[petsc-users] GAMG failure

[Mark Adams]

* Do you set:

    PetscCall(MatSetOption(Amat, MAT_SPD, PETSC_TRUE));
    PetscCall(MatSetOption(Amat, MAT_SPD_ETERNAL, PETSC_TRUE));

Do that to get CG Eigen estimates. Outright failure is usually caused by a
bad Eigen estimate.
-pc_gamg_esteig_ksp_monitor_singular_value
Will print out the estimates as its iterating. You can look at that to
check that the max has converged.

*  -pc_gamg_aggressive_coarsening 0

will slow coarsening as well as threshold.

* you can run with '-info :pc' and send me the output (grep on GAMG)

Mark

On Fri, Mar 24, 2023 at 2:47 PM Jed Brown <jed at jedbrown.org> wrote:

> You can -pc_gamg_threshold .02 to slow the coarsening and either stronger
> smoother or increase number of iterations used for estimation (or increase
> tolerance). I assume your system is SPD and you've set the near-null space.
>
> Blaise Bourdin <bourdin at mcmaster.ca> writes:
>
> > Hi,
> >
> > I am having issue with GAMG for some very ill-conditioned 2D linearized
> elasticity problems (sharp variation of elastic moduli with thin  regions
> of nearly incompressible material). I use snes_type newtonls,
> linesearch_type cp, and pc_type gamg without any further options. pc_type
> Jacobi converges fine (although slowly of course).
> >
> >
> > I am not really surprised that gamg would not converge out of the box,
> but don’t know where to start to investigate the convergence failure. Can
> anybody help?
> >
> > Blaise
> >
> > —
> > Canada Research Chair in Mathematical and Computational Aspects of Solid
> Mechanics (Tier 1)
> > Professor, Department of Mathematics & Statistics
> > Hamilton Hall room 409A, McMaster University
> > 1280 Main Street West, Hamilton, Ontario L8S 4K1, Canada
> > https://www.math.mcmaster.ca/bourdin | +1 (905) 525 9140 ext. 27243
>
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