VecGetValues?

[billy at dem.uminho.pt]

Once I update the ghosts cells values, how do I access them using the global index?

I have used VecGetValues but it says: "Can only get local values, trying 100!"

Billy.

Quoting billy at dem.uminho.pt:

> 
> Thank you very much for your help. :)
> 
> I think I will use VecCreateGhost() then.
> 
> Billy.
> 
> 
> Quoting Barry Smith <bsmith at mcs.anl.gov>:
> 
> > 
> >    If you are using a structured grid you should use DA's to
> > manage all this. You won't need to monkey with IS or VectorScatter
> > or VecCreateGhost. Otherwise, ... see below....
> > 
> > On Sun, 26 Feb 2006, billy at dem.uminho.pt wrote:
> > 
> > > Hi,
> > >
> > > I want to parallelize an unstructured finite volume CFD code. The code
> is
> > > second-order in space, so I only need one layer of overlap between
> domains.
> > The
> > > time advancement is implicit.
> > >
> > > I renumbered the cells in my application to respect PETSc ordering.
> > > Do I need to use AO: Application Orderings? Or is this optional?
> > 
> > You do not have to use AO to manage getting the "PETSc style" ordering,
> > you can do it anyway you like, so long as you have a "PETSc style"
> > ordering.
> > 
> > >
> > > P0             P1
> > > ----------     ----------
> > > | 0| 1| 2| <-> | 9|10|11|
> > > | 3| 4| 5| <-> |12|13|14|
> > > | 6| 7| 8| <-> |15|16|17|
> > > ----------     ----------
> > >
> > > Should I use VecCreateGhost to include the ghost cells or another type
> of
> > vector
> > > like VecCreateSeq?
> > 
> >     Likely you will wish to use VecCreateGhost()
> > 
> > >
> > > P0                P1
> > > -------------     -------------
> > > | 0| 1| 2| 9| <-> |18| 9|10|11|
> > > | 3| 4| 5|10| <-> |19|12|13|14|
> > > | 6| 7| 8|11| <-> |21|15|16|17|
> > > -------------     -------------
> > >
> > > Do I need to create IS: Index Sets?
> > 
> > If you use VecCreateGhost() and friends (VecGhostUpdateBegin/End(), 
> > VecGhostGetLocalForm() etc) you will not need to deal with index sets.
> > >
> > > P0                P1
> > > -------------     -------------
> > > | 0| 1| 2| 9| <-> | 9| 0| 1| 2|
> > > | 3| 4| 5|10| <-> |10| 3| 4| 5|
> > > | 6| 7| 8|11| <-> |11| 6| 7| 8|
> > > -------------     -------------
> > >
> > > Then I should apply VecScatter routines to update ghost cells, right?
> > 
> > If you use VecCreateGhost() then it manages the scatters during the 
> > VecGhostUpdateBegin/End() for you; you do not need to use IS or
> VecScatter.
> > 
> > If you do not use VecCreateGhost() then you need to use IS followed by
> > VecScatterCreate() to build the scatter data structures and 
> > VecScatterBegin/End() to update the ghost values.
> > 
> >     Barry
> > 
> > >
> > > Sorry for asking so many questions, but this is all very confusing to
> me.
> > >
> > >
> > > Billy.
> > >
> > >
> > > Quoting Satish Balay <balay at mcs.anl.gov>:
> > >
> > >> On Fri, 24 Feb 2006, billy at dem.uminho.pt wrote:
> > >>
> > >>>
> > >>> Hello,
> > >>>
> > >>>
> > >>> "VecGetValues - Gets values from certain locations of a vector.
> > Currently
> > >> can
> > >>> only get values on the same processor"
> > >>>
> > >>> How can I get a value of the vector belonging to a different
> processor?
> > >>>
> > >>>
> > >>> I am trying to learn how to adapt my code to work in parallel. Have
> > looked
> > >> at
> > >>> src/snes/examples/tutorials/ex10d/ex10.c, but I am still confused on
> how
> > to
> > >> use
> > >>> petsc with unstructured meshes. Is there any simple step-by-step
> > >> tutorial?
> > >>
> > >> Basically - you'll have to use VecScatter routines to specify the
> > >> communication required. Then use VecScatterBegin/End() to invoke this
> > >> communication. Some of this info is in the tutorial slides [check
> > >> arround slide 150] in:
> > >>
> >
> http://www-unix.mcs.anl.gov/petsc/petsc-as/documentation/tutorials/PetscTu06.pdf
> > >>
> > >> Not usre if there is an unstructured grid example code. [but there are
> > >> examples using VecScatters in src/dm/da/examples/tutorials]
> > >>
> > >> Satish
> > >>
> > >>
> > >
> > >
> > >
> > 
> > 
> 
> 
>