[petsc-users] SuperLU_Dist bug or "intentional error"

[Junchao Zhang]

Could you try the master branch since it seems Stefano fixed this problem
recently?
--Junchao Zhang


On Mon, Mar 9, 2020 at 2:04 PM Rochan Upadhyay <u.rochan at gmail.com> wrote:

> Dear PETSc Developers,
>
> I am having trouble interfacing SuperLU_Dist as a direct solver for
> certain problems in PETSc. The problem is that when interfacing with
> SuperLU_Dist, you need your matrix to be of Type MPISEQAIJ when running MPI
> with one processor. PETSc has long allowed the use of Matrix type MPIAIJ
> for all MPI runs, including MPI with a single processor and that is still
> the case for all of PETSc's native solvers. This however has been broken
> for the SuperLU_Dist option. The following code snippet (in PETSc and not
> SuperLU_Dist) is responsible for this restriction and I do not know if it
> is by design or accident :
>
> In file petsc-3.12.4/src/mat/impls/aij/mpi/superlu_dist/superlu_dist.c
> line 257 onwards :
>
> ierr = MPI_Comm_size(PetscObjectComm((PetscObject)A),&size);CHKERRQ(ierr);
>
>   if (size == 1) {
>     aa = (Mat_SeqAIJ*)A->data;
>     rstart = 0;
>     nz     = aa->nz;
>   } else {
>     Mat_MPIAIJ *mat = (Mat_MPIAIJ*)A->data;
>     aa = (Mat_SeqAIJ*)(mat->A)->data;
>     bb = (Mat_SeqAIJ*)(mat->B)->data;
>     ai = aa->i; aj = aa->j;
>     bi = bb->i; bj = bb->j;
>
> The code seems to check for number of processors and if it is = 1 conclude
> that the matrix is a Mat_SeqAIJ and perform some operations. Only if
> number-of-procs > 1 then it assumes that matrix is of type Mat_MPIAIJ. I
> think this is unwieldy and lacks generality. One would like the same piece
> of code to run in MPI mode for all processors with type Mat_MPIAIJ. Also
> this restriction has suddenly appeared in a recent version. The issue was
> not there until at least 3.9.4. So my question is from now (e.g. v12.4) on,
> should we always use matrix type Mat_SeqAIJ when running on 1 processor
> even with MPI enabled and use Mat_MPIAIJ for more than 1 processor. That is
> use the number of processors in use as a criterion to set the matrix type ?
>
> A an illustration, I have attached a minor modification of KSP example 12,
> that used to work with all PETSc versions until at least 3.9.4 but now
> throws a segmentation fault. It was compiled with MPI and run with mpiexec
> -n 1 ./ex12
> If I remove the "ierr = MatSetType(A,MATMPIAIJ);CHKERRQ(ierr);" it is okay.
>
> I hope you can clarify my confusion.
>
> Regards,
> Rochan
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