[petsc-users] ML and -pc_factor_shift_nonzero

[Jed Brown]

On Fri, 16 Apr 2010 13:51:13 +0200, tribur at vision.ee.ethz.ch wrote:
> ML works now using, e.g., -mg_coarse_redundant_pc_factor_shift_type  
> POSITIVE_DEFINITE. However, it converges very slowly using the default  
> REDUNDANT for the coarse solve.

"Converges slowly" or "the coarse-level solve is expensive"?  I suggest
starting with

  -mg_coarse_pc_type lu -mg_coarse_pc_factor_mat_solver_package mumps

or varying parameters in ML to see if you can make the coarse level
problem smaller without hurting convergence rate.  You can do
semi-redundant solves if you scale processor counts beyond what MUMPS
works well with.

Depending on what problem you are solving, ML could be producing a
(nearly) singular coarse level operator in which case you can expect
very confusing and inconsistent behavior.

Jed