[petsc-users] Copying PETSc Objects Across MPI Communicators

[Matthew Knepley]

On Wed, Oct 25, 2023 at 11:55 PM Damyn Chipman <
damynchipman at u.boisestate.edu> wrote:

> Great thanks, that seemed to work well. This is something my algorithm
> will do fairly often (“elevating” a node’s communicator to a communicator
> that includes siblings). The matrices formed are dense but low rank. With
> MatCreateSubMatrix, it appears I do a lot of copying from one Mat to
> another. Is there a way to do it with array copying or pointer movement
> instead of copying entries?
>

We could make a fast path for dense that avoids MatSetValues(). Can you
make an issue for this? The number one thing that would make this faster is
to contribute a small test. Then we could run it continually when putting
in the fast path to make sure we are preserving correctness.

  Thanks,

    Matt


> -Damyn
>
> On Oct 24, 2023, at 9:51 PM, Jed Brown <jed at jedbrown.org> wrote:
>
> You can place it in a parallel Mat (that has rows or columns on only one
> rank or a subset of ranks) and then MatCreateSubMatrix with all new
> rows/columns on a different rank or subset of ranks.
>
> That said, you usually have a function that assembles the matrix and you
> can just call that on the new communicator.
>
> Damyn Chipman <damynchipman at u.boisestate.edu> writes:
>
> Hi PETSc developers,
>
> In short, my question is this: Does PETSc provide a way to move or copy an
> object (say a Mat) from one communicator to another?
>
> The more detailed scenario is this: I’m working on a linear algebra solver
> on quadtree meshes (i.e., p4est). I use communicator subsets in order to
> facilitate communication between siblings or nearby neighbors. When
> performing linear algebra across siblings (a group of 4), I need to copy a
> node’s data (i.e., a Mat object) from a sibling’s communicator to the
> communicator that includes the four siblings. From what I can tell, I can
> only copy a PETSc object onto the same communicator.
>
> My current approach will be to copy the raw data from the Mat on one
> communicator to a new Mat on the new communicator, but I wanted to see if
> there is a more “elegant” approach within PETSc.
>
> Thanks in advance,
>
> Damyn Chipman
> Boise State University
> PhD Candidate
> Computational Sciences and Engineering
> damynchipman at u.boisestate.edu
>
>
>

-- 
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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