Question

I need to parallelize a method that does an exhaustive pairwise comparison on elements in a list. The serial implementation is straight-forward:

foreach (var element1 in list)
    foreach (var element2 in list)
        foo(element1, element2);

In this case, foo won’t alter the state of element1 or element2. I know it’s not safe to simply do nested Parallel.ForEach statements:

Parallel.ForEach(list, delegate(A element1)
{
    Parallel.ForEach(list, delegate(A element2)
    {
        foo(element1, element2);
    });
});

What would be the ideal way to implement this using the parallel tasks library?

Answer 1

Couldn’t you just have one Parallel and one normal loop? So either

Parallel.ForEach(list, delegate(A element1)
{
  foreach(A element2 in list)
    foo(element1, element2)
});

or

foreach(A element1 in list)
{
  Parallel.ForEach(list, delegate(A element2)
  {
    foo(element1, element2);
  });
}

Should speed it up as well. There was never going to be a thread per cycle anyway, so this would probably be just as fast or slightly slower than nested parallel loops.