[MOAB-dev] [PATCH] Call MPI_Init if using MPI.

[Jed Brown]

Mark Miller wrote:
> Most HPC codes I have worked on have parallel-specific code blocks of
> one form or another.

Are these runtime or compile-time branches?  Which branch is executed
when you have built in parallel and run with mpirun -n 1 ?

> That isn't so much to address MPI-linkage issues in serial as it is to
> address situations in which the actual logic and work to be done is
> different in serial and parallel.

PETSc's stubs must have been getting included inadvertently, there are
no symbols because they are just macros.  An alternative is to namespace
the stubs appropriately.

> In fact PETSC is the ONLY HPC code I am aware of that does this. And, I
> recall many years ago running into MPI name collisions when using it in
> Ale3d. For some reason, Ale3d's MPI_Init() call wound up resolving to
> PETSC's dummy implementation.

Others that I use daily include include Hypre, MUMPS, and ML (and I
think other Trilinos packages).

There is also this package

http://people.sc.fsu.edu/~burkardt/c_src/mpi_stubs/mpi_stubs.html

> I think it is a problem in general for different MPI implementations to
> 'live' in the same executable, even if one of them is a 'serial dummy'.
> And, there isn't necessarily any reason to expect that a parallel
> application is always going to use a given parallel-capable library, in
> parallel as well. HDF5 is a great example. We  at LLNL don't use is it
> in parallel though it is used from parallel apps. If HDF5 had taken the
> dummy MPI approach, that would present a serious problem.

The only problem here is if MPI types are part of the serial interface.
The MPI standard does not define an ABI so, e.g. sizeof(MPI_Comm) ==
sizeof(int) with MPICH2 but sizeof(MPI_Comm) == sizeof(void*) with Open
MPI.  As long as any symbols provided by the stubs (if they even define
any) don't use the MPI namespace, there should be no problem.

Jed

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