[petsc-users] converting parallel matrix MATMPIJ to MATAIJ

[Matthew Knepley]

On Fri, Sep 6, 2013 at 4:25 PM, Lukasz Kaczmarczyk <
Lukasz.Kaczmarczyk at glasgow.ac.uk> wrote:

> Hello,
>
> I solve system of eq. generated by finite element method.
>
> I  apply some projection matrix to stiffness matrix K,
> P=I-CT[(CTC)^-1]C
> where C  is some not square matrix.
>
> Resulting stiffness matrix K' has form
> K' = PT K P,
> with that at hand I solve problem K' *x = f'
>
> I manage to build shell matrix where I use sub ksp solver to get solution
> for (CTC)*b = C*x, where [ b = (CTC^-1*C*x)] . Using penalised matrix for
> preconditioner, where  K_prec = alpha*CCT + K, where alpha is penalty I can
> get solution in efficient way.
>
> Now I like to avoid penalty parameter, in order to do that I will need to
> apply penalty matrix for each individual finite element  matrix before it
> is assembled into K. No problem with that, using scattering it can be done.
>
> Problem is with  solution (CTC)*b = C*x, C and CTC matrices are parallel,
> since I have parallelised assembly functions, problem (CTC)*b = C*x need to
> be solved on each processor independently without communication. It is not
> problem, but to do that I need to transform C and CTC matrix form MATMPIAIJ
> to MATAIJ.
>

I think this might be what you want:


http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/Mat/MatGetSubMatrices.html

   Thanks,

      Matt


> I know that MatConvert will not do it. I wonder it is any other way that
> form very beginning to assemble matrix C as a serial matrix.
>
> Regards,
> Lukasz
>
>
>
>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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