[petsc-users] Correlation between da_refine and pg_mg_levels

[Justin Chang]

Yeah based on my experiments it seems setting pc_mg_levels to $DAREFINE + 1
has decent performance.

1) is there ever a case where you'd want $MGLEVELS <= $DAREFINE? In some of
the PETSc tutorial slides (e.g., http://www.mcs.anl.gov/
petsc/documentation/tutorials/TutorialCEMRACS2016.pdf on slide 203/227)
they say to use $MGLEVELS = 4 and $DAREFINE = 5, but when I ran this, it
was almost twice as slow as if $MGLEVELS >= $DAREFINE

2) So I understand that one cannot refine further than one grid point in
each direction, but is there any benefit to having $MGLEVELS > $DAREFINE by
a lot?

Thanks,
Justin

On Thu, Mar 30, 2017 at 2:35 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
>   -da_refine $DAREFINE  determines how large the final problem will be.
>
>   By default if you don't supply pc_mg_levels then it uses $DAREFINE + 1
> as the number of levels of MG to use; for example -da_refine 1 would result
> in 2 levels of multigrid.
>
>
> > On Mar 30, 2017, at 2:17 PM, Justin Chang <jychang48 at gmail.com> wrote:
> >
> > Hi all,
> >
> > Just a general conceptual question: say I am tinkering around with SNES
> ex48.c and am running the program with these options:
> >
> > mpirun -n $NPROCS -pc_type mg -M $XSEED -N $YSEED -P $ZSEED
> -thi_mat_type baij -da_refine $DAREFINE -pc_mg_levels $MGLEVELS
> >
> > I am not too familiar with mg, but it seems to me there is a very strong
> correlation between $MGLEVELS and $DAREFINE as well as perhaps even the
> initial coarse grid size (provided by $X/YZSEED).
> >
> > Is there a rule of thumb on how these parameters should be? I am
> guessing it probably is also hardware/architectural dependent?
> >
> > Thanks,
> > Justin
>
>
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