[petsc-users] Setting/creating Mats for SNES use

[Dave May]

Damn - Jed scooped me :D

On 2 November 2015 at 22:19, Dave May <dave.mayhem23 at gmail.com> wrote:

>
>
> On 2 November 2015 at 21:49, Justin Chang <jychang48 at gmail.com> wrote:
>
>> So when would I use one over the other?
>>
>> - If I wanted to solve a problem using a direct solver or an iterative
>> solver without a preconditioner, I would use A = J?
>>
>
> Yes.
>
>
>>
>> - The documentation for SNESSetJacobian() says that AMat and PMat are
>> usually the same, but if I used something like GAMG would I need to create
>> two different objects/Mats?
>>
>
> I would say "maybe".
>
> If the Jacobian (here A) was defined via a matrix-free finite difference
> approximation, but you wish to use a non-trivial preconditioner, you might
> wish to assemble J. J might be the Picard linearized operator (for example).
>
> Another use case where A != J might arise is if you define A with a high
> order spatial discretization (probably matrix free) and you use a low order
> discretization to define the preconditioner which will ultimately be passed
> to GAMG.
>
>
>
>>
>> Thanks,
>> Justin
>>
>> On Mon, Nov 2, 2015 at 1:39 PM, Jed Brown <jed at jedbrown.org> wrote:
>>
>>> Justin Chang <jychang48 at gmail.com> writes:
>>>
>>> > Hi all,
>>> >
>>> > In my DMPlex program, I have these lines:
>>> >
>>> > Mat A,J;
>>> >
>>> > ...
>>> >
>>> > ierr = DMSetMatType(dm, MATAIJ); CHKERRQ(ierr);
>>> > ierr = DMCreateMatrix(dm, &J); CHKERRQ(ierr);
>>> > A = J;
>>> >
>>> > ierr = DMSNESSetFunctionLocal(dm, ...); CHKERRQ(ierr);
>>> > ierr = DMSNESSetJacobianLocal(dm, ...); CHKERRQ(ierr);
>>> > ierr = SNESSetJacobian(snes, A, J, NULL, NULL); CHKERRQ(ierr);
>>> > ierr = SNESSetFromOptions(snes); CHKERRQ(ierr);
>>> >
>>> > ...
>>> >
>>> > ierr = SNESSolve(snes, NULL, x); CHKERRQ(ierr);
>>> >
>>> > ...
>>> > ierr = MatDestroy(&J); CHKERRQ(ierr);
>>> >
>>> >
>>> > For the line "A = J;",
>>>
>>> This means you have two handles referring to the same object.
>>>
>>> > what exactly is the difference, if any, between that and "ierr =
>>> > MatDuplicate(...)"
>>>
>>> This creates a new object.
>>>
>>> > or "ierr = MatCopy(...)"?
>>>
>>> The second argument needs to be a valid Mat to call this function.
>>>
>>> > Do these different options somehow affect memory usage/performance?
>>>
>>> Yes.
>>>
>>> > Say I am solving a standard poisson equation using either GAMG and/or
>>> > HYPRE.
>>> >
>>> > Thanks,
>>> > Justin
>>>
>>
>>
>
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