[petsc-users] Block LU preconditioning

[Jed Brown]

On Fri, Jan 13, 2012 at 08:22, TAMAIN Patrick 207314
<Patrick.TAMAIN at cea.fr>wrote:

> I somehow can’t figure out how to do this in practise with PETSC. I tried
> to configure the KSP like this****
>
> call KSPSetOperators(myKSP,A,B,SAME_PRECONDITIONER,ierr) ****
>
> and then run PETSC with the following options:****
>
> -pc_type asm -sub_pc_type lu -ksp_type bcgsl****
>
> but the computation time becomes awful, far more than if I try to do
> separately the LU decomposition of the blocks of B with MUMPS.
>

Make sure the subdomains are chosen to align with the direction you want to
couple strongly (and perhaps span the domain if that is the method you
want). There are APIs to set parallel subdomains that would couple on a
subcommunicator.

I might consider using the original operator and just setting up the domain
with this structure. The fill will be reasonably low if you have long,
skinny domains.
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