VecGetValues?

[billy at dem.uminho.pt]

I establish the ghost cells and I have a vector gx created with VecCreateGhost()
which looks like this:

---gx---:
Process [0]
0  0.875
1  0.625
2  0.875
3  0.625
Process [1]
4  0.375
5  0.125
6  0.375
7  0.125
Process [2]
8  0.375
9  0.125
10 0.375
11 0.125
Process [3]
12 0.875
13 0.625
14 0.875
15 0.625

Then I can use VecGhostGetLocalForm to get the local vector with ghost cell values:

---lx---:
0 0.875
1 0.625
2 0.875
3 0.625
4 0.375
5 0.875
6 0.375
7 0.625

Then if I can access the ghost cell using local index:

xl[4]: 0.375000

I have several global vectors like gx and if I have local representation for
each one of them, isn't this a duplication of memory?

Can I access the ghost cell value with creating the local vector?
(VecGhostGetLocalForm)


Billy.


Quoting Matthew Knepley <knepley at mcs.anl.gov>:

> billy at dem.uminho.pt writes:
> 
> > 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!"
> 
>   We do not have access to local values through global indices expect through
> the DA.
> This is something we might provide with the unstructured update in the next
> release, but it is really
> more common to refer to them through local indices, as in FEM methods.
> 
>      Matt
> 
> > 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
> >> > >>
> >> > >>
> >> > >
> >> > >
> >> > >
> >> > 
> >> > 
> >> 
> >> 
> >> 
> >
> >
> >
> >
> 
> -- 
> "Failure has a thousand explanations. Success doesn't need one" -- Sir Alec
> Guiness
> 
>