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

[Matthew Knepley]

On Tue, Aug 12, 2014 at 11:30 AM, Mari Pecha <pecha.mari at gmail.com> wrote:

>  So, I've tried options with VecScatterCreateToZero but I got error
> message
>
> [0]PETSC ERROR: Nonconforming object sizes
> [0]PETSC ERROR: Vector wrong size 10 for scatter 2 (scatter reverse and
> vector to != ctx from size)
>
> I'm not sure what is wrong. Could you help me? My code I've add to
> attachements.
>

Your vector arguments for the scatter are reversed. You are scattering from
the local vector with pixel data
to the global vector.


> Thank you for your time
> Mari
>
> P.S: I use PETSc 3.5 and SCATTER_BACKWARD is changed to SCATTER_REVERSE.
>

Yes, that is right.

    Matt


> On 12.8.2014 17:53, Matthew Knepley wrote:
>
>  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
>
>
>


-- 
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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