VecGetValues?

[Matthew Knepley]

billy at dem.uminho.pt writes:

      If you look inside the code, we do not duplicate memory for the local rep,
jsut wrap it with a seq vector.

     Matt

> 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
>> 
>> 
>
>
>
>

-- 
"Failure has a thousand explanations. Success doesn't need one" -- Sir Alec Guiness