[petsc-dev] gamg regression

[Mark Adams]

On Tue, Jun 16, 2015 at 12:21 PM, Stephan Kramer <s.kramer at imperial.ac.uk>
wrote:

> Dear petsc-devs
>
> I spent quite some time chasing a regression using gamg. This is solving a
> 2D Stokes problem for velocity only in a cylindrical domain with free slip
> and large viscosity contrasts - for which we found gamg to perform very
> well. That is, everything worked fine for petsc 3.5 but stopped working on
> petsc-master giving KSP_DIVERGED_INDEFINITE_PC. At first I thought it was
> to do with how I setup the nullspace (we were using KSPSetNullSpace
> before), but am convinced I'm doing the right thing now with
> MatSetNullSpace. I also found this earlier thread:
>
> https://lists.mcs.anl.gov/mailman/htdig/petsc-dev/2015-April/017535.html
>
> and set back the defaults using -pc_gamg_coarse_eq_limit 800
> -pc_gamg_square_graph 100. This did resolve a lot of the changes I saw in
> the output of -ksp_view (comparing v3.5.2 and master), but did not fix the
> problem. I did see one last important difference though: with petsc 3.5.2.
> it was using PREONLY+LU at the coarse level, whereas with master it seems
> to be using GMRES+LU. Changing this back to PREONLY+LU everything works
> again as before.
>
> So my question: is the change from PREONLY+LU to GMRES+LU intentional?
>

I did have problems problems with MG setting this to GMRES and I had to
hard wire it ... it used to set PREONLY as the default ...


>
> Second question (less important): I didn't manage to change this at the
> command line. I would have thought -mg_coarse_ksp_type preonly should have
> done the trick but seemed to be ignored. Instead I had to do it with some
> calls in the code:
>
>
Damn, this is a bug.   I will look at this and figure out how to make
PREONLY the default and get -mg_coarse_ksp_type working.

Mark



> call PCSetup(pc, ierr) ! this is the GAMG pc
> call PCMGGETCoarseSolve(pc, ksp_coarse, ierr)
> call KSPSetType(ksp_coarse, PREONLY, ierr)
>
> Cheers
> Stephan
>
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