[petsc-dev] [petsc-users] gamg failure with petsc-dev

Jed Brown jed at jedbrown.org
Thu Aug 14 22:25:56 CDT 2014

Stephan Kramer <s.kramer at imperial.ac.uk> writes:

> An update on this. We have been using mark/gamg-zerod branch and it 
> fixes for us the issue with zero diagonals in the coarsened operators 
> (making the sor smoother, but bizarrely not the jacobi smoother fail). 
> We in fact have some cases where cheby+jacobi does not converge 
> (indefinite pc), 

The most common cause of this is inaccurate eigenvalue bounds.  You can
try using more iterations for estimation.

> but cheby+sor (with the mark/gamg-zerod branch) works well, so we'd be
> very much interested in getting this (or something similar) merged in
> master. Maybe the lv[i-Istart]==0.0 thing isn't entirely robust? We'd
> be happy to contribute.
> As an aside, we also changed the ordering of DOFs as suggested, so that 
> we provide the right block structure to gamg. However, as soon as we 
> actually set the block size (MatSetBlockSizes) the convergence 
> deteriorates substantially (going from ~50 to ~650 iterations). Without 
> setting the block size but with the new ordering, the n/o iterations is 
> roughly the same as before (when our dofs were not interlaced). Any idea 
> what might be going wrong?

Are you setting the block size and the rigid body modes
(MatSetNearNullSpace, perhaps with MatNullSpaceCreateRigidBody)?  Are
those modes set using interlaced ordering?
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