[petsc-users] DIVERGED_INDEFINITE_PC in algebraic multigrid

[Jed Brown]

Michele Rosso <mrosso at uci.edu> writes:

> Hi,
>
> I am solving an inhomogeneous Laplacian in 3D (basically a slightly 
> modified version of example ex34).
> The laplacian is discretized by using a  cell-center finite difference 
> 7-point stencil with periodic BCs.
> I am solving a time-dependent problem so the solution of the laplacian 
> is repeated at each time step with a different matrix (always SPD 
> though) and rhs. Also, the laplacian features large magnitude variations 
> in the coefficients. I solve by means of CG + GAMG as preconditioner.
> Everything works fine for a while until I receive  a 
> DIVERGED_INDEFINITE_PC message. 

What is changing as you time step?  Is there a nonlinearity that
activates suddenly?  Especially a bifurcation or perhaps a source term
that is incompatible with the boundary conditions?  You could try
-mg_levels_ksp_type richardson -mg_levels_pc_type sor.  Can you
reproduce with a small problem?

The configuration looks okay to me.

> Before checking my model is incorrect I would like to rule out the
> possibility of improper use of the linear solver.  I attached the full
> output of a serial run with -log-summary -ksp_view
> -ksp_converged_reason ksp_monitor_true_residual. I would appreciate if
> you could help me in locating the issue.
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