What you are looking for is:
https://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatCreateBAIJ.html
indoe is optimization for AIJ type - where you might not have a simple block
size like 3 that you have.
BAIJ will perform better than AIJ/with-inode
To verify indoe usage - you can run the code with '-log_info' option - and do a
'grep inode'
Satish
On Wed, 6 May 2020, Thomas S. Chyczewski wrote:
>
> All,
>
> I'm relatively new to PETSc and have relied pretty heavily on the example
> codes included in the distribution to figure out the finer points of using
> the PETSc library that I couldn't deduce from the manual. One thing I can't
> figure out is how to solve block matrix systems and I couldn't find an
> example in Fortran. I'm writing a 2D incompressible CFD solver so I have a
> 3x3 block Imax*Jmax system. The closest I've come to finding an example is
> ex19.c in the snes directory, but that is in c and for the nonlinear solver.
>
> I have been able to run PETSc but unwrapping the block matrix into a
> monolithic system. But the manual says "Block matrices represent an important
> class of problems in numerical linear algebra and offer the possibility of
> far more efficient iterative solvers than just treating the entire matrix as
> black box." However, in the FAQs I saw a comment that PETSc scans the AIJ
> matrices for rows that have the same column layout and can deduce if it's a
> block system and use the more efficient solvers. I also saw in the archives
> for this email list a thread where it seems workaround for building fields in
> a Fortran code is discussed ("Back to struct in Fortran to represent field
> with dof > 1"), so I'm beginning to suspect building a block system in
> Fortran might not be straight forward.
>
> All that being said, my questions:
>
> Is there a significant advantage to building the block system as opposed to
> the analogous monolithic system if PETSc can figure out that it's a block
> system? Can you confirm that PETSc does figure this out?
> If there is an advantage to loading the matrix as a block matrix, is there an
> example Fortran code that builds and solves a linear block system?
>
> Thanks,
> Tom C
>
>
