PETSc Global Vector to local process vector

[Barry Smith]

    Your message is unclear.

    DAGlobalToNaturalAllCreate() will give you a scatter to get ALL  
the values
on EACH process in the "natural ordering".

    Do you wish to get ALL the values onto a SINGLE process (and nothing
on the rest)? If so, here is how you do it.

     Vec natural,allonme;
     VecScatter tozero;

     DACreateNaturalVector(da,&natural);
     VecScatterCreateToZero(natural,&tozero,&allonme);

    /* now to get the values on the that 0th process */

     DAGlobalToNaturalBegin(da,vec,INSERT_VALUES,natural);
     DAGlobalToNaturalEnd(da,vec,INSERT_VALUES,natural);
      
VecScatterBegin(tozero,natural,allonme,INSERT_VALUES,SCATTER_FORWARD);
   VecScatterEnd(tozero,natural,allonme,INSERT_VALUES,SCATTER_FORWARD);

    Now process 0 will have a sequential vector that contains the  
entire global vector.
All other processes will have a sequential vector of size 0. ALL  
processes, of course,
must call the DAGlobalToNatural.... and VecScatter... routines.

    It is possible to reverse this operation too, to get values from  
process 0
out to the other processes.

    If this was not your question, then please clarify what you wish  
to do.

    Barry


On Nov 25, 2008, at 4:46 PM, Ignacio Martínez wrote:

> Hello,
>
> I'm a beginner user, and I have a problem:
>
> I have a routine in Fortran and I just want to parallelize few lines
> with PETSc, I'm using DA in order to gather the results, from local
> vectors to a global vector.
> It works fine. But I need to return those values to the Fortran
> routine, and I only get access to local process values.
>
> I try to create a sequential vectors with an scatter to get that
> values. But, PETSc returns an error, something like: "Cannot create
> VECSEQ on more than one process!" (I try to create the vector only in
> one process).
>
> Exactly in using DAGlobalToNaturalAllCreate and  VecScatterBegin to
> get global values in to a process.
>
> And my question is if someone has tried to do something like this, and
> how has done it. Or if could you give me some orientation?
>
> Thank you in advance,
> Ignacio
>