[petsc-dev] non-symmetric permutation in PCFieldSplit

[Matthew Knepley]

On Thu, Feb 23, 2012 at 1:05 PM, Jungho Lee <julee at mcs.anl.gov> wrote:

> I'd like to solve a system with a block structure with PCFieldSplit:
>
> [A B;C D][x;y] = [v;w]
>
> (I wrote it this way to simplify notation but x and y are in fact
> interlaced, i.e., I'm solving a permuted version of the system above,
> where the variables are laid out as x_1 y_1 x_2 y_2 ...., so the
> system is fieldsplit-friendly)
>
> Applying PCFieldSplit to the equivalent system
>
> [D C;B A][y;x] = [w;v]
>
> can be done with command line options (-pc_fieldsplit_0_fields 1
> -pc_fieldsplit_1_fields 0), but I'm also interested in solving
>
> [C D;A B][x;y] = [v;w] (to be able to explore different
> preconditioning strategies), which currently doesn't seem to be
> supported. At first glance it seems like this can be easily solved by
> adding extra IS (is) and PetscInt* (fields) objects in
> _PC_FieldSplitLink, read in integer arrays for rows and columns (the
> symmetric case where the rows and columns have the same indices is
> already handled by -pc_fieldsplit_%d_fields), fill in the extra IS
> object, and allow the user to use these two different IS's in the
> MatGetSubMatrix calls in PCSetUp_FieldSplit. Is it really as simple as
> that, or am I missing something here?
>

No, I think that should definitely be available. However, I also think FS
needs a
complete rewrite. There is way to much crud in there specific to co-located
discretizations on structured grids which should be moved to the DA. FS
should
only takes ISes, which would make the code a LOT simpler. Then the DA has
all
the nice routines for easily creating ISes for it. I am putting in the code
for
DMComplex now, and it seems clear the interface for all DMs should be the
same.

   Matt


> Thanks,
> Jungho

-- 
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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