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

[leejearl]

Hi Matt,

Thanks for your helps. I have another question， but I think I should
ask the question in a new subject.


leejearl


> > 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/>;
> >
> >
> >
> >
> >
> >
>