[dune-pdelab] P2 in parallel

[Volker Schauer]

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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Volker Schauer (Dipl.Phys)

Institute of Applied Mechanics (Civil Engineering), Chair 1
University of Stuttgart
Pfaffenwaldring 7
D-70569 Stuttgart
GERMANY

Phone ++49 (0)711-68560044
email: schauer at mechbau.uni-stuttgart.de