[gmx-users] Effect of pressure coupling frequency on gpu simulations

[Trayder Thomas]

Thanks Mark,
That really helped to clarify how everything is interacting around the
verlet scheme.
What statistics do you recommend examining between nstpcouple settings?
Pressure/box size variation is the obvious one but I was curious whether
you had something else in mind.
-Trayder


On Thu, May 23, 2013 at 4:18 AM, Mark Abraham <mark.j.abraham at gmail.com>wrote:

> On Wed, May 22, 2013 at 6:32 AM, Trayder <trayder.thomas at monash.edu>
> wrote:
>
> > Hi all,
> > I've been running 5fs timestep simulations successfully without gpus
> > (united-atom, HEAVYH). When continuing the same simulations on a gpu
> > cluster
> > utilising the verlet cutoff-scheme they crash within 20 steps. Reducing
> the
> > timestep to 2fs runs smoothly, however I noticed the message:
> >
> >
> >
> > Making this change manually led to crashing simulations as nstcalclr,
> > nsttcouple and nstpcouple default to the value of nstlist. After defining
> > them all separately I was able to determine that the simulation exploding
> > was dependent entirely on nstpcouple and by lowering it to 5 (from the
> > default 10) I was able to run simulations at a 5fs timestep.
> >
> > So, my questions: Is lowering nstpcouple a legitimate solution or just a
> > bandaid?
> >
>
> P-R does not cope well with situations where the box size changes enough
> (e.g. you should normally avoid it during equilibration). nstpcouple != 1
> means that you simulate on an NVE manifold for a period of time (maybe with
> some T changes if nsttcouple != nstpcouple), and I'd suppose the longer
> that interval the bigger the chance of a build-up of pressure that P-R will
> then try to relieve by changing the box size. Larger nstlist and dt will
> exacerbate this, of course. I would recommend you experiment and see how
> far you can push things and keep statistics that still resemble those with
> small nstpcouple. Larger nstpcouple helps reduce the frequency with which
> global communication occurs, and that affects your simulation rate... life
> is complex!
>
> It would be nice if we were able to compute heuristics so that mdrun could
> anticipate such a problem and warn you, but off-hand that seems a tricky
> problem...
>
> The simulation runs with nstcalclr and nsttcouple set to 50 along with
> >
>
> nstcalclr should have no effect - it works only with the group scheme,
> which does not work on GPUs.
>
>
> > nstlist. Is nstlist the only setting that should be increased when
> > utilising
> > gpus?
> >
>
> Yes, AFAIK. The point is that nstlist is the interval between neighbour
> searches, and (at the moment at least) that's only done on the CPU. The
> Verlet kernels cheerfully compute lots of zero-strength interactions
> outside the cutoff (by design), and depending on the relative performance
> of your hardware it is normally more efficient to bump nstlist up (and
> rlist accordingly, to provide a larger buffer for diffusion of particles)
> and compute more zeroes than it is to search for neighbours more often.
>
> Mark
>
>
> >
> > Thanks in advance,
> > -Trayder
> >
> > P.S. The working mdp file:
> >
> >
> >
> >
> >
> >
> >
> > --
> > View this message in context:
> >
> http://gromacs.5086.x6.nabble.com/Effect-of-pressure-coupling-frequency-on-gpu-simulations-tp5008439.html
> > Sent from the GROMACS Users Forum mailing list archive at Nabble.com.
> > --
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