[petsc-users] Problems applying multigrid

[Ellen Price]

Hi PETSc users!

I was playing around with getting faster convergence for my code and
decided to give multigrid a try. For some context, I am using the beuler
timestepper, so the SNES/KSP/PC hierarchy is contained within that. The
problem is defined on a regular grid, a DMDA in dimension 2 with 5 dofs. It
has DM_BOUNDARY_GHOSTED boundaries.

The way I understand it, with these options:
-pc_type mg -pc_mg_galerkin both -pc_mg_levels 2
I should be able to at least *try* multigrid on this problem at a basic
level without changing anything else (please correct me if I'm wrong!),
even if I don't immediately get good convergence.

I know multigrid can be finicky (I've seen a lot of mailing list posts
about it), but I've run into an error that might be internal, or at least
not obvious. Here is a sample of the output:

[12]PETSC ERROR: Arguments are incompatible
[12]PETSC ERROR: Processor's coarse DMDA must lie over fine DMDA
    i_c 314 i_f 628 fine ghost range [499,628]

Does anyone know what would cause this? A quick Google search just brought
up the PETSc source code where the error is issued.

Thanks,
Ellen Price
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