[petsc-dev] Discussion about time-dependent optimization moved from PR

[Jed Brown]

Matthew Knepley <knepley at gmail.com> writes:

>> It's a recipe for confusion.  Either the parameters are never passed
>> explicitly or they are always passed explicitly and should not be stored
>> redundantly in the context, thus perhaps enabling some sort of higher
>> level analysis that dynamically changes parameter values.  I would go
>> with the former for now.
>
>
> I want to say again how much I dislike ad hoc memory layouts through
> contexts and the like. We have a perfectly good layout descriptor (DM)
> that should be used to describe data layout.

This is an independent change from the adjoint work and I think it's out
of scope right now.  If we change it, it should go in its own PR.  I
don't like having one PR with a smattering of non-essential changes to
old interfaces bundled together with new conventions and new
functionality.

Putting the parameters in a vector would enable finite differencing of
the RHSFunction to obtain its dependence on parameters.  That might have
high (non-scalable in number of parameters) cost, but would be less
expensive than finite differencing an entire model.  Consider the
scenario of 100 parameters and 1e6 state variables (at each time step).
If we have the ability to apply the transpose of the Jacobian with
respect to model state, we could run an adjoint simulation and only need
100 RHSFunction evaluations per stage, rather than 100 solves.