VecGetValues and tutorail?

[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
> >>
> >>
> >
> >
> >
> 
>