[petsc-users] TSSetPostStep for projected methods

[Gideon Simpson]

Yup, implemented that with DM.  Works as desired.  Thanks.

-gideon

> On Feb 13, 2017, at 8:03 AM, Matthew Knepley <knepley at gmail.com> wrote:
> 
> On Sun, Feb 12, 2017 at 3:21 PM, Barry Smith <bsmith at mcs.anl.gov <mailto:bsmith at mcs.anl.gov>> wrote:
> 
> > On Feb 12, 2017, at 11:30 AM, Gideon Simpson <gideon.simpson at gmail.com <mailto:gideon.simpson at gmail.com>> wrote:
> >
> > I’ve begun working on implementing a projected time stepping method for solving y’ = f(y), where g(y(t)) is an invariant of the flow.  Per a previous email exchange, it was suggested that I use TSSetPostStep to perform the projection routine, calling TSGetSolution obtain the current iterate, which will be corrected with the projection.  However, I noticed two things:
> >
> > 1.  The documentation for TSGetSolution says "This vector not changed until the solution at the next timestep has been calculated.”  Does that mean that if I make a change to the solution in a PostStep function, it won’t be captured until the next step?
> 
>    No.
> 
> >  What happens at the final time step?
> 
>    Your function should still get called.
> 
> >
> > 2.  The projection is nonlinear, requiring the use of a SNES.  I had originally thought that I would create/configure/destroy the SNES within the main routine of the code, and pass this to the PostStep through a user data structure.  However, the TSSetPostStep function seems to only take functions which are exclusively functions of the TS.  Likewise, I would need to create a residual vector, r, for the SNESSetFunction.  Is there a way to pass these data structures, so that they can be reused. or would they have to be created/destroyed within the PostStep function at each iterate?  How costly would that be?
> 
>    After you create the SNES and the TS call PetscObjectCompose() to attach the SNES to the TS then inside your post-step you can use PetscObjectQuery() or if you want to pass in more than a SNES use PetscContainerCreate().
> 
> An alternative to what Barry suggests, which will definitely work, is to use the DM.
> 
>   DMCreateShell(comm, &dm);
>   DMSetApplicationContext(dm, ctx);
>   TSSetDM(ts, dm);
> 
> In your function
> 
>   TSGetDM(ts, &dm);
>   DMGetApplicationContext(dm, &ctx);
> 
> I prefer this to the Container route.
> 
>   Thanks,
> 
>      Matt
>  
> >
> >
> > -gideon
> >
> 
> 
> 
> 
> -- 
> What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
> -- Norbert Wiener

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