[gpaw-users] Different MPI worlds ASE vs. GPAW: small fix, big fix or migrating to mpi4py?

[Gaël Donval]

Hi,

The test `generic/hydrogen.py` hangs with the pristine python
interpreter with PARALLEL support provided in this MR:

   https://gitlab.com/gpaw/gpaw/merge_requests/403

This is caused by the ASE DB access part of the test:

 * gpaw.mpi.rank gives the right rank.
 * ase.parallel.rank always returns 0 ("DummyMPI()" is used because
   _gpaw is then not built-in).

I can submit an MR in ASE to fix that last point but I'm really
starting to wonder if we shouldn't:

 * at the very least separate mpi.c from _gpaw.c to avoid having MPI
   coupling all other the place;
 * migrate to mpi4py and keep it well-separated as well.

>From Python's perspective, all the MPI stuff in `_gpaw` does not exist:
it is only ever used in `gpaw.mpi` precisely to provide MPI. Yet it is
everywhere: I'd really like to get rid of it.

What do you think bout it?

Gaël