[petsc-users] Regarding ksp ex42 - Citations

[Barry Smith]

  Took a little more time than I expected but the branch barry/extend-pcmg-galerkin now supports 

PCMGSetGalerkin() and -pc_mg_galerkin now take PC_MG_GALERKIN_BOTH,PC_MG_GALERKIN_PMAT,PC_MG_GALERKIN_MAT, PC_MG_GALERKIN_NONE as arguments instead of PetscBool
This allows computing either mat, or pmat or both via the Galerkin process

so you should be able to provide A and M with KSPSetOperators() and then run with -pc_mg_galerkin both to get both generated on the coarse meshes via the Galekin process.  Note that if you use the additional option -pc_use_amat false it will use only the M for both mat and pmat in the multigrid process (while A is only used for the outer Krylov solver definition of the operator.) For some problems this is actually a better approach.


  Please let me know if you have any difficulties with it.

Barry

> On Jul 22, 2016, at 3:42 AM, domenico lahaye <domenico_lahaye at yahoo.com> wrote:
> 
> Dear Barry, 
>  
>   Thank you for your suggestion. 
>  
>   I will be happy to test drive the new code when available. 
> 
>   Kind wishes, Domenico. 
> 
> 
> 
> From: Barry Smith <bsmith at mcs.anl.gov>
> To: Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> 
> Cc: domenico lahaye <domenico_lahaye at yahoo.com>; PETSc Users List <petsc-users at mcs.anl.gov>
> Sent: Friday, July 22, 2016 1:41 AM
> Subject: Re: [petsc-users] Regarding ksp ex42 - Citations
> 
> 
>   I'll add support for handling both A and M via Galerkin. It is easy to write the code, picking a good simple API that doesn't break anything is more difficult.  I'm leaning to change PCMGSetGalerkin(PC,PetscBool) to PCMGSetGalerkin(PC, PCMGGalerkinType) where 
> 
> typedef enum { PC_MG_GALERKIN_BOTH,PC_MG_GALERKIN_PMAT,PC_MG_GALERKIN_MAT, PC_MG_GALERKIN_NONE
> } PCMGGalerkinType;
> 
> Barry
> 
> 
> 
> > On Jul 21, 2016, at 6:09 AM, Lawrence Mitchell <lawrence.mitchell at imperial.ac.uk> wrote:
> > 
> > 
> >> On 21 Jul 2016, at 10:55, domenico lahaye <domenico_lahaye at yahoo.com> wrote:
> >> 
> >> Apologies for being not sufficient clear in my previous message.
> >> 
> >> I would like to be able to Galerkin coarsen A^h to obtain A^H
> >> and to separately Galerkin coarsen M^h to obtain M^H.
> >> 
> >> So, yes, the way in which I currently (partially) understand your
> >> description of the new DMCreateMatrices would do the job.
> > 
> > If you want to separately coarsen A and M via Galerkin, I think it will be easier to just change the code in PCSetUp_MG to handle the case where A and M are different on the coarse levels.  Effectively you just need to replicate the code that computes the coarse grid "B" matrix to separately compute coarse grid A and B matrices and pass them in to KSPSetOperators.
> > 
> > Cheers,
> > 
> > Lawrence
> > 
> 
>