[petsc-users] Near nullspace for preconditioners other than GAMG?

[Barry Smith]

> On Mar 7, 2015, at 5:27 PM, David Knezevic <david.knezevic at akselos.com> wrote:
> 
> OK, thanks for letting me know.
> 
> I've tried GAMG and ML with MatNullSpaceCreateRigidBody and those both work well for me. (I tried ML after Jed pointed out that ML also uses the near nullspace.)
> 
> I have also tried hypre but the convergence hasn't been as good for the elasticity models I've been using, though I may not have been setting the hypre options in an optimal way for elasticity.
> 
> Another situation I've been meaning to ask about: If the elasticity model includes some "near rigid" regions (very high Young's modulus) all iterative solvers I've tried fare poorly. I guess it's because the highly contrasting stiffnesses give a large condition number. Is there a standard way to get the preconditioner compensate for this?

  Off the top of my head:   a "domain decomposition" answer might be to use a sparse direct solver on that region; of course how to tie that into the rest of the regions and the iterative solver is a broad question. 

   Maybe in the context of AMG all the nodes in that region could be retained in the coarser grids (assuming it is a relatively small part of the domain) and so all those points end up in the coarse grid which is solved with a sparse direct solver?  We don't currently have an interface for you to indicate nodes you want to keep in the coarse grid; Mark would have know if this makes any sense at all.

   Barry

> 
> Thanks,
> David
> 
> 
> 
> On Sat, Mar 7, 2015 at 5:57 PM, Mark Adams <mfadams at lbl.gov> wrote:
> FYI, stüben used classical AMG for elasticity but he has articulated his code for elasticity more than Hypre as I understand it.  Hypre can work OK for elasticity in my experience.  Its worth a try.
> 
> Mark
> 
> On Thu, Mar 5, 2015 at 5:27 PM, David Knezevic <david.knezevic at akselos.com> wrote:
> OK, got it, thanks!
> 
> David
> 
> 
> On Thu, Mar 5, 2015 at 5:08 PM, Jed Brown <jed at jedbrown.org> wrote:
> David Knezevic <david.knezevic at akselos.com> writes:
> > I was just wondering if its possible to achieve the same sort of thing with
> > other AMG solvers (e.g. BoomerAMG)? I assume that MatSetNearNullSpace does
> > nothing for external solvers like hypre, right?
> 
> It is used by ML (smoothed aggregation), but not BoomerAMG (classical
> AMG) which uses an algorithm that doesn't have a natural place for such
> information.  To my knowledge, classical AMG is not widely used for
> elasticity.  It is very robust for M-matrices.
> 
> 
>