SNES Convergence
Barry Smith
bsmith at mcs.anl.gov
Mon Jan 7 15:35:35 CST 2008
Sean,
Can you run the ls version with the additional options -info -
snes_monitor and send
the results? You could also make a run with -snes_mf_operator and see
if that "converges"
in the same way or not (this is another check that the analytic
Jacobian is right/wrong).
Thanks
Barry
On Jan 7, 2008, at 3:30 PM, Sean Dettrick wrote:
> Hi,
>
> I am solving a version of the Grad-Shafranov equation, F(x)=0, which
> but for some extra spatial dependences is similar in form to the 2D
> Bratu equation in snes/examples/tutorials/ex5.c. I started with the
> ex5.c code, introducing just enough changes to model the new
> system. The analytic Jacobian function appears to be correct, with
> a Norm of matrix ratio < 1.e9 (found using -snes_type test).
>
> The problem I am having is that in most cases the SNES solver does
> not converge to a solution x of F(x)=0. Rather, what happens is
> that the fnorm (obtained in a monitor function) converges to some
> large non-zero value, and F(x) seems to get "stuck", i.e. it
> converges to a large non-zero result. Even though it is clearly not
> a solution, the -snes_converged_reason is reported as "Nonlinear
> solve converged due to CONVERGED_TR_DELTA". The intermediate KSP
> steps have -ksp_converged_reason reported as "Linear solve converged
> due to CONVERGED_STEP_LENGTH". I have been typically running with
> parameters -da_grid_x 100 -da_grid_y 101 -snes_converged_reason -
> ksp_converged_reason -snes_type tr -ksp_type gmres -snes_max_it 100.
>
> Does this sound like a familiar scenario with a familiar solution?
> Or can anyone point me to some documentation that describes the SNES
> tr and ls parameters in more detail than the manual.pdf?
> Or can anyone recommend the best SNES and KSP parameters for the
> Bratu example?
>
> Any help or advice would be greatly appreciated.
>
> Thanks,
> Sean Dettrick
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