[petsc-users] PCFIELDSPLIT with MATSBAIJ

[Pierre Jolivet]

(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?

Thanks,
Pierre

> On 26 Aug 2023, at 10:09 PM, Carl-Johan Thore <carljohanthore at 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