"(Sadly) MATSBAIJ is extremely broken, in particular, it cannot be used to
retrieve rectangular blocks in MatCreateSubMatrices, thus you cannot get the
A01 and A10 blocks in PCFIELDSPLIT.
I have a branch that fixes this, but I haven’t rebased in a while (and I’m AFK
right now), would you want me to rebase and give it a go, or must you stick to
a release tarball?"
Ok, would be great if you could look at this! I don't need to stick to any
particular branch.
Do you think MATNEST could be an alternative here? My matrix is
[A00 A01;
A01^t A11]
so perhaps with MATNEST I can make use of the block-symmetry at least, and then
use MATSBAIJ for
A00 and A11 if it's possible to combine matrix types which the manual seems to
imply.
Kind regards
Carl-Johan
> On 26 Aug 2023, at 10:09 PM, Carl-Johan Thore <carljohanth...@gmail.com>
> wrote:
>
> Hi,
>
> I'm trying to use PCFIELDSPLIT with MATSBAIJ in PETSc 3.19.4.
> According to the manual "[t]he fieldsplit preconditioner cannot
> currently be used with the MATBAIJ or MATSBAIJ data formats if the
> blocksize is larger than 1". Since my blocksize is exactly 1 it would seem
> that I can use PCFIELDSPLIT. But this fails with "PETSC ERROR: For symmetric
> format, iscol must equal isrow"
> from MatCreateSubMatrix_MPISBAIJ. Tracing backwards one ends up in
> fieldsplit.c at
>
> /* extract the A01 and A10 matrices */ ilink = jac->head;
> PetscCall(ISComplement(ilink->is_col, rstart, rend, &ccis)); if
> (jac->offdiag_use_amat) { PetscCall(MatCreateSubMatrix(pc->mat,
> ilink->is, ccis, MAT_INITIAL_MATRIX, &jac->B)); } else {
> PetscCall(MatCreateSubMatrix(pc->pmat, ilink->is, ccis,
> MAT_INITIAL_MATRIX, &jac->B)); }
>
> This, since my A01 and A10 are not square, seems to explain why iscol is not
> equal to isrow.
> From this I gather that it is in general NOT possible to use
> PCFIELDSPLIT with MATSBAIJ even with block size 1?
>
> Kind regards,
> Carl-Johan
