[petsc-users] Data Distribution from zero-rank process

[Matthew Knepley]

On Tue, Aug 12, 2014 at 9:22 AM, Mari Pecha <pecha.mari at gmail.com> wrote:

>  Okay. I understand my explanation it sees to be chaotic. I try to
> describe my problem on simple simple.
>
> Process with rank zero loads grayscale image from OpenCV library then I'd
> like to distribute values of pixels to grid (DMDA). And I can't find any
> solution for this problem.
>

The right thing to do is to create a custom scatter for this case. However,
you can do this in two parts. First,
read it in on proc 0, and then scatter to all procs using


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Vec/VecScatterCreateToZero.html

where you SCATTER_BACKWARD. Then use


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/DM/DMDANaturalToGlobalBegin.html

to permute from the natural ordering that you read in to the PETSc
ordering. You could do this in one
step if you allow each process to read its piece of the vector
independently.

   Thanks,

      Matt


>
> Thank you
> Mari
>
> On 12.8.2014 16:12, Matthew Knepley wrote:
>
>  On Tue, Aug 12, 2014 at 8:53 AM, Mari Pecha <pecha.mari at gmail.com> wrote:
>
>> Good morning,
>>
>> I'm Mari and I'm beginner PETSc user and I'd like to ask about tough
>> problem, especially for me.
>> I solve problem of data-set distribution, which are saved only on
>> zero-rank process, to DMDA global vector. It means that every process
>> stores a appropriate  subset of data-set. I used
>> DMDAGlobalToNaturalAllCreate and DMDANaturalAllToGlobalCreate function, and
>> then VecScatterBegin and VecScatterEnd functions ... see please code below
>>
>
>  I have no idea what you really want to do. How about describing your
> problem with
> 2 procs and 3 values per proc.
>
>
>>     ...
>>
>>     ierr = DMDAGlobalToNaturalAllCreate( da, &tolocalall   );
>> CHKERRQ(ierr);
>>     ierr = DMDANaturalAllToGlobalCreate( da, &fromlocalall );
>> CHKERRQ(ierr);
>>
>>     if( rank == 0  )
>>     {
>>
>>         ierr = VecCreateSeq( PETSC_COMM_SELF, SIZE, &localall );
>>
>>         ierr = VecScatterBegin( tolocalall, x, localall, ADD_VALUES,
>> SCATTER_FORWARD_LOCAL ); CHKERRQ(ierr);
>>         ierr = VecScatterEnd(   tolocalall, x, localall, ADD_VALUES,
>> SCATTER_FORWARD_LOCAL ); CHKERRQ(ierr);
>>
>
>  This will not work since you have a collective call (VecScatterBegin)
> inside if(!rank)
>
>     Matt
>
>
>>         ierr = VecGetArray( x, &vlocal ); CHKERRQ(ierr);
>>
>>         PetscInt s;
>>         VecGetSize( localall, &s );
>>
>>         ierr = VecView( localall, PETSC_VIEWER_STDOUT_SELF ); CHKERRQ(
>> ierr );
>>
>>         //create data on zero rank process
>>         for ( int i = 0; i < s; i++) *vlocal++ = i;
>>
>>         ierr = VecRestoreArray( localall, &vlocal ); CHKERRQ(ierr);
>>
>>         ierr = VecScatterBegin(fromlocalall, localall, x, ADD_VALUES,
>> SCATTER_FORWARD_LOCAL ); CHKERRQ(ierr);
>>         ierr = VecScatterEnd(  fromlocalall, localall, x, ADD_VALUES,
>> SCATTER_FORWARD_LOCAL ); CHKERRQ(ierr);
>>     }
>>
>>     ....
>>
>> But the piece of code gets and distributes only vector-values belong to
>> zero-rank process. So, I haven't any idea how can I solve my problem only
>> with PETSc functions. I'd like to get all values from global vector to
>> zero-rank process and put back all data-set only from zero-rank to global
>> vector. Can you help me, please?
>>
>> Thanks for your response
>> Mari
>>
>
>
>
>  --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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