[petsc-users] PCFIELDSPLIT with MATSBAIJ

[Pierre Jolivet]

> On 26 Aug 2023, at 11:16 PM, Carl-Johan Thore <carl-johan.thore at liu.se> wrote:
> 
> "(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?

Well, your A00 and A11 will possibly be SBAIJ also, so you’ll end up with the same issue.
I’m using both approaches (monolithic SBAIJ or Nest + SBAIJ), it was crashing but I think it was thoroughly fixed in https://gitlab.com/petsc/petsc/-/commits/jolivet/feature-matcreatesubmatrices-rectangular-sbaij/
It is ugly code on top of ugly code, so I didn’t try to get it integrated and just used the branch locally, and then moved to some other stuff.
I’ll rebase on top of main and try to get it integrated if it could be useful to you (but I’m traveling right now so everything gets done more slowly, sorry).

Thanks,
Pierre

> 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 <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
> 

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