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

[Matthew Knepley]

On Wed, Jul 18, 2018 at 10:15 PM David Knezevic <david.knezevic at akselos.com>
wrote:

> On Wed, Jul 18, 2018 at 3:34 PM, Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> 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
>>
>
> Thanks! Calling MatSetPreallocationXAIJ inside FormJacobian works for me.
>
> So I guess this means that we create a new sparsity pattern "from
> scratch" each time we call MatSetPreallocationXAIJ and set the values in
> the matrix?
>

Yes.

  Matt


> 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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