[petsc-users] ILU and Block Gauss Seidel smoothing

[Jed Brown]

On Mon, Jun 20, 2011 at 16:26, Gianluca Meneghello <gianmail at gmail.com>wrote:

> My goal is to perform some relaxation sweeps for each ordering at each
> level of a multigrid process, probably using ksptype PREONLY and
> pctype ILU.
> Is that possible?
>

Yes, but you would need to perform a separate factorization for each
ordering. Also, the orderings that are not aligned with the grid in memory
will have poor performance. This is a general problem with using different
orderings.

Is there the equivalent of -pc_sor_its with ILU
> (-pc_ilu_its maybe)?
>

-ksp_type richardson is the same thing, put it inside -pc_type bjacobi for
-pc_sor_lits. (-pc_sor_its is just a lower overhead way to do that cycling).


> I also have the problem that in order to build the ordering I would
> need to have access to a structure containing some grid informations,
> and it seems I cannot pass that structure to the YourOrdering function
> you suggested.
>

You can PetscObjectCompose() your structure to the Mat. You might need
PetscContainerCreate() to wrap your struct.


>
> I guess a solution for me could be to build the IS from an external
> function (not used by petsc) and then attach them directly to the mat
> structure. I also guess that the one to use are the ones at line 36 of
>

No, those slots are not public. Use PetscObjectCompose().
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