[Beowulf] g77 limits...

Florent Calvayrac Florent.Calvayrac at univ-lemans.fr
Thu Feb 23 01:22:09 PST 2006

Robert G. Brown wrote:
> Dear List,
> What are the current limits on the size of arrays that can be allocated
> in g77?  I know that this is almost a FAQ, but I cannot look back at the
> archives because it is so time dependent an answer.  In particular, can
> an x64 box with a modern linux kernel and a modern g77 allocate 4-8 GB
> arrays (presuming, of course, that one uses a long int for an index)?  I
> have seen references to "using an offset" in a few google hits (that
> were not very informative) -- does that basically mean doing pointer
> arithmetic in fortran?

it works without too much worrying about long ints
 in g77 if you use the compile switch  "-mcmodel=medium" ou -mcmodel=large.

(at least it is my experience on an Opteron64 box with Centos4.0 and  
16Go of RAM)

>   d) Where commercial compilers aren't a viable option (although I
> suggested them) -- the software has to build and be usable by e.g.
> researchers in countries where there simply is no money to spend on
> compilers.

isn't the Intel compiler available free of charge for students or demo 
purposes if
you give up support  ?

Besides I found that for the ifc the
"-mcmodel=medium -i_dynamic"
flag combination   is helpful for large static arrays where heap 
problems can appear

> The last suggests that it would be ideal if large arrays were at least
> approximately "transparent" -- so that the software would build on 32
> bit systems and be runnable there with smaller arrays but would also
> build and run on x64 big-memory systems without the need for extensive
> instrumentation of the code.

Works for me with g77.

Florent Calvayrac  | 
Directeur du SC Informatique Ressources Num.  de l'Universite du Maine
Lab. de Physique de l'Etat Condense UMR-CNRS 6087 
Inst. de Rech. en Ingenierie Molec. et Matx Fonctionnels  FR CNRS 2575 

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