Okay, got it. What are the options for setting GAMG as the coarse solver? On Thu, Mar 30, 2017 at 3:37 PM, Matthew Knepley <[email protected]> wrote:
> On Thu, Mar 30, 2017 at 3:04 PM, Justin Chang <[email protected]> wrote: > >> 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 >> > > Depending on how big the initial grid is, you may want this. There is a > balance between coarse grid and fine grid work. > > >> 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? >> > > Again, it depends on the size of the initial grid. > > On really large problems, you want to use GAMG as the coarse solver, which > will move the problem onto a smaller number of nodes > so that you can coarsen further. > > Matt > > >> Thanks, >> Justin >> >> On Thu, Mar 30, 2017 at 2:35 PM, Barry Smith <[email protected]> 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 <[email protected]> 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 >>> >>> >> > > > -- > What most experimenters take for granted before they begin their > experiments is infinitely more interesting than any results to which their > experiments lead. > -- Norbert Wiener >
