dolfin team mailing list archive

[Niclas Jansson <njansson@xxxxxxxxxx>]

Anders Logg <logg@xxxxxxxxx> writes:

> On Thu, Feb 03, 2011 at 09:03:58PM +0100, Niclas Jansson wrote:
>> Anders Logg <logg@xxxxxxxxx> writes:
>>
>> > On Tue, Feb 01, 2011 at 11:57:28AM -0000, Anders Logg wrote:
>> >> Question #143645 on DOLFIN changed:
>> >> https://answers.launchpad.net/dolfin/+question/143645
>> >>
>> >> Anders Logg proposed the following answer:
>> >> On Tue, Feb 01, 2011 at 11:50:37AM -0000, Garth Wells wrote:
>> >> > Question #143645 on DOLFIN changed:
>> >> > https://answers.launchpad.net/dolfin/+question/143645
>> >> >
>> >> >     Status: Open => Answered
>> >> >
>> >> > Garth Wells proposed the following answer:
>> >> > I don't think so.
>> >>
>> >> I think it should be possible, at least "in principle". I'll comment
>> >> more later. I'm in a meeting now.
>> >
>> > Sorry for my late reply.
>> >
>> > What you can do is to get the "overlap":
>> >
>> > std::map<uint, std::vector<uint> >* overlap = mesh.data().vector_mapping("overlap");
>> >
>> > This gives you a mapping for each shared vertex to its neighboring
>> > processes (the processes it's shared with).
>> >
>> > You can check if a vertex is shared by
>> >
>> >   overlap->find(i) != overlap->end()
>> >
>> > There is no functionality for checking edges, cells etc. Such
>> > operations must be expressed in terms of their vertices.
>> >
>> > It would be easy to add some shortcuts like
>> >
>> >   bool Vertex::is_ shared() const;
>> >
>> > and even
>> >
>> >   bool MeshEntity::is_shared() const;
>> >
>> > etc if that helps. They could just look at the overlap.
>> >
>> > If that is costly, we could store another MeshFunction that holds the
>> > appropriate data (or add as part of ParallelData).
>> >
>> >
>>
>> Perfect! That should be enough to get me started porting some code.
>
> Great!
>
>> How about owners of mesh entities? If I remember correctly mesh entities
>> were not owned by a specific rank. If one could determine if a vertex
>> is owned, ghosted or shared, it would make life much simpler when
>> implementing various parallel mesh algorithms.
>
> Isn't the rule that when a vertex is shared, it's owned by the lowest
> rank process?
>

Ok, if so that should be enough. 

> I don't know how we handle ghosted entities (or if we even have
> ghosted entities). We have ghosted dofs stored in Vectors (handled by
> PETSc/Epetra) but I'm not sure how it's handled. Garth knows a great
> deal more about that.
>

True, but I don't like to start messing around with dofs for pure mesh
algorithms.


Niclas