OK, thanks a lot Jed for your quick answer. I have a second question on multigrid : I define a matrix block size (equal to 3). Is this mandatory for any type of multigrid preconditioner (BoomerAMG, ML and GAMG) ? Or is it only required with ML and GAMG ?
Best regards Natacha On Fri, Jul 18, 2014 at 3:07 PM, Jed Brown <[email protected]> wrote: > Natacha BEREUX <[email protected]> writes: > > > Dear all, > > > > I am testing three multigrid preconditionners: BoomerAMG, ML and GAMG on > a > > 3D elasticity problem (a cube with prescribed displacement on upper and > > lower faces). > > I set the null space by calling MatNullSpaceCreateRigidBody and then > > MatSetNearNullSpace). > > > > I see a difference (about half iterations) for ML and GAMG but nothing > > happens with BoomerAMG (same number of iterations with and without > setting > > the NearNullspace). > > That's expected because BoomerAMG does not use any such information > (though the algorithm might be extended in the future so that those > vectors become seed vectors for Bootstrap AMG, for example; I don't know > if the BoomerAMG developers have any interest in implementing this). >
