[petsc-users] Better PCGAMG Settings in 2D?

Mark Adams mfadams at lbl.gov
Fri Jul 20 11:38:53 CDT 2018

On Thu, Jul 19, 2018 at 9:44 PM Weston, Brian Thomas <weston8 at llnl.gov>

> Hi Mark,
> I’m using the default settings with GAMG for 2D problems and it’s
> performing much worse than BoomerAMG for my problem. I was told, however,
> that GAMG’s defaults were optimized for 3D problems, so I was wondering if
> you knew of better settings for 2D or which knobs might be most significant
> to play with? Is pc mg levels the way to control the number of aggressive
> coarsening levels? I tried using unsmoothed aggregation, but it was only
> marginally better than the default.

Are you looking at iteration count or solve time?

This is a 5-point stencil. You want to square the graph:
'-pc_gamg_square_graph 20' (20 is the number of levels to square the graph,
which is infinity really).

You can use:

-mg_levels_ksp_type richardson
-mg_levels_pc_type sor

and one processor this is a prefect smoother, just for debugging.

And you can run with -info and grep on GAMG and send me the result (about
20 lines). This should give me an idea of what is wrong if these ideas
don't work.


> The discretization is a essentially a 2nd-order cell-centered
> finite-volume scheme and I’m primarily solving a pressure-velocity Schur
> complement system.
> best,
> Brian
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