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Subject: Re: [OMPI users] ScaLapack and BLACS on Leopard
From: Gregory John Orris (gregory.orris_at_[hidden])
Date: 2008-03-06 18:25:55


All,
I really didn't want to start a new thread discussing the virtues and
vices of every compiler, since this is hardly my forte and the
opportunity to offend someone is fairly high, whilst making myself
look clownish....
What I should have said was that "for my organization one cannot
justify the cost of buying, for example Intel's compiler."
Aside from some political/economic/legal reasons for this being true,
we have a lot of very specific code that has been optimized over the
years by hand. While there are clearly speed advantages for Intel's
compilers for many many problems, in our view, for our problems and
our "in house" software, the speedup has not been sufficient to
warrant the cost of a compiler for several hundred to several thousand
computers (yes there are that many developers). Besides, timing claims
and speedups are also dependent upon the version of gcc/gfortran one
uses, and what memory allocation routine, etc, etc.... There have been
significant improvements in gcc from 4.1 through 4.3 that have been
well documented. And there have also been accusations that intel
"chooses" problems to accentuate their claims of supremacy. But
doesn't every vendor do this? I admit that given an infinite amount of
money I would go with the Intel compiler for all of our development
work. Since we are more of a "proof or concept" organization and ship
out production runs to real centers (with intel compilers), even a
factor of 2 (which is our typical experience) is not significant
enough. If it were to become, as you state roughly 8 times faster, it
might be a different story. As it is, only legacy code is using
fortran and if one moves to C/C++ the differences we've seen have been
miniscule.

Bottom line, it works for my configuration right now and both me and
the other users are happy.

Thanks all, for help, advice, and a provocative discussion.

Regards,
Greg

On Mar 6, 2008, at 5:11 PM, Michael wrote:

>
> On Mar 6, 2008, at 12:49 PM, Doug Reeder wrote:
>> Greg,
>>
>> I would disagree with your statement that the available fortran
>> options can't pass a cost-benefit analysis. I have found that for
>> scientific programming (e.g., Livermore Fortran Kernels and actual
>> PDE solvers) that code produced by the intel compiler runs 25 to 55%
>> faster than code from gfortran or g95. Looking at the cost of adding
>> processors with g95/gfortran to get the same throughput as with
>> ifort you recover the $549 compiler cost real quickly.
>>
>> Doug Reeder
>
>
> I've a big fan of g95, but actually I'm seeing even greater
> differences in a small code I'm using for some lengthy calculations.
>
> With 14 MB of data being read into memory and processed:
>
> Intel ifort is 7.7x faster then Linux g95 on MacPro 3.0 GHz
> Intel ifort is 2.9x faster then Linux g95 on Dual Opteron 1.4 GHz
> Intel ifort is 1.8x faster then Linux g95 on SGI Altix 350 dual
> Itanium2 1.4 GHz
> OS X g95 is 2.7x faster then Linux g95 on a MacPro 2.66 GHz (same
> hardware exactly)
>
> The complete data set is very large, 56 GB, but that is 42 individual
> frequencies, where as the 14 MB is a single frequency, data averaged
> over areas, so get a favor of the answer but not exactly the right
> answer. I played around with compiler options, specified the exact
> processor type within the limits of gcc and I gained only factions of
> a percent.
>
> A co-worker saw factor 2 differences between Intel's compiler and g95
> with a very complicated code.
>
> Michael
>
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