[petsc-users] KSP convergence problem

[John Mousel]

Can you comment on a GASM type approach to find a solution for the null
space? I notice that the null vectors that successfully make the true
residual drop are only complicated in a very thin band around the
interface. This band is easy to identify using a level set. Other than
that, the null space vector has a low frequency variation. My thought was
to break the matrix into two sub-matrices, and somehow apply GAMG as a
preconditioner on the far matrix, and ILU on the interface-adjacent matrix.
Is this dumb or a complete misunderstanding of GASM?


On Wed, Mar 20, 2013 at 5:08 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

>
> On Wed, Mar 20, 2013 at 5:04 PM, John Mousel <john.mousel at gmail.com>wrote:
>
>> I've wanted to scrap this approach for a long time, but moving away from
>> these GFM-type treatments is not a choice that I've been allowed to follow
>> through on for various reasons which are out of my control.
>
>
> Unless there are some clever tricks to characterize the null space or to
> keep preconditioners compatible with the null space, the folks making the
> decisions might have to reconsider. It doesn't matter how sexy a method
> looks if it requires a solve and that solve cannot be done efficiently.
>
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