[MITgcm-devel] Re: [MITgcm-cvs] MITgcm/tools/build_options CVS Commit

[Constantinos Evangelinos]

On Friday 11 September 2009 11:03:26 am David Ferreira wrote:
> Hi Dimitris,
> We are looking into this. At this point, with -O3, "-fp-model precise"
> or "-mp"
> is needed otherwise the models crashes (Chris tells me "-ftz" is
> included into "-mp").
> Also, it turns out that the verification experiments with cube-sphere
> fail the restart
> test with "-fp-model precise", but not with "-mp".
> I'm running testreports with -O1/2/3, to see clarify a bit where it gets
> bad.
> "-O3 -ftz" or even "-O2" could end up being good compromise.
> david

On Itanium systems -O3 implies -ftz. It may very well be that is the case for 
-O2 as well. -fp-model-precise appeared to be equivalent to -mp but in fact 
seems to set -no-ftz explicitly so that -O3 -fp-model-precise (-ftz 
overridden) ends up being different from -O3 -mp (-ftz not overridden). -mp/-
fp-model-precise slows things down anyway (but not by a factor of 50 that 
denormal handling causes) so ideally one should find the source code file that 
requires it and only use it there. We are forced to use -mp on ECCO as well.

> Dimitris Menemenlis wrote:
> > Would it be make sense to also remove "-fp-model precise",
> > i.e., make the second line the default, and possibly add "-ftz", to
> > linux_ia64_ifort+mpi_altix_nas ?
> >
> > As Chris discovered, the "-fp-model precise" slows down
> > seaice_evp.F by a factor of ~50.

Constantinos
-- 
Dr. Constantinos Evangelinos
Department of Earth, Atmospheric and Planetary Sciences
Massachusetts Institute of Technology