[petsc-dev] [petsc-users] Problem with AMG packages

[Matthew Knepley]

On Wed, Oct 9, 2013 at 5:47 PM, Mark F. Adams <mfadams at lbl.gov> wrote:

> >
> > (referring to your previous email about the kind of discretization) I'm
> > using P_2 finite elements on tetras. The mesh is so small that it might
> as
> > well be partitioned lexicographically (but in this case it was done by
> > Metis)
> >
>
> Well, This seems fine.  Higher order elements can confuse AMG sometimes
> but the math is decent in hypre and ML and I think it would be about the
> same with GAMG with the parameter that I gave you.  Something is going
> wrong.  We are trying to find it but not having any luck.
>
> There are many parameters, including stupid things like solver parameters
> and stupider things like bugs, that can kill an iterative solver and we are
> trying to figure out what is wrong here.  You should be running with about
> 1/10 or 1/100, or even 1/1000, the number of processors that you are using
> here.  If you eventually want to run in this regime then that is fine but
> for debugging it helps to be in a more normal regime, because then you (or
> us) can start to see secondary things like super slow flop rates that can
> provide a hint of what is going wrong.  It is also better to start in
> serial so there are less moving parts -- I have to think that these
> terrible ML and hypre setup times would go away in serial … not that that
> is a solution but it is knowledge.
>
> Also, keep in mind that MG is better at scaling so for small problems
> direct solvers or simple solvers can be faster.  Also AMG has significant
> setup costs (but they should not be as high as you are seeing) and this
> pushes the "cross over" point higher if you are just looking at one solve.
>  These costs are amortized for multiple solves but these are just the basic
> complexity issues with solvers.  MG may not always win on Poisson but it
> should, just about always.


We can run this exact case with SNES ex12.

   Matt


>
> Mark
>
> > Now that at least either GAMG or BoomerAMG are not stuck for small
> > matrices I'll try to go bigger, and hopefully won't have to bother you
> > again, thanks a lot for all your pointers.
> >
> > Pierre
> >
> > PS: about src/ksp/ksp/examples/tutorials/ex56.c, Dirichelet (line 6)
> > should read Dirichlet.
>
> Thanks,
>



-- 
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
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