[petsc-dev] BDF implementation

[Lisandro Dalcin]

On 31 March 2016 at 01:07, Abhyankar, Shrirang G. <abhyshr at anl.gov> wrote:
>
>>
>>Oh! Very good point. Then, the only way to know is the postevent
>>callback flagging the timestepper about it.
>
> The restart will only work when TSEvent is used. What about without
> TSEvent?
>

Well, I was thinking only about the issues related to TSEvent. What
other use cases you have in mind?

>>Well, A
>>VecGetArray()/RestoreArray() on the solution would still work, even if
>>the postevent callback does not actually modify the state. However,
>>this is sounds like a bad API for users to flag the restart.
>
> How about an additional check on RHS function vector state, similar to
> what¹s been done with the solution vector state?
>

I think that makes no sense. Let me put an example. Suppose you have this  ODE:

dU/dt = -U + (t<1) ? 0 : 1

Then you code it all in IFunction (i.e, do not use RHSFunction at all)
and use an implicit solver. Then we use TSEvent to force the solver
pass through  t=1, we do not change state, but the continuity of the
solution changes anyway because of the time-dependent term. If we are
integrating with let say BDF(p) p>=2, the solver has to be restarted.
In this use case, I think the only way for the solver to know a
restart is needed, is the postevent function flagging the solver about
it.

In other use cases of events, you are only interesting in accurate
detection of them for the solver to not "step-over" the event time,
but then you do not change state, rhs, nothing, and you can continue
timestepping without need of restart, BDF(p) p>=2 can happily continue
keeping its previous history.


-- 
Lisandro Dalcin
============
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