[petsc-dev] resurrecting the finite element energy functions thread

[Geoffrey Irving]

On Thu, Jan 23, 2014 at 6:43 AM, Matthew Knepley <knepley at gmail.com> wrote:
> On Wed, Jan 22, 2014 at 4:16 PM, Geoffrey Irving <irving at naml.us> wrote:
>>
>> On Tue, Jan 21, 2014 at 9:52 PM, Jed Brown <jed at jedbrown.org> wrote:
>> > Lisandro Dalcin <dalcinl at gmail.com> writes:
>> >> I thought to do that in PetIGA, but then realized that a reducer+eval
>> >> at quadrature points+MPI_MAX sounds weird (though I can imagine some
>> >> use cases). A reduce with MPI_MAX is a lucky consequence of computing
>> >> integrals through quadrature.
>> >
>> > Suppose we want to know the max stress in a structure to determine
>> > whether it will be damaged by a given loading scenario?  The quadrature
>> > points are not sufficient to determine a max stress for the discrete
>> > solution, but it is a convergent method for determining the max stress
>> > of the continuum structure.
>>
>> So I wrote PetscFEIntegrateScalars, but then realized that I have no
>> idea how to organize the DM level.  Residuals and jacobians can be
>> only be "turned on" via DMSNESSetFunctionLocal, and then accessed via
>> the SNES.  This would work in the specific case of an objective, but
>> not for the general case where we're integrating some arbitrary number
>> of PetscScalars.
>>
>> Where should the outer "integrate a bunch of scalars over a DM with a
>> bunch of PetscFE" objects go, and what should it be called?  The
>> naming conventions at this level are rather obscure.
>>
>> Relatedly, is it going to be a problem if I want to use PetscFE
>> routines outside an SNES, such as inside a TAO optimization problem?
>> Should I make a dummy SNES and then point TAO to SNESComputeObjective
>> and such, or is there a cleaner way?
>
>
> 1) I have been putting functions like this (DMPlexComputeL2Diff) in DM
> because there seemed to be no other place.
>
> 2) Nope you can use them anywhere

It looks like the local versions are usable from anywhere, but the
global versions including communication are not.  I.e.,
DMPlexComputeResidualFEM does no communication, and its global version
is the internal function SNESComputeFunction_DMLocal plus a pointer to
DMPlexComputeResidualFEM.  Is this how I should expose the scalar
integration as well: local version only, with the user responsible to
doing the field synchronization before and the reduce afterwards?

Geoffrey