And MKL has the same issues for dense , see for example here https://gitlab.com/petsc/petsc/-/blob/master/src/mat/impls/dense/seq/dense.c#L2186 <https://gitlab.com/petsc/petsc/-/blob/master/src/mat/impls/dense/seq/dense.c#L2186> If A is 0x8 and B is 8x5 then C is correct to be of size 0x5. The rows and columns of the resulting matrix have to follow the rules.
> On May 31, 2020, at 12:54 AM, Mark Adams <mfad...@lbl.gov> wrote: > > 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@mcs.anl.gov <mailto:petsc-dev@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
