[petsc-users] GAMG for the unsymmetrical matrix

[Mark Adams]

On Thu, Apr 6, 2017 at 7:39 AM, Kong, Fande <fande.kong at inl.gov> wrote:
> Thanks, Mark and Barry,
>
> It works pretty wells in terms of the number of linear iterations (using
> "-pc_gamg_sym_graph true"), but it is horrible in the compute time. I am
> using the two-level method via "-pc_mg_levels 2". The reason why the compute
> time is larger than other preconditioning options is that a matrix free
> method is used in the fine level and in my particular problem the function
> evaluation is expensive.
>
> I am using "-snes_mf_operator 1" to turn on the Jacobian-free Newton, but I
> do not think I want to make the preconditioning part matrix-free.  Do you
> guys know how to turn off the matrix-free method for GAMG?

You do have an option to use the operator or the preconditioner
operator (matrix) for the fine grid smoother, but I thought it uses
the PC matrix by default. I don't recall the parameters nor do I see
this in the view output.  Others should be able to help.