[petsc-users] KSP convergence problem

[Jed Brown]

On Wed, Mar 20, 2013 at 5:14 PM, John Mousel <john.mousel at gmail.com> wrote:

> 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?


What is the dimension of the null space?

"Bootstrap AMG" might be applicable to your problem, but it has yet to be
implemented in a library. It is still expensive to construct, but may be
better than the alternatives (except that using a sane spatial
discretization is still likely better).
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