PETSc Global Vector to local process vector

[Barry Smith]

    Looks ok.

On Nov 25, 2008, at 5:54 PM, Ignacio Martínez wrote:

>
> My apologies, I want all values in all processor, following your  
> indications I guess I need to use something like:
>
>   Vec natural,allonme;
>   VecScatter tozero;
>
>   DACreateNaturalVector(da,&natural);
> /***********************************************************/
>   VecScatterCreateToAll(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);
>
> And all process should have all data. Is this right?
>
> Thank you very much,
> Ignacio
>
> 2008/11/26 Barry Smith <bsmith at mcs.anl.gov>:
> >
> >   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
> >>
> >
> >
>