[petsc-users] Correlation between da_refine and pg_mg_levels

[Barry Smith]

  -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