a task to create a multithreaded application. We chose to make a multithreaded implementation of merge sort.

We however can’t manage to make it work faster than the serial implementation.

I have already tried the following:

• implementation with unlimited threads (code example 1) (was extremely slow)
• implementation with limited threads (code example 2) (4 threads max – still really slow)
• implementation using Parallel.Invoke (code example 3) (still slower)
• complex implementation also with a parallel merge function (just shamefully slow)

When I use the analyze tools in Visual Studio (Instrumentation part) I found the timings for the functions called and the threaded solution is always extremely slower than the serial implementation.

I can’t see any possible reason for this.

(for example: with 5000000 numbers to sort; serial implementation: 16.717,17; parallel: 20.259,97; results with just 1 extra thread)

I tested it on both machines I own:

• Intel Core 2 Quad Q9450 @ 2.66Ghz
• Intel Core i7 Q720 @1.60Ghz

I can’t for my life figure out how this is possible, shouldn’t this just speed up the process?

I would be really greatefull if somebody would be able to help me out.

code example 1:

ParallelMerge pMerge = new ParallelMerge(T, p1, q1 -1, p2, q2-1, A, p3);
Thread thread = new Thread(new ThreadStart(pMerge.parallel_merge));
thread.Start();

ParallelMerge pMerge2 = new ParallelMerge(T, q1 + 1, r1, q2, r2, A, q3 + 1);
pMerge2.parallel_merge();
thread.Join();

code example 2:

if(depthRemaining > 0)
{
   ParallelMerge pMerge = new ParallelMerge(T, p1, q1 -1, p2, q2-1, A, p3);
   thread thread = new Thread(new ThreadStart(pMerge.parallel_merge));
   thread.Start();
   ParallelMerge pMerge2 = new ParallelMerge(T, q1 + 1, r1, q2, r2, A, q3 + 1);
   pMerge2.parallel_merge(); 
   thread.Join();
}
else
{
   ParallelMerge pMerge = new ParallelMerge(T, p1, q1 -1, p2, q2-1, A, p3);
   pMerge.parallel_merge(); 
   ParallelMerge pMerge2 = new ParallelMerge(T, q1 + 1, r1, q2, r2, A, q3 + 1);
   pMerge.parallel_merge(); 
}

code example 3:

if (depthRemaining > 0)
{
   Parallel.Invoke(
      () => threaded_merge_sort(getallen, p, q, depthRemaining-1));

   threaded_merge_sort(getallen, q + 1, r, 0);
}
else
{
   threaded_merge_sort(getallen, p, q, 0);
   threaded_merge_sort(getallen, q+1, r, 0);
}

Answer 1

it seems the problem wasn’t with the code but with the analysing tools from VS.

-Arne Claerebout