[petsc-users] Efficiently build a matrix from two asymmetric diagonal block matrices

[Matthew Knepley]

On Thu, Jul 21, 2022 at 6:28 AM Emile Soutter <emile.soutter at corintis.com>
wrote:

> Dear all,
>
> I am struggling with the simple following problem : Having a first matrix
> B1 of size n1xm1, a second matrix B2 of size n2 x m2, build a matrix M of
> size (n1+n2)x(m1+m2) where the blocks B1 and B2 are the "diagonal" of M
> (M[0:n1,0:m1]=B1, M[n1:(n1+n2),m1:(m1+m2)]=B2). In my case, the blocks B1
> and B2 are obtained from another routine, directly in the petsc matrix form
> (or pyop2.Sparsity form). However the blocks are not squared (n1,n2,m1,m2
> are all different integers). The operation is easy to do with the SetValues
> option. However, it takes a large amount of time (too much) when the system
> becomes large. I struggle to do it efficiently and in parallel. What method
> do you recommend to use to do this as fast as possible?
>
> Thanks you for any tips,
>

I think it depends on what you want to do with the final matrix. If you
only want MatMult, then I think you can just use MatNest

  https://petsc.org/main/docs/manualpages/Mat/MatCreateNest/

which will wrap up the submatrices. However, if you want to manipulate the
values (factorization, relaxation, etc) then you need
to assemble a monolithic matrix. For this you could create the global
matrix, and then use

  https://petsc.org/main/docs/manualpages/Mat/MatCreateLocalRef/

to get a submatrix to assemble directly into, which you pass to your
assembly routine. Clearly this is more complicated, but
sometimes necessary.

  Thanks,

      Matt


> Emile
>


-- 
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/ <http://www.cse.buffalo.edu/~knepley/>
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