[petsc-users] unreliable AMG in PETSc

[Arne Morten Kvarving]

On 23/10/14 15:53, Jed Brown wrote:

hi jed,
> Arne Morten Kvarving <arne.morten.kvarving at sintef.no> writes:
> Do you mean if the matrix is _not_ marked SPD?  Interestingly, CG is
no i mean to warn if the matrix is marked SPD, main KSP is CG
and Richardson is used on the multigrid ksps. i guess it falls into user 
app space
and is hard to do on a general level since, as you say, cg might survive 
slight asymmetries (or
an user could be filtering out the naughty modes, like the hydrostatic).

> often tolerant of weak asymmetry, such as you get with nonsymmetric
> enforcement of Dirichlet boundary conditions.  That tolerance can be
> considered robust when the first application of the preconditioner
> solves those equations contributing to asymmetry, in which case further
> iterations take place in a "benign" subspace in which the operator is
> SPD (so long as the preconditioner does not undo this).
>
> We don't have a good system for offering advice or things to check and
> PETSc's convention is always to make solvers silent unless information
> is requested.  We could have -ksp_view note when a matrix has not been
> marked SPD.  Or we could (with much more effort) create a new feature
> "-ksp_analyze" that runs some fairly expensive tests and offers advice.
thanks. i kinda supected this was the policy.

cheers

arnem