[gmx-users] different results when using different number cpus

[Berk Hess]

Hi,

With Gromacs and (nearly) all other MD packages you will never be able
to get binary identical results when running on different number of CPUs.
Since MD is chaotic, the results can be very different.

Berk.


>From: Carsten Kutzner <ckutzne at gwdg.de>
>Reply-To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>To: Discussion list for GROMACS users <gmx-users at gromacs.org>
>Subject: Re: [gmx-users] different results when using different number cpus
>Date: Wed, 05 Dec 2007 14:10:06 +0100
>
>Hi Dechang,
>
>it is normal that results are not binary identical if you compare the
>"same" MD system on different numbers of processors. If you use PME then
>you will probably get slightly different charge grids for 2 and for 16
>processors - since the charge grid has to be divisible by the number of
>CPUs in x- and y-direction. Even if you manually set the grid dimensions
>to be the same for both cases, your simulations could diverge when using
>version 3.x of the FFTW. This version has a build-in timer and chooses
>the fastest of several algorithms which could be another even in two
>runs on the same number of processors - depending on the timing results.
>With different algorithms you get slight differences in the last digit
>of the computed numbers (rounding / truncation / order of evaluation)
>which will then grow during the simulation and lead to diverging
>trajectories. Of course the averaged properties of the simulation are
>unaffected by those differences and should be the same if averaged long
>enough.
>You could use FFTW 2.x and manually set the FFT grid size to the same
>value for the 2 and 16 CPU case - but I am not shure if this is enough
>to get binary identical results.
>You could also repeat your simulations several times with (slightly)
>different starting conditions (maybe different starting velocities) to
>get a better picture of the average behaviour of your system. If in all
>16 processor cases you see the proteins diverge and in all 2 processor
>cases you see them converge, I would guess something is wrong.
>
>Hope that helps,
>   Carsten
>
>
>Dechang Li wrote:
> >  Dear all,
> >
> > ¡¡¡¡I used Gromacs3.3.1 to do a simulation about two proteins in 
>water(tip3p).
> > I run two similar simulations, one for 2 cpus, while the other for 16 
>cpus.
> > The two simulations have the same .gro, .top, and the same .mdp files. I 
>found
> > the results were not the same. In the 2 cpus simulation, the two 
>proteins
> > run closer and closer. But they run apart in the 16 cpus simulation.
> >    Is that normal the different results when using different number 
>cpus? The
> > size of my simulation box is 9*7*7.
> >
> >
> >
> >
> >
> >
> >
> > Best regards,
> >
> > 2007-12-5
> > ¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡¡
> >
> > =========================================
> > Dechang Li, PhD Candidate
> > Department of Engineering Mechanics
> > Tsinghua University
> > Beijing 100084
> > PR China
> >
> > Tel:   +86-10-62773779(O)
> > Email: li.dc06 at gmail.com
> > =========================================¡¡¡¡¡¡¡¡¡¡
> >
> >
> > ------------------------------------------------------------------------
> >
> > _______________________________________________
> > gmx-users mailing list    gmx-users at gromacs.org
> > http://www.gromacs.org/mailman/listinfo/gmx-users
> > Please search the archive at http://www.gromacs.org/search before 
>posting!
> > Please don't post (un)subscribe requests to the list. Use the
> > www interface or send it to gmx-users-request at gromacs.org.
> > Can't post? Read http://www.gromacs.org/mailing_lists/users.php
>
>--
>Dr. Carsten Kutzner
>Max Planck Institute for Biophysical Chemistry
>Theoretical and Computational Biophysics Department
>Am Fassberg 11
>37077 Goettingen, Germany
>Tel. +49-551-2012313, Fax: +49-551-2012302
>http://www.mpibpc.mpg.de/research/dep/grubmueller/
>http://www.gwdg.de/~ckutzne
>_______________________________________________
>gmx-users mailing list    gmx-users at gromacs.org
>http://www.gromacs.org/mailman/listinfo/gmx-users
>Please search the archive at http://www.gromacs.org/search before posting!
>Please don't post (un)subscribe requests to the list. Use the
>www interface or send it to gmx-users-request at gromacs.org.
>Can't post? Read http://www.gromacs.org/mailing_lists/users.php

_________________________________________________________________
Play online games with your friends with Messenger 
http://www.join.msn.com/messenger/overview