Justin Chang <[email protected]> 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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