[Users] logic of scheduling SelectBoundConds in McLachlan?

[Roland Haas]

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.

> 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.

Yours,
Roland

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