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

[Dave May]

On Thu 21. Jul 2022 at 14:06, Matthew Knepley <knepley at gmail.com> wrote:

> 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.
>


As an assembled matrix is sought, a follow up suggestion might be to first
create a MatNest representation and pass this to MatConvert to convert the
Nest Mat into an MPIAIJ Mat.

The MatNest object is pretty light weight and doesn’t use much memory as it
refers (via a pointer) to the original matrices. Hence this two step
approach might be appropriate.

Cheers
Dave

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/>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20220721/b0147bb1/attachment.html>