[petsc-dev] attempt at understanding PetscFEM

[Geoffrey Irving]

On Mon, Nov 18, 2013 at 4:39 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
> Geoffrey Irving <irving at naml.us> writes:
>
>> The seems to be a typo in the abstract:
>>
>>     This effect typically limits order to at most *quadratic*, despite
>> the favorable accuracy and stability properties offered by *quadratic*
>> and higher order discretization
>
> I guess it sounds funny, but quadratic is the crossover point: too
> expensive according to many people, but required by others.

The first real simulation I'm going to run once I get set up will need
accurate bending, so I'm now in the latter camp.

>> The index notation was the whole point, incidentally, since it
>> transparently defines the order of array dimensions in the code.
>
> Okay, that is a convention that I think you got right, but there are
> vectorization reasons to change it in some settings, so if you're going
> to build something significant, it would be worth hedging against index
> ordering changes.

Changing it later will be easy if I have working code to compare against.

>> With the current setup I think they need to be arrays of function
>> pointers since different fields can live in different function spaces
>> and have different quadrature rules.  However, the g0,g1,g2,g3 could
>> be single arrays, and you could merge all f0,f1,g0,g1,g2,g3,g4 into a
>> single array of function pointers for simultaneous residual+Jacobian
>> evaluation.  Compacting it into a single function pointer always would
>> require forcing the use of a single global quadrature rule.  I don't
>> know if that would ever be a problem.
>
> Terms would be grouped by quadrature rules, with one function per
> quadrature rule.  The user would be free to split that evaluation into
> parts.  If you have two quadrature rules and both need the material
> properties, the properties need to be evaluated at both quadrature
> rules.

That seems reasonable.  I don't imagine the support for multiple
quadrature rules would get used very much.

Geoffrey