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

[Jed Brown]

Are you using an older version of PETSc?

On Wed, Sep 19, 2012 at 2:56 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:

> No Jacobian output showed when I did this,
>
> ./my-project-opt -i input.i -snes_compare_explicit
>
> Any suggestion?
>
> Ling
>
>
>
> On Wed, Sep 19, 2012 at 1:12 PM, Zou (Non-US), Ling <ling.zou at inl.gov>wrote:
>
>>
>>
>> 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
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20120919/33da0b5b/attachment-0001.html>