[Beowulf] Re: the comparison between OpenMP and MPI

[Florent Calvayrac]

Peter St. John wrote:
> Rich,
> Two things; the small and obvious point, "cheapest" isn't the only 
> motivation for open source, but you know that.
> What surprised me was "...can handle more complex codes" and "...to 
> compile correctly". By compiling correctly, you mean, achieving the 
> desired performance characteristics for the target executable? In my 
> experience compilers are reliably logically correct. I once tracked a 
> bug to the symbolic debugger :-), but never to the compiler itself 
> (although I've always been able to use mature compilers). Compiler 
> writers pretty much define "language law". (And I'm sure the ones at 
> Intel are just as proud as the ones at IBM and CMU.)
> As for complexity, I've written things that exceeded the available 
> stack depth, but really I don't understand a program being too complex 
> for a compiler. Too long, sure. Everything has resource limitations. 
> But not too complex. So I'd be very amused to see some examples, maybe 
> of local complexity, I wouldn't be able to read the 100k lines of 
> fortran myself :-)
> Peter
>
I once discovered a bug in the Cray Fortran compiler for T3E.
Without optimization the code was running fine but at O2/O3
results were wrong. It turned out that some code lines I had autogenerated
from Maple were implying a large number of variables, exceeding
 the number of registers available and wrapping on the first register.
Without optimization no registers were used so the code was giving
correct results and at O1 code was optimized on a line per line basis
and not procedure-wide.

So even commercial code can be wrong...



-- 

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