[Beowulf] compilers vs mpi?

Mark Hahn hahn at mcmaster.ca
Tue Jul 20 11:54:59 PDT 2010

>> between the application and MPI.  that is, I would like to be able
>> to compile MPI (say, OpenMPI) with gcc, and expect it to work correctly 
>> with apps compiled with other compilers.  I guess I'm reasoning by analogy 
>> to normal distro libs.
> I haven't built OpenMPI this way,
> but you may try to link statically with commercial compiler libraries
> (say -static-intel, -Bstatic_pgi),

I'd rather build with gcc if possible.  I guess I'd be surprised if 
there were compute-intensive-enough parts of MPI to justify using some
other compiler.  (please, if anyone has any quantitative observations 
on the quality of current compilers, let me/list know!)

> Yes, they do recommend compiler homogeneity.
> However, I have built hybrids gcc+ifort
> and gcc+pgf90 and both work fine.
> (I have the homogeneous versions also.)

oh.  so the idea here is that the C part of OpenMPI has an ABI
which is compatible with basically all the other C compilers,
such as would be used to compile app-side code.  but that the fortran 
side has to be matched, library and app sides?  if that's the case,
then would it make sense to factor out the fortran interface?

> Fortran77 never had these features anyway, and I guess
> mpif77 doesn't check if you are passing an integer
> where it should be a real, or if your argument list is shorter
> than the function requires.

so if I have f90 code that uses an mpi header (not .mod interface),
does that mean there's no function signature checking at all?
as far as I know, my organization has never done .mod-based MPI,
so maybe this is why we're facing the issue now, after 10 years 
and 4k users ;)

>> PS: we have a large and diverse user base, so tend to have to support gcc, 
>> intel, pathscale and pgi. 
> ... and don't forget Open64!  :)

well, that's an interesting point.  I haven't quite figured out who is doing
the canonical release for Open64 nowadays (highest ver number seems to be 
from AMD).  have you done any comparisons?

>> we even have people who want to use
>> intel's damned synthetic 128b FP over MPI :(
> It's hard to keep the customer satisfied.
> You give them the sky, they want the universe.

for me, the real problem is knowing whether the user understands that 
synthetic 128b FP is drastically slower than 64b hardware FP.  has anyone
tried to do a comparison?

thanks, mark.

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