[petsc-users] gamg failure with petsc-dev

[Jed Brown]

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

> 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?

Do you set the block size?  Can you reproduce by modifying
src/ksp/ksp/examples/tutorials/ex49.c (plane strain elasticity)?
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