[petsc-users] DMSTAG Gathering Vector on single process

[Colton Bryant]

Ah excellent! I was not aware of the ability to preallocate the objects and
migrate them each time.

Thanks!
-Colton

On Wed, Dec 6, 2023 at 5:18 PM Matthew Knepley <knepley at gmail.com> wrote:

> On Wed, Dec 6, 2023 at 5:54 PM Colton Bryant <
> coltonbryant2021 at u.northwestern.edu> wrote:
>
>> Hello,
>>
>> I am working on a code in which a DMSTAG object is used to solve a fluid
>> flow problem and I need to gather this flow data on a single process to
>> interact with an existing (serial) library at each timestep of my
>> simulation. After looking around the solution I've tried is:
>>
>> -use DMStagVecSplitToDMDA to extract vectors of each component of the flow
>> -use DMDACreateNaturalVector and DMDAGlobalToNatural to get the
>> components naturally ordered
>> -use VecScatterCreateToZero to set up and then do the scatter to gather
>> on the single process
>>
>> Unless I'm misunderstanding something this method results in a lot of
>> memory allocation/freeing happening at each step of the evolution and I was
>> wondering if there is a way to directly perform such a scatter from the
>> DMSTAG object without splitting as I'm doing here.
>>
>
> 1) You can see here:
>
> https://petsc.org/main/src/dm/impls/stag/stagda.c.html#DMStagVecSplitToDMDA
>
> that this function is small. You can do the DMDA creation manually, and
> then just call DMStagMigrateVecDMDA() each time, which will not create
> anything.
>
> 2) You can create the natural vector upfront, and just scatter each time.
>
> 3) You can create the serial vector upfront, and just scatter each time.
>
> This is some data movement. You can compress the g2n and 2zero scatters
> using
>
>   https://petsc.org/main/manualpages/PetscSF/PetscSFCompose/
>
> as an optimization.
>
>   Thanks,
>
>      Matt
>
>
>> Any advice would be much appreciated!
>>
>> Best,
>> Colton Bryant
>>
>
>
> --
> 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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