[petsc-users] Nonzero I-j locations

[Smith, Barry F.]

   This is an interesting idea, but unfortunately not directly compatible with libMesh filling up the finite element part of the matrix. Plus it appears MatGetLocalSubMatrix() is only implemented for IS and Nest matrices :-(

   You could create  a MATNEST reusing exactly the matrix from lib mesh as the first block and call MatConvert() to MPIAIJ format. This is easier I guess then coding the conversion yourself. (Still has the memory and copy issues but if that is the best we can do). Note that MatNest() requires that all its matrices live on all the ranks of the MPI_Comm, so for your A B and C you will need to declare them on the MPI_Comm with zero rows and columns for most ranks (maybe all but one).

   Barry


    



> On May 29, 2019, at 6:43 PM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Wed, May 29, 2019 at 7:30 PM Manav Bhatia via petsc-users <petsc-users at mcs.anl.gov> wrote:
> 
> I would not choose Nest if you want to eventually run MUMPS, since that will not work. I would
> try to build your matrix using
> 
>   https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetLocalSubMatrix.html
> 
> obtained from your bigger matrix. This is our interface to assembling into portions or your matrix as if
> its the entire matrix.
> 
>   Thanks,
> 
>     Matt
>  
>   Also, I am currently using MUMPS for all my parallel solves. 
> 
>    I would appreciate any advice. 
> 
> Regards,
> Manav
> 
> 
> > On May 29, 2019, at 6:07 PM, Smith, Barry F. <bsmith at mcs.anl.gov> wrote:
> > 
> > 
> >  Manav,
> > 
> >  For parallel sparse matrices using the standard PETSc formats the matrix is stored in two parts on each process (see the details in MatCreateAIJ()) thus there is no inexpensive way to access directly the IJ locations as a single local matrix. What are you hoping to use the information for? Perhaps we have other suggestions on how to achieve the goal.
> > 
> >   Barry
> > 
> > 
> >> On May 29, 2019, at 2:27 PM, Manav Bhatia via petsc-users <petsc-users at mcs.anl.gov> wrote:
> >> 
> >> Hi, 
> >> 
> >>   Once a MPI-AIJ matrix has been assembled, is there a method to get the nonzero I-J locations? I see one for sequential matrices here: https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetRowIJ.html , but not for parallel matrices. 
> >> 
> >> Regards,
> >> Manav
> >> 
> >> 
> > 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener
> 
> https://www.cse.buffalo.edu/~knepley/