Switch from C++ MPI interface to C interface

[mdjurfeldt]

The C++ interface to MPI has been deprecated.

On OpenMPI installations >= 2.0.1 under OSX, the C++ interface is disabled.

MUSIC should switch to using the C interface.

Notifying @heplesser.

[orjanekeberg]

Unfortunately, this also affects the MUSIC API, because Setup::communicator() and Runtime::communicator() both return MPI::Intracomm which will have to change.

[mdjurfeldt]

Next release is MUSIC 2.0. I guess an API change could be acceptable. During the path towards release, we could maintain two branches, one with C++ MPI API and one without.

Not sure how good idea it is, but Setup::communicator () and Runtime::communicator () could support both the MPI C++ API (on platforms where it exists) and the MPI C interface through the use of coercion rules. That way, we could have a period where the C++ variant is deprecated, and the need of two branches is obviated.

I should also mention here that it seems rather easy to recompile OpenMPI with the C++ enabled on the OSX platform.

[heplesser]

@mdjurfeldt Thanks for pointing out the (almost) obvious. With

brew install open-mpi --with-cxx-bindings

one can install current MPI versions with C++ bindings under OSX.

[lekshmideepu]

@heplesser : To me it says Error: invalid option: --with-cxx-bindings

[heplesser]

@lekshmideepu The open-mpi formula may have changed over the past 2.5 years, so that the option is no longer available. Maybe it is no longer needed, and C++ bindings are built automatically?

[lekshmideepu]

@heplesser
From the OpenMPI release notes:

The MPI C++ bindings -- which were removed from the MPI standard in
v3.0 -- are no longer built by default and will be removed in some
future version of Open MPI. Use the --enable-mpi-cxx-bindings
configure option to build the deprecated/removed MPI C++ bindings.

[mdjurfeldt]

@heplesser @lekshmideepu It is probably time that I release a version of MUSIC which uses the C MPI bindings directly.

[opoplawski]

FYI - openmpi 5.0.0 has been released which removed the C++ bindings.

[sanjayankur31]

@mdjurfeldt @heplesser : what can we do to help here? MUSIC won't build on the latest Fedora releases (currently rawhide) with OpenMPI until this is fixed.

[mdjurfeldt]

@sanjayankur31 @heplesser MUSIC switched to using MPI C bindings mid 2022 and the 1.2 releases have these changes. I will look into why it doesn't build with OpenMPI 5.0.0.

[sanjayankur31]

Ah, great, thanks very much @mdjurfeldt . I'll also go check again. It could be other issues causing our build to fail too.

[mdjurfeldt]

@sanjayankur31 @heplesser Thank you, Sanjay. Once we have sorted this out, I'll close this issue (should have been done long ago!).

[sanjayankur31]

We're just waiting for Cython 3.0.6 to land:

#77 (comment)

I'll let you know as soon as this is done and confirmed.

[sanjayankur31]

We have a fixed build now, so I think this can be closed ✨

[sanjayankur31]

Although, we have this issue open at the mo, still investigating what's going on here:

nest/nest-simulator#3057