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

David Knezevic david.knezevic at akselos.com
Sat Mar 7 17:27:07 CST 2015

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

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?


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.
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