[petsc-dev] Meaning of PETSc matrices with zero rows but nonzero columns?

[Mark Adams]

Failing for empty matrices sucks of MKL.
I don't know of any reason that we care how many columns are in a
matrix with no rows. I see no reason not to let it stay the way it is, that
is with the number of columns that it should have if it had rows.
I would vote for just doing what you need to do to get the test suite to
pass and have a MKL test with zero rows.
A GAMG test should give you an empty local marix on the coarse grid on rank
!= 0.

On Sat, May 30, 2020 at 2:07 PM Mills, Richard Tran via petsc-dev <
petsc-dev at mcs.anl.gov> wrote:

> All,
>
> I'm working on adding support for matrix products to AIJMKL, and I'm
> uncertain about some issues surrounding empty matrices. PETSc will happily
> let me multiply an empty matrix with another (and this arises in the
> sequential matrix-matrix multiplication routines when running with multiple
> MPI ranks and using MPIAIJ), but MKL does not like empty matrices (or
> matrices with no nonzeros), so I've got code in various places in the
> existing AIJMKL routines to handle these cases without calling MKL.
>
> I'm not quite sure what needs to be done in, say,
> MatMatMultSymbolic_SeqAIJMKL_SeqAIJMKL(). In the SeqAIJ version, if a
> matrix A is passed in that has zero rows (that is, A->rmap->N = 0), and
> matrix B has N columns (B->cmap->N = N), then a matrix C with zero rows and
> N columns is created. My question boils down to "Does it mean anything in
> PETSc to have a matrix with 0 rows but a nonzero number of columns"? It is
> less complicated if I handle the empty matrix cases by creating a matrix
> with 0 rows and 0 columns, but I am not sure if this breaks something. (I'm
> also not sure what a "matrix" with 0 rows even means.) Also not sure if
> there is any other info that I need to preserve in the result matrix C, of
> if it is OK to handle the case of any empty A or B by always producing the
> same empty matrix C.
>
> --Richard
>
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