[petsc-dev] TSSet{Pre,Post}Step

[Barry Smith]

   Jed,

    Feel free to change to what makes sense to you. At this moment you  
are in the drivers seat for TS.


    Barry

On Mar 2, 2010, at 1:37 PM, Jed Brown wrote:

> We have some nonsense going on with TSSolve and TSStep (which do the
> same thing, so one needs to be killed).  Both of these only call
> TSStep_XXX once, and the stepping loop is handled internally.  So
> TSSetPreStep and TSSetPostStep are pretty worthless, you're just  
> setting
> a callback to be run immediately after *you* call TSStep, so you might
> as well call these yourself.  Does anyone actually use these?
>
> I think these should be run before and after *each* step, not each  
> call
> to TSStep.  The reason I brinng this up is that a viable use case is  
> to
> run semi-implicit integration where F_fast is integrated implicitly  
> and
> F_slow is done explicitly.  One way to do this is to set a PostStep  
> that
> integrates the explicit system up to the end of the step.  Another  
> is to
> bury explicit integration in function evaluation for the explicit
> system, but you need to know when to complete the step.  Dana says he
> has actually done this to get second order accuracy for a stiff system
> while still doing transport explicitly, though it sounds like that may
> have been as much for political reasons as because the fully coupled
> implicit system, would actually perform worse if someone was to write
> the implicit code for it and split preconditioning appropriately.
>
> I'd like to make Pre and PostStep meaningful before the release (this
> should be trivial once we agree on where they should sit).  I'd also
> like to hear comments about how TS should behave, at some later  
> point, I
> want to clean it up a bit, and probably get rid of all the
> implementation specializations for linear systems (which can be
> abstracted as a special case of a nonlinear system so that the user
> interface doesn't change).
>
> I'd also like to add a TSInterpolate to evaluate the state at  
> arbitrary
> points inside a step (to support the partly explicit treatment above,
> and because it will be necessary if we want to integrate continuous
> adjoints [1] and delay differential equations --- we don't support any
> of this stuff yet, but I want to think about what the interfaces would
> look like.)
>
> Jed
>
> [1] As far as I know, existing software to integrate continuous  
> adjoints
> (e.g. CVODES and IDAS from Sundials) use an interpolation scheme  
> that is
> not compatible with the forward integrator.  The fact that naive
> interpolation of states can produce something very different (think of
> concentrations in a reactive flow, in which case the average of two
> subcritical mixtures could be explosive) is one reason why discrete
> adjoints are attractive.  Availability of compatible high-order
> interpolation would make continuous adjoints attractive as well  
> (unless
> I'm missing something).