[Dune-devel] Semantics of {ghost,overlap}Size

[Martin Nolte]

Hi,

actually, I wonder whether anybody uses these methods at all (except for 
checking the return value for 0).

Otherwise, I totally agree with Markus that semantics of the methods are 
completely unclear:
- They are not mentioned in the papers on dune-grid.
- The documentation of the interface does not give a definition
   (what is the size of a "region"? Why is this integral?).
- The tests do not verify the result of these methods at all
   (though it seems the information can be obtained by grid traversal).

A mere hint in the documentation of YaspGrid is absolutely insufficient.

Implementation node: SPGrid only implements these methods for codim 0, 
returning the number of elements in the corresponding partition. Up to now 
nobody complained about this.

Given the current status, we might just replace these methods by capabilities 
like hasGhosts or hasOverlap.

Anyway, in my opionion (as stated on the wish list for the developer meeting) 
the parallel part of the dune-grid interface should be revised in the near 
future. I think there are several points worth discussion. I was already 
thinking about a "small" meeting of those directly involved in the parallel 
interface (both on the user's and on the developer's side). What do the others 
feel?

Best,

Martin

On 11/07/2013 11:39 AM, Andreas Dedner wrote:
> Hi.
> I've wondered about that as well and am thinking that the method really only
> makes sense for codim 0 (where it the semantics seems clear enough).
>
> In DUNE everything boils down to codim 0 anyway (i.e. what leaf entities
> are for example). So if I have an overlap of 2 codim 0 entities then
> I would expect that the corresponding entities of higher codimension are
> there
> as well....
>
> Does anybody use that method for c>0?
>
> Best
> Andreas
>
> PS: should these method be deprecated on the grid class?
>
> On 07/11/13 10:33, Markus Blatt wrote:
>> Hi,
>>
>> I recently looked at the documentation off these methodsbut this did
>> not help me understand what they are doing. It reads "Return size of
>> ghost region for a given codim on the leaf grid" but it is not clear
>> to me what this size is.
>>
>> In the documentation of yaspgrid it says that size is the distance in
>> graph, which is not really clearer. I assume that means the smallest
>> path between any two entities on different borders of that
>> region. Which still leaves the question open what the graph
>> of an enitity set of a given codim is. The vertices are of course the
>> codim entities. Seems like the edges would be the codim 1 entities for
>> elements and for all other entities with codim c the ones with codim c+1.
>>
>> Is there a precise definition somewhere?
>>
>> Markus
>
>
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-- 
Dr. Martin Nolte <nolte at mathematik.uni-freiburg.de>

Universität Freiburg                                   phone: +49-761-203-5630
Abteilung für angewandte Mathematik                    fax:   +49-761-203-5632
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