[Getdp] Floating potentials in 3D: long execution times

[Christophe Geuzaine]

To the list: these performance issues should be fixed in SVN. If you use 
global quantities please le us know if you encounter any problems with 
the new version.



On 27/05/11 08:33, Christophe Geuzaine wrote:
>
> Hi John - Thanks for the feedback. The two issues you mention (slow
> generation of GroupsOfNodesOf and slow assembly) are indeed known, but
> seem to be especially bad in your case. How many nodes do you have on
> the floating potential surfaces?
>
>
> On 25/05/11 17:54, John_V wrote:
>> I am using GetDP to calculate the potential in a 3D geometry of mixed
>> conductors and insulators. The calculation is repeated frequently with
>> different charge distributions, so CPU time is important. Even though my
>> mesh has ~400k nodes, with some experimenting I have been able to get
>> good solutions in about 90 seconds, provided all the conducting surfaces
>> have Dirichlet boundary conditions.
>>
>> I need also to be able to deal with floating conductors, however. My
>> first attempt at this was to treat the floating conductor as if it were
>> a dielectric with very large dielectric constant. The problem with this
>> approach is that it degrades the condition of the matrix. I have to use
>> the ILUP preconditioner (~12 minutes to compute) instead of the faster
>> ILU0 one (0.3 seconds).
>>
>> The discussion of floating potentials in the manual gave me hope of a
>> better alternative. Bernhard Kubicek's "Capacitor2D" example on the Wiki
>> showed the way, The method is very elegant, based I think on basis
>> functions for groups of nodes as described in the paper by Dular,
>> Legros, and Nicolet in IEEE Trans. Magnet. 34 (1998). I have implemented
>> this for my 3D geometry.
>>
>> As I hoped, doing it this way let me return to the fast ILU0
>> precondition. However, time gained here is lost elsewhere. Execution
>> times are VERY large. It requires a total of 57 minutes CPU time for
>> this method on my mesh.
>>
>> The two largest contributors to the 57 minutes are preprocessing
>> (especially generating GroupsOfNodesOf, 27.5 minutes) and generating the
>> system of equations (29 minutes).
>>
>> What is GetDP doing during all that time? Are these operations
>> inherently this time-consuming?
>>
>> John
>>
>>
>>
>> _______________________________________________
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>> getdp at geuz.org
>> http://www.geuz.org/mailman/listinfo/getdp
>
>


-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine