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Subject: Re: [OMPI users] Prototypes for Fortran MPI_ commands using 64-bit indexing
From: Jeff Squyres (jsquyres) (jsquyres_at_[hidden])
Date: 2013-10-30 17:49:20


On Oct 30, 2013, at 4:35 PM, Jim Parker <jimparker96313_at_[hidden]> wrote:

> I have recently built a cluster that uses the 64-bit indexing feature of OpenMPI following the directions at
> http://wiki.chem.vu.nl/dirac/index.php/How_to_build_MPI_libraries_for_64-bit_integers

That should be correct (i.e., passing -i8 in FFLAGS and FCFLAGS for OMPI 1.6.x).

> My question is what are the new prototypes for the MPI calls ?
> specifically
> MPI_RECV
> MPI_Allgathterv

They're the same as they've always been.

The magic is that the -i8 flag tells the compiler "make all Fortran INTEGERs be 8 bytes, not (the default) 4." So Ralph's answer was correct in that all the MPI parameters are INTEGERs -- but you can tell the compiler that all INTEGERs are 8 bytes, not 4, and therefore get "large" integers.

Note that this means that you need to compile your application with -i8, too. That will make *your* INTEGERs also be 8 bytes, and then you'll match what Open MPI is doing.

> I'm curious because some off my local variables get killed (set to null) upon my first call to MPI_RECV. Typically, this is due (in Fortran) to someone not setting the 'status' variable to an appropriate array size.

If you didn't compile your application with -i8, this could well be because your application is treating INTEGERs as 4 bytes, but OMPI is treating INTEGERs as 8 bytes. Nothing good can come from that.

If you *did* compile your application with -i8 and you're seeing this kind of wonkyness, we should dig deeper and see what's going on.

> My review of mpif.h and mpi.h seem to indicate that the functions are defined as C int types and therefore , I assume, the coercion during the compile makes the library support 64-bit indexing. ie. int -> long int

FWIW: We actually define a type MPI_Fint; its actual type is determined by configure (int or long int, IIRC). When your Fortran code calls C, we use the MPI_Fint type for parameters, and so it will be either a 4 or 8 byte integer type.

-- 
Jeff Squyres
jsquyres_at_[hidden]
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