[Dune] parallel DUNE grid for implementing a spherical shell (UGGrid)

[Eike Mueller]

Hi Oliver & Markus,

thanks a lot again, I will try this.

The reason I only want one cell per processor on the macro grid is that this grid is the coarsest level in my geometric 
multigrid algorithm, and I can then get a very nice hierachy of fine grids by additional refinement. I then don't have to pull 
any data together on fewer processors when prolongating/restricting (I only need a relatively small number of coarse levels, as 
my problem it increasingly well conditioned as it is coarsened).

Cheers,

Eike

Oliver Sander wrote:
> Am 06.11.2012 08:44, schrieb Markus Blatt:
>> Hey Eike,
>>
>> On Mon, Nov 05, 2012 at 07:17:48PM +0000, Eike Mueller wrote:
>>>> And loadbalancing would only be needed for the macrogrid, e.g., not 
>>>> dynamic.
>>>>
>>> That's right, in the code I would refine the grid until there is
>>> only cell per processor (this is the macrogrid). Then I would call
>>> loadBalance, followed by further grid refinement. So for example
>>> with 24 processors, I would subdivide each of the six cells in the
>>> original grid into four cells, then loadbalance that grid and refine
>>> further.
>> Actually, this approach could be the root of the problem. The
>> loadbalancing is a heuristic algorithm and normally one always gets
>> some load imbalance here. But if you just have as many cells as
>> processor, then naturally some will end up with no cells at all.
> This is true, but it is not the whole truth.
> 
> The default load balancing strategy of UG is Recursive Coordinate 
> Bisection.
> This means roughly that the grid bounding box is partitionend into 
> axis-aligned
> cells, and these cells are assigned to processors.  I reckon (I didn't 
> check)
> that since your grid is a hollow sphere, some cells simply remain empty.
> 
> UG offers several other strategies, but all this really is hardly tested.
> Have a look at the lbs method in ug/parallel/dddif/lb.c:533 for some
> alternatives.
> 
> good luck,
> Oliver
> 
>>
>> How about doing some more refinement before load balancing?
>>
>> Cheers,
>>
>> Markus
>>
> 
> 
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-- 
Dr Eike Mueller
Research Officer

Department of Mathematical Sciences
University of Bath
Bath BA2 7AY, United Kingdom

+44 1225 38 5633
e.mueller at bath.ac.uk
http://people.bath.ac.uk/em459/