[petsc-users] How Can I scatter a global vector to a local vector without using DMGlobaltoLocalBegin/DMGlobaltoLocalEnd?

[Matthew Knepley]

On Tue, Jan 15, 2019 at 4:51 AM leejearl via petsc-users <
petsc-users at mcs.anl.gov> wrote:

> Hi, Matt
>
> > > Hi, all Petscer:
> > >   I am very sorry for disturbing you for my questions.
> >
> >
> > I like your questions.
> >
> >
> > > I have a
> > > problems as follow.
> > >   1. A DMPlex object dm is created.
> > >   2. A global vector and a local vector are created using the
> > following
> > > routines DMCreateGlobalVector and DMCreateLocalVector.
> > >   3. I have initialed global Vector. Because the local vector have
> > some
> > > overlap cells, I want to scatter the data from global to local.
> > >   4. If I don't use the routines
> > > DMGlobaltoLocalBegin/DMGlobaltoLocalEnd, how can I scatter it?
> >
> >
> > To understand the question better, why would you not use
> > DMGlobalToLocal()?
> I have create a PetscSection for the dm object. The section is used to
> store the macroscopic variables $\rho$, u,  v, $\rho$E. A pair of
> global and local vector are created using DMCreateGlobalVector and
> DMCreateLocalVector.
>
> I also needed a vector to store the derivatives of the macroscopic
> variables, but I have the difficulty to set two PetscSection to a dm
> object.


You are right. You can't use 2 Sections for 1 DM. However, this one is easy.
Use DMClone() to get another DM. They will share the expensive topology
information, but the new one can have the different Section. Then all the
GlobalToLocal will be created automatically.

  Thanks,

    Matt


> The first section has four variables on the cell point, and the
> variable number of second section is  not the same as the first
> section. I have no
> idea about this problem, and I think that whether I can scatter it by
> myself without using  DMGlobalToLocal()?
>
> Can you point me out how can I creat two PetscSection whin a dmplex
> object, and how can I using DMGlobalToLocal() for the two pair of
> vectors whin a dmplex object?
> >
> > > I have
> > > scatter the vectors using VecScatterBegin and VecScatterEnd, and
> > the IS
> > > is created using the localId and the glolbalId (Obtained by
> > > DMPlexGetCellNumbering). I got a wrong result.
> > >
> >
> > DMPlexGetCellNumbering() gives you are _arbitrary_ numbering of the
> > cells,
> > not the
> > global numbering of unknowns.
> >
> >
> > >   Can anyone give me some advices, and does it have some to do with
> > the
> > > natural ids?
> > >
> >
> > 1) You can get the local ids from the local Section (DMGetSection),
> > and the
> > global ids
> > from the global Section (DMGetGlobalSection). You loop over all the
> > points
> > in the mesh
> > and get the offset/size pairs. However, this is error-prone (since
> > you can
> > have constraints)
> > and probably unnecessary.
> >
> > 2) If you look at the code for DMGlobalToLocalBegin():
> >
> >
>
> https://bitbucket.org/petsc/petsc/src/650136806607101f8eb495880e9efa33f2f75729/src/dm/interface/dm.c#lines-2201
> >
> > you see it just call PetscSFBcastBegin(), and there is a similar
> > BcastEnd()
> > in GlobalToLocalEnd(). Thus all
> > we are doing is using the SF directly. That is the same as using a
> > Scatter.
> >
> >   Thanks,
> >
> >    Matt
> Thank you very much. I have do it yestoday following the code for
> DMGlobalToLocalBegin().
> >
> > >   Any helps are approciated. Thanks.
> > >
> > > leejearl
> > >
> > >
> >
> > --
> > 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/>
>
>
>
>
>
>

-- 
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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