[Users] logic of scheduling SelectBoundConds in McLachlan?

Erik Schnetter schnetter at cct.lsu.edu
Tue Feb 19 12:19:55 CST 2013


Did you set the parameters that Ian Hinder suggested? This will make
interpreting the timing output much easier.

Time is not only spent in routines, but also in infrastructure tasks not
listed here (regridding etc.). Ian Hinder implemented a tree-form timer
output that makes it much easier to see how much time is spent on what
task. I don't recall the options to select this -- Ian, is this
Carpet::output_timer_tree_every and friends?


On Mon, Feb 18, 2013 at 2:59 PM, Kelly, Bernard J. (GSFC-660.0)[UNIVERSITY
OF MARYLAND BALTIMORE COUNTY] <bernard.j.kelly at nasa.gov> wrote:

> Hi Roland, and thanks for your reply. I'm still a bit confused, I confess
> (see below) ...
> On 2/16/13 12:38 AM, "Roland Haas" <roland.haas at physics.gatech.edu> wrote:
> >Hello Bernard,
> >
> >> Hi. Can someone explain to me why ML_BSSN calls SelectBoundConds within
> >> MoL_PostStep? It seems like the kind of once-off routine that would
> >>happen
> >> near the start of a simulation, rather than something that has to be
> >> performed every single timestep.
> >In the "new" boundary/symmetry interface (ie using thorn Boundary and
> >Symbase) one has to Select the variables for boundaries each time before
> >ApplyBCs is scheduled. There is a routine in Boundaries that clears the
> >selection.
> OK; but can you tell me *why* this is? Why should we ever have to
> re-specify what kind of boundary conditions we use during a simulation,
> any more than we re-specify the evolution equations? Perhaps I don't
> really understand what "select the variables" means here.
> >
> >> I wouldn't mind, but while trying to understand why ML_BSSN was evolving
> >> so slowly on one of our machines, I looked at the TimerReport files, and
> >> saw that SelectBoundConds was taking *much* more time (like 20 times as
> >> long) than the actual RHS calculation routines.
> >The long time is most likely caused by the fact that the boundary
> >selection routine tends to be the one calling SYNC which means it is the
> >one that does an MPI wait (if there is load imbalance) and communicates
> >data for buffer zone prolongation etc.
> So it might be spending most of the time waiting for other cores to catch
> up? But if it's really waiting for prior routines to finish on other
> processors, then on the handful of cores where SBC appears significantly
> *quicker* than usual (e.g. ~50,000 seconds instead of ~100,000) I should
> see earlier routines taking correspondingly *longer*, right? But I don't.
> I'm attaching TimerReport files for two cores on the same (128-core)
> evolution. Core 000 is typical. Line 3591 (the most up-to-date instance of
> "large" SBC behaviour) shows about 100K seconds spent cumulatively over
> the simulation so far. Core 052 shows only about half as much time used in
> the same routine, but I can't see what other EVOL routines might be taking
> up the slack.
> (Note, BTW, that what I'm running isn't vanilla ML_BSSN, but a locally
> modified version called MH_BSSN. The scheduling and most routines are
> almost identical to McLachlan)
> Bernard
> >
> >Yours,
> >Roland
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Erik Schnetter <schnetter at cct.lsu.edu>
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