Where should we put collectives?

[rogerpearce]

@steiltre @bwpriest

Quick discussion point. I want to add a prefix_sum collective and am realizing how many collective helpers we have added to the comm. Do you think we should move this out to keep comm from getting so cluttered?

Current list that we have in com::

• all_reduce_sum
• all_reduce_max
• all_reduce_min
• all_reduce

I want to add prefix_sum, but I suspect these will also be very handy in the future:

• bcast
• is_same
  • We could shorten the all_reduce_* to just min/max/sum/and/or
• reduce (w/o the all)

Example usage would be:

{
   #include <ygm/collective.hpp>
   // .....
   size_t sum = ygm::sum(42, comm);
}

EDIT: We could start this by not actually moving the comm::all_reduce* but keeping them and marking them as deprecated and remove later.

[bwpriest]

I like the idea of moving collectives into standalone functions that take a comm as an argument.

[rogerpearce]

OK, I'll start this with prefix_sum and we can later decide when we want to move all_reduce. Lots of code uses all_reduce_sum.

[rogerpearce]

This will require adding a method to get the underlying MPI_Comm from ygm::comm, which is something that @ppete asked me for anyways.

Do we like?

ygm::comm::get_mpi_comm()