[petsc-users] DIVERGED_INDEFINITE_PC in algebraic multigrid

[Michele Rosso]

Jed,

thanks for your reply.
By using the options you suggested, namely /-mg_levels_ksp_type 
richardson -mg_levels_pc_type sor/, I was able to
solve without bumping into the DIVERGED_INDEFINITE_PC message. 
Nevertheless, the number of iterations increases drastically as the 
simulation progresses.
The Poisson's equation I am solving arises from a variable-density 
projection method for incompressible multi-phase flows.
At each time step the system matrix coefficients change as a consequence 
of the change in location of the heavier phase; the rhs changes
in time because of the change in the velocity field. Usually the 
black-box multigrid or the deflated conjugate gradient method are used 
to solve efficiently this type of problem: it is my understanding - 
please correct me if I am wrong - that AMG is a generalization of the 
former.
The only source term acting is gravity; the hydrostatic pressure is 
removed from the governing equation in order to accommodate periodic 
boundary conditions: this is more a hack than a clean solution. Could it 
be the reason behind the poor performances/ DIVERGED_INDEFINITE_PC 
problem I am experiencing?
Thanks,

Michele



On 05/17/2014 12:26 AM, Jed Brown wrote:
> 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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