[petsc-users] How to fetch right values of a local vector

[Matthew Knepley]

On Fri, Feb 23, 2024 at 9:54 AM 袁煕 <yuanxi at advancesoft.jp> wrote:

> Hello, I cannot fetch right values of a local vector after a second time
> calling DMSetLocalSection.   My program runs like ==================== call
> PetscSectionSetup(section1,ierr) call
> DMSetLocalSection(dm_mesh,section1,ierr)call PetscSectionDestroy(section1,ierr)call
>
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> Hello,
>
> I cannot fetch right values of a local vector after a second time calling
> DMSetLocalSection.  My program runs like
>
> ====================
> call PetscSectionSetup(section1,ierr)
> call DMSetLocalSection(dm_mesh,section1,ierr)
> call PetscSectionDestroy(section1,ierr)
>
> call DMCreateGlobalVector(dm_mesh,vec_a,ierr)
>
> call VecDestroy(vec_a,ierr)
> call DMClearLocalVectors(dm_mesh, ierr);
>

You also need to wipe out the SF,

  call DMSetSectionSF(dm, PETSC_NULL_PETSCSF, ierr)


> call PetscSectionSetup(section2,ierr)
> call DMSetLocalSection(dm_mesh,section2,ierr)
>
> call DMCreateGlobalVector(dm_mesh,vec_a,ierr)
> call DMGetLocalVector(dm_mesh,lvec,ierr);
> call DMGlobalToLocal(dm_mesh,vec_a,INSERT_VALUES,lvec,ierr)
> call VecGetArrayReadF90(lvec,tempv,ierr)
> call VecGetArrayReadF90(vec_a,tempv1,ierr)
> ========================
>
> In this program, I got two Fortran style arrays, *tempv* and *tempv1*,
> from a Vec *vec_a*. When only 1 cpu is used, those two arrays should be
> completely the same. Am I right? But I found there are differences as
> follows:
>
> *  In array *tempv*,  dofs newly introduced by section2 are zero valued.
> Looks like those values are ignored by DMGlobalToLocal. But in array
> *tempv1*, all dofs values are set correctly.
>
> Did I do something wrong in the above code?
>

We do not usually reset the section on a DM. Instead, when we want to use a
different function space,
we clone the DM, using DMClone(), and make a new function space on the new
DM.

  Thanks,

     MAtt

Much thanks for your help.
>
> X. Yuan, Ph.D. in Solid Mechanics
>


-- 
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

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