[Dune] Preconditioner

[Mike Rosing]

Howdy Christian,

Recompiling everytime I change size is annoying, but it's also not
that big a deal. Worst case if I want different sizes I can just use
different named variables - so I can see how to get there from here.

I just have to change my thought process to the reality of the code.

I did try flattening the matrix to BCRSMatrix<double> but I must have
done something wrong because DUNE aborted with a NaN error. So I think
being safe is the way to go.

Thanks!
Mike

On Tue, 9 Sep 2025, Christian Engwer wrote:

> Date: Tue, 09 Sep 2025 07:55:16 +0200
> From: Christian Engwer <christian.engwer at uni-muenster.de>
> To: dune at lists.dune-project.org, Mike Rosing <eresrch at sdf.org>
> Subject: Re: [Dune] Preconditioner
> 
> Dear Mike,
>
> I'm afraid also we DUNE people need to dig deeper into this.
>
> The issue here is that you're using Matrix for the blocks, which have 
>dynamic size. The usual pattern is to use FieldMatrix, which has static 
>size and a default constructor. The algorithms use this to decide where 
>to invest directly.
>
> The genetic algorithms usually expect BCRSMatrix to contain dense data, 
>but Matrix dies not where that everything below is dense.
>
> The safe way would be to use BCRSMatrix<FieldMatrix<double,n,n>> and 
>BlockVector<FieldVector<double, n>>
>
> Do you really need the dynamic size?
>
> I didn't test myself, but perhaps BCRSMatrix<DynamicMatrix<double,n,n>> 
>and BlockVector<DynamicVector<double, n>> work. I would expect relaxation 
>methods to work, but not AMG. Für ILU it might be a problem, that the 
>constructor of DynamicMatrix needs the size information.
>
> Best
> Christian
>
> Am 6. September 2025 21:04:48 MESZ schrieb Mike Rosing <eresrch at sdf.org>:
>> I figured out how to get the MatrixAdaptor to be happy with my structures:
>>
>> using Occupy = Matrix<double>;
>> using ElementVec = BlockVector<double>;
>>
>>  MatrixAdapter<BCRSMatrix<Occupy>, BlockVector<ElementVec>,
>>                    BlockVector<ElementVec>> linearOperator(stiffns);
>>
>> but none of the preconditioners are happy with this.
>> The ILU preconditioner gives the attached error output.
>>
>> Does it make any sense to attempt to create a preconditioner that can deal with this structure, or should I just copy all the data into a flat matrix and vector with no sub-structure? Because I haven't gotten to call the solver yet, so this might be a lot deeper than I want to know about.
>>
>> Thanks,
>>
>>
>> Mike Rosing
>> Engineering Research
>> Madison WI USA
>> Elliptic Curve Crypto:  http://mng.bz/D9NA
>

Mike Rosing
Engineering Research
Madison WI USA
Elliptic Curve Crypto:  http://mng.bz/D9NA