[petsc-users] Insert one sparse matrix as a block in another

[Matthew Knepley]

On Mon, Dec 12, 2022 at 5:24 PM Jed Brown <jed at jedbrown.org> wrote:

> The description matches MATNEST (MATCOMPOSITE is for a sum or product of
> matrices) or parallel decompositions. Also consider the assembly style of
> src/snes/tutorials/ex28.c, which can create either a monolithic or block
> (MATNEST) matrix without extra storage or conversion costs.
>

I will just say a few words about ex28. The idea is that if you are already
calling MatSetValues() to assemble your submatrices, then
you can use MatSetValuesLocal() to remap those locations into locations in
the large matrix, using a LocalToGlobalMap. This allows
you to choose either a standard AIJ matrix (which supports factorizations
for example), or a MatNest object that supports fast extraction
of the blocks.

  Thanks,

    Matt


> Mark Adams <mfadams at lbl.gov> writes:
>
> > Do you know what kind of solver works well for this problem?
> >
> > You probably want to figure that out first and not worry about
> efficiency.
> >
> > MATCOMPOSITE does what you want but not all solvers will work with it.
> >
> > Where does this problem come from? We have a lot of experience and might
> > know something.
> >
> > Mark
> >
> > On Mon, Dec 12, 2022 at 1:33 PM Peder Jørgensgaard Olesen via
> petsc-users <
> > petsc-users at mcs.anl.gov> wrote:
> >
> >> Hello
> >>
> >>
> >> I have a set of sparse matrices (A1, A2, ...) , and need to generate a
> >> larger matrix B with these as submatrices. I do not know the precise
> sparse
> >> layouts of the A's (only that each row has one or two non-zero values),
> >> and extracting *all* values to copy into B seems incredibly wasteful.
> How
> >> can I make use of the sparsity to solve this efficiently?
> >>
> >>
> >> Thanks,
> >>
> >> Peder
> >>
> >>
> >>
> >> Peder Jørgensgaard Olesen
> >> PhD student
> >> Department of Civil and Mechanical Engineering
> >>
> >> pjool at mek.dtu.dk
> >> Koppels Allé
> >> Building 403, room 105
> >> 2800 Kgs. Lyngby
> >> www.dtu.dk/english
> >>
> >>
>


-- 
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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