[petsc-users] Changing the jacobian matrix during SNES iterations

[Matthew Knepley]

On Wed, Jul 18, 2018 at 3:25 PM David Knezevic <david.knezevic at akselos.com>
wrote:

> On Wed, Jul 18, 2018 at 1:59 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Wed, Jul 18, 2018 at 1:31 PM David Knezevic <
>> david.knezevic at akselos.com> wrote:
>>
>>> I'm using SNES for a finite element contact solve, in which the sparsity
>>> pattern of the jacobian can change from one Newton iteration to the next
>>> (since the nodes on the contact surface move).
>>>
>>> In order to handle this I figured the best way would be to destroy the
>>> jacobian matrix and re-allocate it with a new sparsity pattern inside each
>>> call to FormJacobian, does that seem like a reasonable approach in this
>>> context?
>>>
>>
>> Yes.
>>
>>
>>> Also, I recall from an earlier discussion that this matrix re-allocation
>>> inside FormJacobian is supported by SNES, but I just wanted to confirm that?
>>>
>>
>> Yes.
>>
>>
>>> Also, I was wondering if there is any example where the matrix is
>>> re-allocated inside SNES iterations so that I can make sure that I do it
>>> correctly?
>>>
>>
>> No, unfortunately. Contributions always welcome :)
>>
>
>
> OK, as a test case I'd like to modify snes/tutorials/ex1.c to destroy and
> reallocate the jacobian matrix with the same sparsity pattern
> inside FormJacobian1 (once I can do that with the same sparsity pattern,
> then it should be straightforward to do the same thing with a modified
> sparsity pattern). To do that, I tried adding the following code inside
> FormJacobian1:
>
>   ierr = MatDestroy(&B);
>

You do not want to destroy the matrix. There would be no way to get another
Mat back out of the function.


>   ierr = MatCreate(PETSC_COMM_WORLD,&B);CHKERRQ(ierr);
>

And do not recreate it.


>   ierr = MatSetSizes(B,PETSC_DECIDE,PETSC_DECIDE,2,2);CHKERRQ(ierr);
>

Nor reset the sizes. However, you do want to call

  MatSetPreallocationXAIJ();

which should work


>   ierr = MatSetFromOptions(B);CHKERRQ(ierr);
>   ierr = MatSetUp(B);CHKERRQ(ierr);
>

  THanks,

    Matt



> That gives a segfault, so I gather something else is needed here, any
> suggestions on what I need to do? I guess I need to do something else in
> order to make sure that the SNES uses the newly created matrix (e.g. update
> the original Mat J)?
>
> Thanks,
> David
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.caam.rice.edu/~mk51/>
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