[petsc-users] converting parallel matrix MATMPIJ to MATAIJ

[Barry Smith]

  I would use MatGetRedundantMatrix()


On Sep 6, 2013, at 4:52 PM, Matthew Knepley <knepley at gmail.com> wrote:

> 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
> 
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> -- 
> 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