[dune-pdelab] P2 in parallel

[Bernd Flemisch]

Hey Volver, PDELab,

sorry, I don't have time to look into your code at the moment. But I had 
a look at 
dune-pdelab-howto/src/convection-diffusion/nonoverlappingsinglephaseflow.cc 
and adapted it to P2. I think it runs into the same problems as you do 
with your code. A sequential run works fine, while a parallel run on two 
processes produces completely wrong results. And the problem seems to 
come from the edge degrees of freedom.

Maybe a PDELab-developer has a chance to look at it, a patch against 
r491 is attached. I can also have a closer look in the first week of 
January.

Kind regards
Bernd


On 12/22/2011 02:40 PM, Volker Schauer wrote:
> ok, so although I don't want to bother anybody and even if Chistmas is
> coming soon, anyway, I will do it now.
> So are in the code I sent to the mailing list last Friday some basic
> assumptions wrong, does it give vanishing residuals also for you or do
> you just agree that there might be a problem with the P2 elements in
> parallel ?
> Since others also seem to use the pdelab in parallel, and don't face
> this problem it might also be just a wrong application of the methods,
> but where ?
> So please give at least some comments, because somehow due to this mad
> deterministic behaviour of these modern computers, I feel I will
> come back next year and the problem won't have gone.
>
> Many thanks
>
> Volker
>
>
>
> -------- Weitergeleitete Nachricht --------
> Von: Volker Schauer<schauer at mechbau.uni-stuttgart.de>
> An: dune-pdelab at dune-project.org
> Betreff: Re: [dune-pdelab] P2 in parallel - small correction
> Datum: Fri, 16 Dec 2011 15:22:46 +0100
>
> So here is a test area for the Poisson problem. The solver definition is
> actually the ingredients of the class
>
> Dune::PDELab::ISTLBackend_NOVLP_CG_SSORk<PGOS>,
>
> I extracted these, since I am using the Poisson problem repeatedly and
> the preconditioner is changing the matrix, so that it cannot be reused
> in the upper case (a fact that cost me a lot of time to find out!)
>
> If one now changes the P_order to 1 everything works fine on one as well
> as on 2 processes, however for P_order = 2 on 2 processes the residuum
> does not vanish any more.
>
>
> @ Bernd -  you were probably right, but when I read your
> mail I had almost completed the set up, thanks Bernd.
>
>
> Volker
>
>
> -------- Weitergeleitete Nachricht --------
> Von: Bernd Flemisch<bernd at iws.uni-stuttgart.de>
> Kopie: dune-pdelab<dune-pdelab at dune-project.org>
> Betreff: Re: [dune-pdelab] P2 in parallel - small correction
> Datum: Fri, 16 Dec 2011 13:05:53 +0100
>
> Hey Volker,
>
> would it not be easier to adapt the nonoverlappingsinglephaseflow
> example from the pdelab howto to get a test case? There
> (src/convection-diffusion/nonoverlappingsinglephaseflow.cc), I think you
> only have to set k equal to 2 in the Pk3DLocalFiniteElementMap in lines
> 314 or 376.
>
> Kind regards
> Bernd
>
> On 12/16/2011 12:33 PM, Volker Schauer wrote:
>> Hi Markus
>>
>> The norm of the residual (calculated with novlpSP.dot) which on a single
>> process goes to values less then 10^-10 gives me now a value at the size
>> of>= 36000 and changing the tolerance of the solver basically does not
>> effect this.
>>
>> I did not look at the residual in a greater detail, so maybe I should
>> check if it is large e.g. only on boundary nodes ? But how would I check
>> that, since I do not really know/understand the assignment of indices in
>> gridvectors like the residual to the entities of the grid with that
>> strange offset in Dune and so I do not know which of my indices belongs
>> to a boundary node. (If somebody could give here a "short" explanation,
>> I think this would be valueable by its own, also for other users).
>> So I could iterate over the entities of e.g. codim 3 and find that one's
>> partition type is border, but now how do I find which index in the
>> grid-vector is assigned to that entity ?
>>
>>
>> So I will try to extract the Poisson problem and send it to the mailing
>> list.
>>
>> Thanks for your help (all pdelab members) - I really appreciate that
>>
>> Volker
>>
>>
>>
>> -------- Weitergeleitete Nachricht --------
>> Von: Markus Blatt<markus at dr-blatt.de>
>> Reply-to: Markus Blatt<markus at dr-blatt.de>
>> An: Volker Schauer<schauer at mechbau.uni-stuttgart.de>
>> Kopie: dune-pdelab<dune-pdelab at dune-project.org>
>> Betreff: Re: [dune-pdelab] P2 in parallel - small correction
>> Datum: Fri, 16 Dec 2011 11:46:27 +0100
>>
>> Hi,
>>
>> On Fri, Dec 16, 2011 at 11:34:36AM +0100, Volker Schauer wrote:
>>> It would be helpful if someone says that he has already used the
>>> communication from PDELab to set up the Poisson or any other PDE
>>> successfully with P2 elements (I suppose this was tested in PDELab when
>>> the parallel use was implemented?). Would it help you or anybody else if
>>> I extract the Poisson problem (which is embedded in a larger code doing
>>> some more stuff) and send it to the mailing list ?
>>>
>> Yes, that would help in two ways: giving us an easy test case and
>> checking whether there is a mistake in the usage of PDELab.
>>
>> You were saying that your residual is not not zero. Would do you mean
>> by zero? Our solvers only achieve a relative residual reduction, so
>> this might happen if your intial residual is large.
>>
>> Have you looked at the residual? If so, where does is deviate from the
>> expected values?
>>
>> Cheers,
>>
>> Markus
>>
>>
>
>
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-- 
_____________________________________________________________________

Bernd Flemisch                               phone: +49 711 685 69162
IWS, Universität Stuttgart                   fax:   +49 711 685 60430
Pfaffenwaldring 61                  email: bernd at iws.uni-stuttgart.de
D-70569 Stuttgart                  url: www.hydrosys.uni-stuttgart.de
_____________________________________________________________________

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