[petsc-dev] MatCreateTranspose semantics

[Jed Brown]

I would just use MatTranspose here.  We've had limited demand for more
sophisticated operations on objects of type MATTRANSPOSE and there
probably isn't much benefit in fusing the parallel version here anyway.

Jeff Hammond <jeff.science at gmail.com> writes:

> I am trying to understand how to use a transposed matrix view along the
> lines of Numpy (
> https://github.com/ParRes/Kernels/blob/master/PYTHON/transpose-numpy.py#L99
> ).
>
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateTranspose.html
> says
>
> "Creates a new matrix object that behaves like A'"
>
> But later it seems to suggest that this is only true when performance a
> matrix-vector product:
>
> "Rather the new matrix object performs the matrix-vector product by using
> the MatMultTranspose() on the original matrix"
>
> I got the idea to use this view concept from
> https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatTranspose.html,
> which says:
>
> "Consider using MatCreateTranspose() instead if you only need a matrix that
> behaves like the transpose, but don't need the storage to be changed."
>
> When I try to use the transpose view in MatAXPY using this code:
>     Mat AT;
>     ierr = MatCreateTranspose(A, &AT); // create transpose view of A
>     ierr = MatAXPY(B, one, AT, SAME_NONZERO_PATTERN);
>
> I get an error suggesting that I am doing something bad:
>
> [0]PETSC ERROR: --------------------- Error Message
> --------------------------------------------------------------
> [0]PETSC ERROR: Object is in wrong state
> [0]PETSC ERROR: Not for unassembled matrix (Y)
> [0]PETSC ERROR: See https://www.mcs.anl.gov/petsc/documentation/faq.html
> for trouble shooting.
> [0]PETSC ERROR: Petsc Release Version 3.13.1, unknown
> [0]PETSC ERROR: ./transpose-petsc on a  named jrhammon-nuc by jrhammon Mon
> Jun  1 15:38:20 2020
> [0]PETSC ERROR: Configure options --prefix=/home/jrhammon/PRK/deps/petsc
> --with-blaslapack-dir=/opt/intel/inteloneapi/mkl/2021.1-beta06
> --with-mpi-dir=/opt/intel/inteloneapi/mpi/2021.1-beta06 --with-cxx=0
> --with-fc=0
> [0]PETSC ERROR: #1 MatAXPY() line 74 in
> /home/jrhammon/PRK/deps/petsc-src/src/mat/utils/axpy.c
> [0]PETSC ERROR: #2 main() line 170 in transpose-petsc.c
> [0]PETSC ERROR: PETSc Option Table entries:
> [0]PETSC ERROR: -i 10
> [0]PETSC ERROR: -n 100
> [0]PETSC ERROR: ----------------End of Error Message -------send entire
> error message to petsc-maint at mcs.anl.gov----------
>
> What do I need to do to use a transpose view properly outside of M*V?
>
> Thanks,
>
> Jeff
>
> --
> Jeff Hammond
> jeff.science at gmail.com
> http://jeffhammond.github.io/