[petsc-dev] MatTransposeMatMult() bug

[Pierre Jolivet]

Thanks Hong, this has been fixed by Stefano there: https://gitlab.com/petsc/petsc/-/commit/38faa66a3f98cdf432182095b5ca9cf1fc38724a?merge_request_iid=3717 <https://gitlab.com/petsc/petsc/-/commit/38faa66a3f98cdf432182095b5ca9cf1fc38724a?merge_request_iid=3717>
Pierre

> On 17 Mar 2021, at 8:27 PM, Zhang, Hong <hzhang at mcs.anl.gov> wrote:
> 
> Pierre,
> Do you mean a possible bug in C=AtB MatTransposeMatMult()?
> Can you provide a stand-alone test without hpddm that reproduces this error? 
> Hong
> From: petsc-dev <petsc-dev-bounces at mcs.anl.gov> on behalf of Pierre Jolivet <pierre at joliv.et>
> Sent: Wednesday, March 17, 2021 4:31 AM
> To: For users of the development version of PETSc <petsc-dev at mcs.anl.gov>
> Subject: [petsc-dev] MatTransposeMatMult() bug
>  
> Hello,
> While trying out Stefano’s PCApplyMat_MG() code (*), we stumbled upon weird numerical errors when reusing a Mat for both MatProduct_AB and MatProduct_AtB.
> This reminded me that there has been a long-standing issue with MatTransposeMatMult(), see https://www.mcs.anl.gov/petsc/petsc-current/src/ksp/pc/impls/hpddm/hpddm.cxx.html#line608 <https://www.mcs.anl.gov/petsc/petsc-current/src/ksp/pc/impls/hpddm/hpddm.cxx.html#line608>, that I never looked into.
> I’ve now been trying to figure this out, because this has side effects in multiple places (PCMG and PCHPDDM at least), and thus could impact user-code as well?
> With this commit: https://gitlab.com/petsc/petsc/-/commit/03d8bd538039defc2fcc3e37d523735c4aaceba0 <https://gitlab.com/petsc/petsc/-/commit/03d8bd538039defc2fcc3e37d523735c4aaceba0>
> +
> $ mpirun -n 4 src/ksp/ksp/tutorials/ex76 -ksp_converged_reason -pc_type hpddm -pc_hpddm_levels_1_eps_nev 20 -ksp_type preonly -mat_type aij -load_dir ${DATAFILESPATH}/matrices/hpddm/GENEO -rhs 2 -pc_hpddm_coarse_correction balanced -C_input_mattransposematmult -D_output_mattransposematmult
> I’m seeing that C is nonzero, but D is full of zeros.
> Mat Object: 4 MPI processes
>   type: mpidense
> 5.7098316584361917e-08 1.0159399260517841e-07
> 1.5812349976211856e-07 2.0688121715350138e-07
> 2.4887556933361981e-08 4.8111092300772958e-08
> 1.4606298643602107e-07 1.7213611729839211e-07
> […]
> Mat Object: 4 MPI processes
>   type: mpidense
> 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00
> 0.0000000000000000e+00 0.0000000000000000e+00
> […]
> 
> If one switches to a MatType which has no MatProduct_AtB implementation with B of type MPIDense (reminder: in that case, the product is computed column-by-column), e.g., -mat_type sbaij, one gets the expected result.
> Mat Object: 4 MPI processes
>   type: mpidense
> 7.2003197398135299e-01 9.5191869895699011e-01
> 6.1793966541680234e-02 9.3884397585488877e-01
> 1.0022337823233585e-02 2.4653068080134588e-01
> 1.4463931936094099e-01 8.6111517670701687e-01
> 
> Is there a bug somewhere with the MatAIJ implementation, or am I doing something which is not allowed by the MatProduct() machinery?
> 
> Thanks,
> Pierre
> 
> (*) https://gitlab.com/petsc/petsc/-/merge_requests/3717 <https://gitlab.com/petsc/petsc/-/merge_requests/3717>
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