[petsc-dev] Broken MatMatMult_MPIAIJ_MPIDense

[Pierre Jolivet]

Jed,
I’m not sure how easy it is to put more than a few lines of code on GitLab, so I’ll just send the (tiny) source here, as a follow-up of our discussion https://gitlab.com/petsc/petsc/merge_requests/2069#note_220229648 <https://gitlab.com/petsc/petsc/merge_requests/2069#note_220229648>.
Please find attached a .cpp showing the brokenness of C=A*B with A of type MPIAIJ and B of type MPIDense when the LDA of B is not equal to its number of local rows.
It does [[1,1];[1,1]] * [[0,1,2,3];[0,1,2,3]]
C should be equal to 2*B, but it’s not, unless lda = m (= 1).
Mat Object: 2 MPI processes
  type: mpidense
0.0000000000000000e+00 1.0000000000000000e+00 2.0000000000000000e+00 3.0000000000000000e+00
0.0000000000000000e+00 1.0000000000000000e+00 2.0000000000000000e+00 3.0000000000000000e+00

If you change Bm here https://www.mcs.anl.gov/petsc/petsc-dev/src/mat/impls/aij/mpi/mpimatmatmult.c.html#line549 <https://www.mcs.anl.gov/petsc/petsc-dev/src/mat/impls/aij/mpi/mpimatmatmult.c.html#line549> to the LDA of B, you’ll get the correct result.
Mat Object: 2 MPI processes
  type: mpidense
0.0000000000000000e+00 2.0000000000000000e+00 4.0000000000000000e+00 6.0000000000000000e+00
0.0000000000000000e+00 2.0000000000000000e+00 4.0000000000000000e+00 6.0000000000000000e+00

Unfortunately, w.r.t. MR 2069, I still don’t get the same results with a plain view LDA > m (KO) and a view + duplicate LDA = m (OK).
So there might be something else to fix (or this might not even be a correct fix), but the only reproducer I have right now is the full solver.

Thanks,
Pierre

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