Re: [petsc-users] Solving Poisson equation with multigrid

[Michele Rosso]

Hi,

I confirm that
        -pc_type gamg  -pc_mg_cycle_type v  -pc_gamg_agg_nsmooths 1

produces the correct shift in PETSc 3.4.
So my problem is solved.   I will upgrade to 3.4 in my productive 
machine installation.
It is a Cray machine (Blue Waters). Assuming I want to use the Cray 
compiler, which options should I use for ./configure
for the installation.

Thank you


On 05/29/2013 09:05 PM, Jed Brown wrote:
Michele Rosso <mro...@uci.edu> writes:

Hi,

I proceeded as Matt suggested. I am running without nullspace
(Dirichlet's BCs) with the following options:

-pc_type gamg  -pc_mg_cycle_type v  -pc_gamg_agg_nsmooths 1
-mg_coarse_sub_pc_factor_shift_type NONZERO -ksp_view -options_left

The results are fine and the run finishes. The output of -ksp_view does
not say anything about the coarse solver being shifted.
With petsc-3.4, it has these lines:

           tolerance for zero pivot 2.22045e-14
           using diagonal shift on blocks to prevent zero pivot

You can try a proposed fix in the branch 'jed/fix-gamg-coarse'.



Hong, I think this diagnostic output is unintuitive for a user that
doesn't know this relation.  Shall we add the enum names to the output
lines below?

     if (factor->info.shifttype==(PetscReal)MAT_SHIFT_POSITIVE_DEFINITE) {
       ierr = PetscViewerASCIIPrintf(viewer,"  using Manteuffel 
shift\n");CHKERRQ(ierr);
     }
     if (factor->info.shifttype==(PetscReal)MAT_SHIFT_NONZERO) {
       ierr = PetscViewerASCIIPrintf(viewer,"  using diagonal shift to prevent zero 
pivot\n");CHKERRQ(ierr);
     }
     if (factor->info.shifttype==(PetscReal)MAT_SHIFT_INBLOCKS) {
       ierr = PetscViewerASCIIPrintf(viewer,"  using diagonal shift on blocks to 
prevent zero pivot\n");CHKERRQ(ierr);
     }