[gmx-users] Power6 @ 4.7 Ghz vs Xeon E5472 @ 3.00GHz

[Berk Hess]

Ah, I noticed just now that the Xeon model is in the subject line.
So it is not a Nehalem Xeon. Then the difference is somewhat larger than I expected.
Maybe using fortran could help a bit.

Berk

From: gmx3 at hotmail.com
To: gmx-users at gromacs.org
Subject: RE: [gmx-users] Power6 @ 4.7 Ghz vs Xeon E5472 @ 3.00GHz
Date: Wed, 17 Feb 2010 11:47:38 +0100










> Date: Wed, 17 Feb 2010 03:12:57 +1100
> From: Mark.Abraham at anu.edu.au
> To: gmx-users at gromacs.org
> Subject: Re: [gmx-users] Power6 @ 4.7 Ghz vs Xeon E5472 @ 3.00GHz
> 
> On 17/02/10 03:06, pim schravendijk wrote:
> > Dear Gromacs users,
> >
> > I have been installing Gromacs on a Power6 4.7 GHz with Infiniband,
> > GBit-Ethernet
> >
> > before calling configure, I set:
> >
> > export LDFLAGS="-L/gpfs/schraven/fftw3/lib"
> > export CPPFLAGS="-I/gpfs/schraven/fftw3/include"
> > export CC="mpcc_r -q64"
> > export CFLAGS="-O3 -qstrict -qarch=auto -qtune=auto"
> > export CXX="mpCC_r -q64"
> > export CXXFLAGS="-O3 -qstrict -qarch=auto -qtune=auto"
> > export F77="mpxlf_r -q64"
> > ### O3 gives warning: The NOSTRICT option (default at OPT(3)) has the
> > potential to alter the semantics of a program.
> > export FFLAGS="-O3 -qstrict -qnoprefetch -qarch=auto -qtune=auto"
> > export LIBS="-lmass"
> > export FLIBS= "-xlf -xlf90"
> > export OBJECT_MODE=64
> >
> > I run on both machines at 8 cores (so within the board), and find the 8
> > power6 cores to be 4-5 times slower (!) than the 8 intel xeon cores.
> >
> > Does anyone have similar experience between these machines? Are there
> > some things I should check?
> > It's a bit useless to use the power6 at all with these kind of speeds,
> > but they were already here anyway and amazon's 30 day refund on retour
> > has expired already ;)
> > Hopefully I'm doing something wrong and the situation can still be improved?
> 
> This issue might have been discussed on the list before, have a search. 
> I suspect the issue is that there are no optimized inner loops for 
> Power6 as there are for x86. You can see what GROMACS is finding and 
> doing in the configure output. Probably GROMACS is finding nothing 
> useful and falling back on generic C versions of such loops. You should 
> experiment with --enable-fortran to try the generic FORTRAN versions.

This is partly the issue.
The other problem is, if I recall correctly, that someone at IBM told me
that the VMX units have been put in the power6 at a late state in the design process
and that they are therefore not very fast.

Is the factor 4 compared to a Nehalem Xeon?
That is roughly the difference I would expect.

Berk

> 
> Mark
> -- 
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