[petsc-users] Copying PETSc Objects Across MPI Communicators

[Damyn Chipman]

Yeah, I’ll make an issue and use a modified version of this test routine.

Does anything change if I will be using MATSCALAPACK matrices instead of the built in MATDENSE? Like I said, I will be computing Schur complements and need to use a parallel and dense matrix format.

-Damyn

> On Oct 26, 2023, at 10:01 AM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Wed, Oct 25, 2023 at 11:55 PM Damyn Chipman <damynchipman at u.boisestate.edu <mailto: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 <mailto: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 <mailto: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 <mailto: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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