[petsc-users] using '-snes_type test' to test my coded Jacobian

[Zou (Non-US), Ling]

On Wed, Sep 19, 2012 at 1:03 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> On Wed, Sep 19, 2012 at 1:54 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:
>
>> Dear Matt,
>>
>> Thanks again for helping me on the '-snes_type test' issue. This is a
>> great tool which helped us make really good progress recently.
>>
>> I have couple of more questions about the '-snes_type test'. From my
>> understanding, this finite difference method is based on a tiny
>> perturbation from a base solution vector. This base solution vector, from
>> my observation, seems to be [1, 1, 1, ....].  However, this solution vector
>> sometimes is very far from real physics, which causes issue, for example a
>> dependent pressure variable get negative value.
>>
>> I wonder,
>> 1), if this [1, 1, 1, ...] base solution vector is always used during the
>> real simulation, for example when using the finite difference
>> preconditioner. Or this base solution vector will change as solution gets
>> updated?
>>
>
> -snes_type test tries three different states: your user-defined state,
> then constant -1.0, then constant +1.0. It does not solve the system so you
> can't continue stepping, but you can just look at the result from the first
> test.
>

Ahhh....... I see, that's why I always see three comparisons there. The
first one should help me better when dealing with real simulation case.

>
>
>> 2), is it possible to choose a different base solution, for example,
>> [1000, 1000, 1000, 10, 10, 10, 1.e9, 1.e9, 1.e9] as the base solution
>> vector when using the '-snes_type test'  option to test my hand-coded
>> Jacobian.
>>
>
> You can also use -snes_compare_explicit which shows the matrix (only
> linearized around your actual state) and actually solves the system. There
> is also -snes_compare_explicit_draw. If you know that you preallocated
> correctly, -snes_compare_coloring is better. Details here.
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESComputeJacobian.html
>

This is something new to me. I'd take a look at the link. Thanks a lot, Jed.

Best,

Ling
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