Re: [Libmesh-users] using PCGAMG

[Dmitry K. <ka...@mc...>]

GAMG is primarily an algebraic multigrid preconditioner.
The geometric part is somewhat weaker and isn't the primary purpose of
GAMG,
so I would recommend using -pc_gamg_type agg for plain aggregation.

For good performance with elasticity it is important to use smoothed
aggregation,
for example,  -pc_gamg_agg_nsmooths 1
but for that to be effective, it is critical to supply a "nearnullspace" --
basically,
the nullspace of the Neumann operator.  It is the rigid body modes in the
case of elasticity.
If you are able to specify the block size of the matrix, GAMG will try to
guess the correct
nearnullspace for you.  In particular bs=3 will cause GAMG to construct the
rigid body modes
from the coordinates set with PCSetCoordinates.

Now, libMesh doesn't have an API for setting the matrix block size (as far
as I know).
You can hack a bit: extract the PETSc objects and do it yourself, but a
more natural way is
to compute your own rigid body modes in a callback attached to
NonlinearSolver::nearnullspace or NonlinearSolver::nearnullspace_object.  A
nearnullspace is simply an array of NumericVector that
contains a basis for the nearnullspace. libMesh will orthonormalize it for
you (PETSc requires
that).

Hope this helps.
Dmitry.


On Thu, May 9, 2013 at 11:08 AM, Cody Permann <cod...@gm...> wrote:

> On Thu, May 9, 2013 at 9:18 AM, Ataollah Mesgarnejad <
> am...@ti...
> > wrote:
>
> >
> > Cody,
> >
> > On May 9, 2013, at 9:29 AM, Cody Permann <cod...@gm...> wrote:
> >
> >
> >
> >
> > On Thu, May 9, 2013 at 8:07 AM, Ataollah Mesgarnejad <
> > am...@ti...> wrote:
> >
> >> Dear all,
> >>
> >> I was wondering if anyone has any experience using gamg with libMesh
> (I'm
> >> particularly interested in elasticity)?
> >>
> >>
> > It certainly works, however in my very limited experiecne, it's slightly
> > more tricky to use than Hypre as we've found that it requires a few more
> > options.  Gamg is very flexible and part of PETSc so it ties in nicely.
> >
> >
> >
> > That's wonderful. Do I need to add anything in the code itself like block
> > size or ..? Or do you mean the command line options for gamg?
> >
>
> Just the command line options. If you have specific questions, the petsc
> user list is a great place to ask.
>
> Cody
>
> >
> > My main question would be if the geometric part of gamg would work with
> >> the underlying mesh structure in libMesh?
> >>
> >
> > I'm not sure if I'm missing something with this question.  I don't see
> why
> > this preconditioner would care about the mesh structure, it's abstracted
> > well above that level.  From libMesh's perspective, it's just another
> > preconditioner.
> >
> >
> > OK. I'm not very familiar with low level structure in libMesh so I wasn't
> > sure.
> >
> > Thanks,
> > Ata
> >
> >
> > Cody
> >
> >
> >>
> >> I would appreciate any comments/recommendations,
> >> Ata
> >>
> >>
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