[petsc-users] gamg failure with petsc-dev

[Stephan Kramer]

Hi guys,

We have been having some problems with GAMG on petsc-dev (master) for cases that worked fine on petsc 3.4. We're solving a Stokes equation (just the velocity block) for a simple convection in a square 
box (isoviscous). The problem only occurs if we supply a near null space (via MatSetNearNullSpace) where we supply the usual (1,0) (0,1) and (-y,x) (near) null space vectors. If we supply those, the 
smoother complains that the diagonal of the A matrix at the first coarsened level contains a zero. If I dump out the prolongator from the finest to the first coarsened level it indeed contains a zero 
column at that same index. We're pretty confident that the fine level A matrix is correct (it solves fine with LU). I've briefly spoken to Matt about this and he suggested trying to run with 
-pc_gamg_agg_nsmooths 0 (as the default changed from 3.4 -> dev) but that didn't make any difference, the dumped out prolongator still has zero columns, and it crashes in the same way. Do you have any 
further suggestions what to try and how to further debug this?

Cheers
Stephan