[petsc-users] Correlation between da_refine and pg_mg_levels

[Jed Brown]

Justin Chang <jychang48 at gmail.com> writes:

> 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

Smaller coarse grids are generally more scalable -- when the problem
data is distributed, multigrid is a good solution algorithm.  But if
multigrid stops being effective because it is not preserving sufficient
coarse grid accuracy (e.g., for transport-dominated problems in
complicated domains) then you might want to stop early and use a more
robust method (like direct solves).
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