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.abra...@gmail.com>wrote: > On Wed, May 22, 2013 at 6:32 AM, Trayder <trayder.tho...@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. > > -- > > gmx-users mailing list gmx-users@gromacs.org > > http://lists.gromacs.org/mailman/listinfo/gmx-users > > * Please search the archive at > > http://www.gromacs.org/Support/Mailing_Lists/Search before posting! > > * Please don't post (un)subscribe requests to the list. Use the > > www interface or send it to gmx-users-requ...@gromacs.org. > > * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists > > > -- > gmx-users mailing list gmx-users@gromacs.org > http://lists.gromacs.org/mailman/listinfo/gmx-users > * Please search the archive at > http://www.gromacs.org/Support/Mailing_Lists/Search before posting! > * Please don't post (un)subscribe requests to the list. Use the > www interface or send it to gmx-users-requ...@gromacs.org. > * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists > -- gmx-users mailing list gmx-users@gromacs.org http://lists.gromacs.org/mailman/listinfo/gmx-users * Please search the archive at http://www.gromacs.org/Support/Mailing_Lists/Search before posting! * Please don't post (un)subscribe requests to the list. Use the www interface or send it to gmx-users-requ...@gromacs.org. * Can't post? Read http://www.gromacs.org/Support/Mailing_Lists
