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

[Oliver Sander]

Am 12.11.2012 16:01, schrieb Eike Mueller:
> Hi Oliver,
>
> I tried several other strategies, but without any luck. Whatever I do, 
> the algorithm seems to refuse to split up the macro cells, i.e. the 6 
> elements I insert with the grid factory.
>
> I also tried to simplify the problem as much as possible. I now create 
> one unit cube with the grid factory, do not insert any boundary 
> segments and refine the grid by calling globalRefine(refcount), so 
> that I end up with a cartesian unit cube split into 8^refcount 
> elements. I then balance the grid with loadBalance() (i.e. no 
> arguments). I would have thought that that should work. Still, if I 
> refine 1,2 or 3 times (i.e. I should end up with 8,64,512 elements), 
> for an 8 core run only one process stores the entire grid.
This should really work, can you post your test program?

But be careful: If you load-balance small grids then all processors
get all elements, but most of them only as ghosts.  Did you
check that?

cheers,
Oliver

>
> Could this be a problem with the grid factory?
>
> Thanks,
>
> 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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>
>