Jungho will be adding this more generality and looking to simplify by
removing the nfields, fields case from the code. For now we plan to keep the
nfields, fields information because it is useful in the PCView() and provides
much more information than the IS case. Eventually I guess we can augment the
ISView() to provide this information and then totally remove the nfields,
fields from the PC_FieldSplit object.
Barry
On Feb 23, 2012, at 1:14 PM, Matthew Knepley wrote:
> 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
