[petsc-dev] Controlling matrix type on different levels of multigrid hierarchy? (Motivation is GPUs)

[Dave May]

Hi Richard,

On Wed, 12 Jun 2019 at 18:38, Mills, Richard Tran via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> Colleagues,
>
> I think we ought to have a way to control which levels of a PETSc
> multigrid solve happen on the GPU vs. the CPU, as I'd like to keep coarse
> levels on the CPU, but run the calculations for finer levels on the GPU.
> Currently, for a code that is using a DM to manage its grid, one can use
> GPUs inside the application of PCMG by doing putting something like
>
>   -dm_mat_type aijcusparse -dm_vec_type cuda
>
> on the command line. What I'd like to be able to do is to also control
> which levels get plain AIJ matrices and which get a GPU type, maybe via
> something like
>
>   -mg_levels_N_dm_mat_type aijcusparse -mg_levels_N_dm_mat_type cuda
>
> for level N. (Being able to specify a range of levels would be even nicer,
> but let's start simple.)
>
> Maybe doing the above is as simple as making sure that DMSetFromOptions()
> gets called for the DM for each level.
>

I think you'd need really need a unique options prefix for each DM. Either
set by you if you created the hierarchy or internally defined / set of the
dms are constructed during PCSetUp_MG


But I think I may be not understanding some sort of additional
> complications. Can someone who knows the PCMG framework better chime in? Or
> do others have ideas for a more elegant way of giving this sort of control
> to the user?
>

Are you building the DM hierarchy yourself, or is PETSc generation the
coarse DMs for you. If the former, then you can always choose the Mat type
via DMSetMatType().

Cheers
Dave


>
> Best regards,
> Richard
>
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