[petsc-users] Providing Interpolation/Injections to Multigrid infrastructure

[Matthew Knepley]

On Fri, Feb 12, 2016 at 9:20 AM, Paul T. Bauman <ptbauman at gmail.com> wrote:

> Hi Barry,
>
> On Thu, Feb 11, 2016 at 2:45 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>
>>
>> > On Feb 11, 2016, at 1:36 PM, Boris Boutkov <borisbou at buffalo.edu>
>> wrote:
>> >
>> > Hello All,
>> >
>> > I'm currently working on hooking into the PETSc multi-grid
>> infrastructure by creating my own DMShell and providing my own
>> interpolation and injection operators. The issue I am currently facing is
>> that while PCSetUp_MG is running, PETSc begins by attempting to create an
>> interpolation (through DMCreateInterpolation) for the finest grid that I am
>> passing it.
>> >
>> > What would be a good way to try and let the PCSetUp_MG procedure know
>> the first mesh it is receiving is the finest one, so there should be no
>> interpolation provided to it?
>>
>>   Boris,
>>
>>   I do not understand the question, perhaps it is a matter of language.
>
> Say you have two levels total: call them coarse and fine. the PCSetUp_MG
>> will try to create a single interpolation "for/to the fine grid" FROM the
>> coarse grid. This is necessary to do the geometric multigrid. You would
>> then through your DMSHELL provide the function that creates the
>> interpolation matrix.  The PCSetUp_MG will not attempt to create an
>> interpolation "for the coarse mesh" since there is no mesh coarser than the
>> coarse mesh. So
>> PCSetUp_MG() should be trying to create only the needed interpolations,
>> so you should not need to tell them not to try to create certain ones.
>
>
> The information I think we're missing is, more or less, the order of
> operations during the MG setup and how PETSc is expecting us to behave with
> our DMShell implementation. I'll stick with your coarse and fine grid. On
> our side, we can construct the coarse and the fine grids, supplying the DMs
> and accompanying PetscSections, at simulation setup time since we know what
> our grid hierarchy is (we're focusing on hierarchical meshes to start
> with). Then, for DMRefine and DMCoarsen, we can just return the correct DM
> that we've built. (Maybe this the wrong way to do it?) We also have
> implementations of DMCreateInterpolation and DMCreateInjection for moving
> between each of the levels.
>
> Our main confusion is, once we've set up of all that, which of the DMs
> we've constructed do we go ahead and give to SNES? The fine or the coarse?
> Is it expected that we should be checking the DM supplied to, e.g.,
> DMCreateInterpolation for which of the operators PETSc is wanting? We were
> guessing that we should supply the fine DM to SNES (and then not expecting
> DMCreateInterpolation to be called yet).
>

Ah, good question. I think it should be the fine grid, and this is the
default for FAS. Some stuff used to take a coarse grid because
DMDA was easier to refine, but that should all be fixed now.

Also, it looks like Lawrence responded.

   Matt


> Clearly, there's some setup steps we're missing or we're just going about
> this all the wrong way. Is there an existing example of setting up a
> problem using a DMShell? I poked a little but couldn't find one.
>
> Thanks much.
>
> Best,
>
> Paul
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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