[petsc-dev] [petsc-users] gamg failure with petsc-dev

Mark Adams mfadams at lbl.gov
Sun Aug 17 10:10:23 CDT 2014

> The most common cause of this is inaccurate eigenvalue bounds.  You can
> try using more iterations for estimation.
I have also found that CG converges much faster than GMRES (default) so if
you are SPD, I would always use:  '-gamg_est_ksp_type cg' .  Also,
'-pc_gamg_verbose 2' will print out the eigenvalues that GAMG computes for
smoothing.  With -ksp_view you will see the eigenvalues that Cheby is using
(GAMG sets these so they should be the same if you use GAMG with
pc_gamg_nsmooths>0). The largest eigenvalue should be around 2-3.  If it is
too low (<2) then the instability that Jed is referring to would be the

You can also use more iterations, and see if it has converged, with
-gamg_est_ksp_max_it N.  I think the default is 5.
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