Re: [Libmesh-users] PETSC-DM and fieldsplit preconditioners.

[Dmitry K. <ka...@mc...>]

I assume you are referring to using the Schur-complement flavor of
PCFieldSplit?
How many variables do you have?  Are they all C0?  Is the basis nodal? If
so,
you don't necessarily need DMlibMesh:

Assume you have m P1 variables (maybe m==2?). You can lay your degrees of
freedom
in the variable-major order (by this I mean having contiguous nodal
coefficients corresponding
to a given node -- maybe that's called 'node-major':-).
Then use these options:
-pc_type fieldsplit -pc_fieldsplit_block_size m -pc_fieldsplit_0_fields 0
-pc_fieldsplit_1_fields 1\
-pc_fieldsplit_type schur -pc_fieldsplit_schur_factorization_type full

This will put the  0-th (in the zero-base C-speak) variable into the 0-th
split, and the first variable
in the first split.  You can also permute the column blocks, if necessary,
or put more variables
into each split.  You might try -pc_fieldsplit_schur_factorization_type
upper or lower.

The crucial part, however, is to have a good preconditioner for S, which in
the terminology of the notes
you sent is S = D - C inv(A) B.  By default PETSc will use D as the
preconditioner for S.  That may
or may not be good enough.  Otherwise you will need to assemble a
preconditioner for S, things
will get more hairy, and we might need to put more support for that into
libMesh.

Hope this helps.
Dmitry.


On Tue, Nov 26, 2013 at 1:56 PM, Subramanya Sadasiva <po...@ou...>wrote:

>
> Hi Dmitry,
> I am using the DM solver to solve a Cahn Hilliard equation using a C0
> discretization. I would like to use the preconditioner used in
>
>
> http://www.mcs.anl.gov/~anitescu/Presentations/2011/anitescu-2011-SIAM-CSE-DVI.pdf
>
>
> I have been able to use the PETSC-DM solver with the VI solver to solve
> the Cahn Hilliard equation, but I haven’t been able to get a field-split
> preconditioner to work.
>
> Thanks,
> Subramanya
>
>
> On Nov 26, 2013, at 2:07 PM, Dmitry Karpeyev <ka...@mc...> wrote:
>
> DMlibMesh should work, but it's in need of an overhaul and simplification.
> Could you tell me how you intend to use it?  That way we can figure out
> what needs to be done to make it usable.
> Thanks.
> Dmitry.
>
>
> On Thu, Nov 21, 2013 at 1:32 PM, subramanya sadasiva <po...@ou...>wrote:
>
>> Hi , I want to use petsc fieldsplit preconditioners with the petsc-dm
>> solver that has been implemented as part of libmesh.  The last time I tried
>> it, I had trouble because dmcreatefielddecomposition was not being called.
>> Is there someone that could guide me with trying to correct this in
>> libmesh? Thanks, Subramanya
>>
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>
>
>
> --
> Dmitry Karpeev
> Mathematics and Computer Science
> Argonne National Laboratory
> Argonne, Illinois, USA
> and
> Computation Institute
> University of Chicago
> 5735 S. Ellis Avenue
> Chicago, IL 60637
> -----------------------
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