[petsc-users] Really Really Really Weird SNES behaviour

[Matthew Knepley]

On Sun, Jul 14, 2013 at 12:17 AM, subramanya sadasiva
<potaman at outlook.com>wrote:

> Hi,
> I am observing some really really really weird SNES behavior in my SNESVI
> code called through Libmesh. This behaviour appeared after I changed some
> initial conditions and only happens in an optimized build. I am running
> this code on a Macbook pro running os x 10.8..
>
> When the debug code is run, the residuals computed for the initial
> conditions provided give norms which are of the expected magnitude.. so the
> SNES_Monitor output is,
>
> solving the cahn hilliard time step
>   0 SNES Function norm 8.223262421671e-01
>   1 SNES Function norm 3.793806858333e-03
> Nonlinear solve did not converge due to DIVERGED_MAX_IT iterations 1
>
> The output from SNES_Monitor with the optimized code on the other hand is,
>
> solving the cahn hilliard time step
>   0 SNES Function norm 5.153882032022e+19
>   1 SNES Function norm 1.446612980133e+19
> Nonlinear solve did not converge due to DIVERGED_MAX_IT iterations 1
>
>
> Absolutely nothing else has changed in the code except that one code is
> built with a debugging and one with an optimized version of the code.
>
> Any ideas?
>

This sound like an uninitialized variable in your residual evaluation
routine. Very often, debugging code
will initialize variables to 0, whereas optimized leaves them alone.

   Matt


> Subramanya
>



-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener
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