[petsc-users] Configure nested PCFIELDSPLIT with general index sets

[Matthew Knepley]

On Wed, Mar 22, 2017 at 10:03 AM, Natacha BEREUX <natacha.bereux at gmail.com>
wrote:

> Hello,
> if my understanding is correct, the approach proposed by Matt and Lawrence
> is the following :
> - create a DMShell (DMShellCreate)
> - define my own CreateFieldDecomposition to return the index sets I need
> (for displacement, pressure and temperature degrees of freedom) :
> myCreateFieldDecomposition(... )
> - set it in the DMShell ( DMShellSetCreateFieldDecomposition)
> - then sets  the DM in KSP context  (KSPSetDM)
>
> I  have some more questions
> - I did not succeed in setting my own CreateFieldDecomposition in the
> DMShell : link  fails with " unknown reference to «
> dmshellsetcreatefielddecomposition_ ». Could it be a Fortran problem (I
> am using Fortran)?  Is this routine available in PETSc  Fortran interface ?
> \
>

Yes, exactly. The Fortran interface for passing function pointers is
complex, and no one has added this function yet.


> - CreateFieldDecomposition is supposed to return an array of dms (to
> define the fields). I am not able to return such datas.  Do I return a
> PETSC_NULL_OBJECT instead ?
>

Yes.


> - do I have to provide something else to define the DMShell ?
>

I think you will have to return local and global vectors, but this just
means creating a vector of the correct size and distribution.

  Thanks,

     Matt


> Thanks a lot for your help
> Natacha
>
> On Tue, Mar 21, 2017 at 2:44 PM, Natacha BEREUX <natacha.bereux at gmail.com>
> wrote:
>
>> Thanks for your quick answers. To be honest, I am not familiar at all
>> with DMShells and DMPlexes. But since it is what I need, I am going to try
>> it.
>> Thanks again  for your advices,
>> Natacha
>>
>> On Tue, Mar 21, 2017 at 2:27 PM, Lawrence Mitchell <
>> lawrence.mitchell at imperial.ac.uk> wrote:
>>
>>>
>>> > On 21 Mar 2017, at 13:24, Matthew Knepley <knepley at gmail.com> wrote:
>>> >
>>> > I think the remedy is as easy as specifying a DMShell that has a
>>> PetscSection (DMSetDefaultSection) with your ordering, and
>>> > I think this is how Firedrake (http://www.firedrakeproject.org/) does
>>> it.
>>>
>>> We actually don't use a section, but we do provide
>>> DMCreateFieldDecomposition_Shell.
>>>
>>> If you have a section that describes all the fields, then I think if the
>>> DMShell knows about it, you effectively get the same behaviour as DMPlex
>>> (which does the decomposition in the same manner?).
>>>
>>> > However, I usually use a DMPlex which knows about my
>>> > mesh, so I am not sure if this strategy has any holes.
>>>
>>> I haven't noticed anything yet.
>>>
>>> Lawrence
>>
>>
>>
>


-- 
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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