[petsc-users] Composite shell preconditiner

[Jed Brown]

On Fri, Aug 17, 2012 at 7:11 AM, Alexander Grayver
<agrayver at gfz-potsdam.de>wrote:

>  On 17.08.2012 14:52, Jed Brown wrote:
>
>
> See this *ADDITIVE*? You are preconditioning with I+P_{SOR}. You probably
> want multiplicative.
>
> Jed,
>
> It didn't work.
>
> -pc_type composite -pc_composite_pcs shell,sor -pc_composite_type
> multiplicative
> ----->
> Linear solve did not converge due to DIVERGED_ITS iterations 250
>
> KSP Object:() 1 MPI processes
>   type: tfqmr
>   maximum iterations=250, initial guess is zero
>   tolerances:  relative=1e-07, absolute=1e-50, divergence=10000
>   left preconditioning
>   using PRECONDITIONED norm type for convergence test
>
> PC Object:() 1 MPI processes
>   type: composite
>   Composite PC type - MULTIPLICATIVE
>
>   PCs on composite preconditioner follow
> ....
>
> Is this something unexpected?
>

Sorry, I misinterpreted what you want. PC_COMPOSITE_MULTIPLICATIVE
re-evaluates the residual after applying the first stage of the
preconditioner. Do you _really_ want to just apply one preconditioner after
the next? You can use PC_COMPOSITE_SPECIAL, but read this man page and
understand why your preconditioner has that form.
http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/PC/PCCompositeType.html


Barry, it looks like despite the interface, "alpha" has never been used by
this implementation.


> I can try to write minimal example to make sure that this is not related
> to my app itself.
>
> --
> Regards,
> Alexander
>
>
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