[petsc-users] Fwd: Any changes in ML usage between 3.1-p8 -> 3.3-p6?

[Jed Brown]

"Christon, Mark A" <christon at lanl.gov> writes:

> HI Mark,
>
> Thanks for the information.  We thought something had changed and
> could see it the effect, but couldn't quite pin it down.

So did you try running your old tests with

 -mg_levels_sub_pc_factor_zeropivot 1e-12 -mg_levels_sub_pc_factor_shift_amount 1e-12

I would expect this to only make a small difference.

> To be clear our pressure-Poisson equation is a warm and fluffy
> Laplacian, but typically quite stiff from a spectral point of view
> when dealing with boundary-layer meshes and complex geometry — our
> norm.  This is the first-order computational cost in our flow solver,
> so hit's like the recent change are very problematic, and particularly
> so when they break a number of regression tests that run nightly
> across multiple platflorms.
>
> So, unfortunately, while I'd like to use something as Jacobi, it's
> completely ineffective in for the operator (and RHS) in question.

As Mark says, use

  -mg_levels_ksp_type chebyshev -mg_levels_pc_type jacobi

(possibly with more explicit spectrum options).  See Mark's paper

  http://www.columbia.edu/~ma2325/adams_poly.pdf