[petsc-dev] Ghost values in sieve

[Chris Eldred]

Yes I think that makes sense. How do you access the associated PetscSF
for the DMComplex? Is it possible to define multiple PetscSF's for a
given DMComplex, each associated with different PetscSections?

My current understanding of using DMComplex is (for the serial case):

1) Create a DMComplex object, set its cones/supports, etc.
2) Create PetscSections that describe the layout of data across the DMComplex
3) Set a PetscSection as the default section
4) Use DMCreateLocalVector to create the Vec objects needed for the computation

Is this correct?

In the parallel case, what else needs to be done?

-Chris

On Fri, Aug 17, 2012 at 1:24 PM, Matthew Knepley <knepley at gmail.com> wrote:
> On Fri, Aug 17, 2012 at 1:21 PM, Chris Eldred <chris.eldred at gmail.com>
> wrote:
>>
>> I was wondering how ghost values were handled when creating a
>> distributed (global) vector from a specific PetscSection over a
>> DMComplex object- ie what defines the stencils used to figure out
>> which values need to be ghosted? What about local vs. global indexes?
>
>
> This answer will have to be long because the question is mixing levels in
> the interface.
>
> A PetscSection is only an offset structure, so it has no designated fields
> for overlap information. What I do for DM is to create 2 sections: one
> describing the layout of the local vector, and one describing the global
> vector and ghosts. The global section can be created automatically given
> the local section and a PetscSF (successor to VecScatter).
>
> The PetscSF describes the parallel overlap, and also has scatters and
> reductions.
> This is created by the DMComplex instead of a scatter since it can handle
> arbitrary types, and has a better query interface. DMComplex creates a
> (non-overlapping) partition of some stratum of points, usually cells for
> FEM.
> Then the closure(p) U star(p) for each point is added to that partition, and
> the dofs on any shared point are ghosted.
>
> This is all now done using the vanilla DM interface, so DMGlobalToLocal()
> and LocalToGlobal() and cousins work as normal.
>
> Does that make sense?
>
>     Matt
>
>>
>>
>> -Chris
>>
>> --
>> Chris Eldred
>> DOE Computational Science Graduate Fellow
>> Graduate Student, Atmospheric Science, Colorado State University
>> B.S. Applied Computational Physics, Carnegie Mellon University, 2009
>> chris.eldred at gmail.com
>
>
>
>
> --
> 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



-- 
Chris Eldred
DOE Computational Science Graduate Fellow
Graduate Student, Atmospheric Science, Colorado State University
B.S. Applied Computational Physics, Carnegie Mellon University, 2009
chris.eldred at gmail.com