Re: [gentoo-dev] vitrual/mpi

[Anton Starikov]

Ferris McCormick wrote:
> Both sys-cluster/lam-mpi and sys-cluster/mpich provide implementations
> of mpi (message passing interface, I believe) to allow building
> computation clusters, similar to PVM (parallel virtual machine).
That's correct :) But MPI can be used not only with clusters. I have it 
on my home and office desktops, just because I develop software for MPI 
:) And people use it on SMP machines also.

> Unfortunately, lam-mpi and mpich are not quite compatible.  A
> few packages (few = 2 to 5) depend explicitly on lam-mpi in case
> of USE='mpi ...', and apparently cannot use mpich. (dev-libs/fftw-2.x
> series, app-sci/gromacs, app-sci/orsa, app-sci/tree-puzzle)
That's right, these packages depend on lam-mpi, because it is hardcoded 
into ebuilds. One of ideas behind virtual/mpi is to make it possible to 
write ebuild that will support both of them. It is not impossible. 
Theoretically, any application, which can be compiled with LAM can be 
also compiled with MPICH, if application conform MPI standards (at least 
MPI 1.2) and not tuned especially for particular MPI realization (it is 
really exotic case, because it brokes one of ideas behind of  MPI as a 
open standard). So, on a source level (source of application which use 
MPI, of course) - they are the same.
There is only some minor difference about linking. And some exotic in 
case of using some non-standard Fortran features and MPICH. (for ex. if 
you get command-line arguments from Fortran, AFAIK it is not standard 
and not conform MPI standard, with MPICH you will get a bit more, than 
you need, and different on slave and master nodes :)). But AFAIR there 
no any exotic with C/C++.
But linking also could be done without problems. Ebuilds just should 
follow MPI standard and use mpicc instead gcc, mpif77 instead g77 and so 
on. If in ebuilds for LAM_MPI, MPICH (and something else which will 
appeared) it is properly configured, they will install such wrappers, 
which BTW cares about proper linking with proper libraries. AFAIR they 
do it now.
OK, often people really don't care about wrappers and link directly, 
that's true :) But even in such case there is no big problem with 
linking. Just add check in ebuild, which MPI we use. Or, probably in 
some applications autoconf care about it (I think in app-sci/gromacs 
such case).

fftw definetely  can use MPICH. Moreover, MPICH is much more spread than 
LAM, so, I almost sure that it is much more probable to find some 
software which will not work with LAM, rather with MPICH. For example, I 
always test my programs with MPICH, even if I personaly like LAM. So, I 
always keep one machine with MPICH. Because MPICH is some kind of 
portable reference implementation of MPI related to MPI standard developers.

-- 
Anton Starikov

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