[petsc-users] Jacobian check is sensitive to the choice of differencing parameter

Matthew Knepley knepley at gmail.com
Sun Jul 19 10:08:35 CDT 2015

On Sun, Jul 19, 2015 at 9:45 AM, Khai Pham <khaipham at utexas.edu> wrote:

> Dear Petsc-Users,
> The SNES only converges for a small problem. I did the jacobian check. The
> result is sensitive to the choice of differencing parameter.

This looks like you probably have the correct Jacobian, but the problem is
very sensitive. Then convergence might be
hard as you scale up because you have a bad initial guess. Its impossible
to say anything else without knowing more
about your problem.


> With the option "-snes_type test":
> "Norm of matrix ratio 4.11569e-09 difference 7.64426e-07 (user-defined
> state)
> Norm of matrix ratio 2.05846e-08 difference 3.89655e-06 (constant state
> -1.0)
> Norm of matrix ratio 5.77446e-09 difference 1.09307e-06 (constant state
> 1.0)"
> with the option "-snes_type test -mat_fd_type ds" :
> "Norm of matrix ratio 1.05213e-07 difference 1.95418e-05 (user-defined
> state)
> Norm of matrix ratio 2.51463e-07 difference 4.76004e-05 (constant state
> -1.0)
> Norm of matrix ratio 2.81081e-08 difference 5.32071e-06 (constant state
> 1.0)".
> Could you give me any hint where should I check for the error?
> Khai

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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