[petsc-users] TSBDF prr-load higher order solution

Fri Mar 11 11:09:24 CST 2022

Thank you so much,

This is exactly what I need. Just to clarify, these functions that you are
mentioning already exist?

Or you are simply sketching out what is going to be needed?

Thank you,

-Alfredo

On Thu, Mar 10, 2022 at 3:40 PM Zhang, Hong <hongzhang at anl.gov> wrote:

>
> > On Mar 10, 2022, at 2:51 PM, Jed Brown <jed at jedbrown.org> wrote:
> >
> > Would the order be inferred by the number of vectors in
> TSBDFSetStepVecs(TS ts, PetscInt num_steps, const PetscReal *times, const
> Vec *vecs)?
>
> Ah! Yes. num_steps could be used to set bdf->k which is the current order
> of BDF.
>
> Hong (Mr.)
>
> > "Zhang, Hong" <hongzhang at anl.gov> writes:
> >
> >> It is clear to me now. You need TSBDFGetStepVecs(TS ts, PetscInt
> *num_steps, const PetscReal **times, const Vec *vecs) as Jed suggested so
> that you can access the work vectors for TSBDF. In addition, you need to
> save the step size and the current order to jumpstart BDF (without starting
> from BDF-1). Setting the step size is straightforward with TSSetTimeStep().
> To access/recover the order, you need something like
> TSBDF{Set|Get}CurrentOrder(TS ts, PetscInt **order).
> >>
> >> Thanks,
> >> Hong (Mr.)
> >>
> >> On Mar 10, 2022, at 11:35 AM, Alfredo J Duarte Gomez <
> aduarteg at utexas.edu<mailto:aduarteg at utexas.edu>> wrote:
> >>
> >> Hello,
> >>
> >> The solutions y_n and y_n-1 are the same ones as those saved internally
> for the BDF-2.
> >>
> >> The practical problem is for example: run a simulation with a BDF-2 for
> 10 hrs, see that everything looks good and decide that this simulation is
> worth continuing for another 10 hrs. Except now when I start from that last
> solution, the TSBDF starts from a single solution and steps its way up to
> the BDF-2 (numerical noise appears), instead of simply being able to load
> the last two solutions from my HDF5 output file and continue as if it had
> not stopped.
> >>
> >> Maybe I confused you by referencing the TSBDF_Restart, instead of the
> "starting" procedure for higher order BDFs, I thought they were the same
> but maybe not.
> >>
> >> Thank you,
> >>
> >> -Alfredo
> >>
> >>
> >>
> >> On Thu, Mar 10, 2022 at 11:04 AM Zhang, Hong <hongzhang at anl.gov<mailto:
> hongzhang at anl.gov>> wrote:
> >>
> >>
> >> On Mar 10, 2022, at 10:05 AM, Alfredo J Duarte Gomez <
> aduarteg at utexas.edu<mailto:aduarteg at utexas.edu>> wrote:
> >>
> >> Hello Zhang and Hong,
> >>
> >> Thank you for your reply.
> >>
> >> As I described, I simply wanted to be able to restart a higher order
> BDF from a previous solution.
> >>
> >> For example, if I want to restart a BDF-2 solution I can simply load
> times (n is current time step ) t_n-1, t_n, load solutions y_n, and yn-1
> from a restart file and continue integration with a BDF-2 formula as if it
> never stopped.
> >>
> >> Are y_n and yn-1 from the restart file different from the states saved
> internally for BDF-2? Are you trying to modify these states yourself? A
> restart is needed typically when you have discontinuities in the system, so
> the solutions before the discontinuity point have to be discarded. If you
> simply want to modify the states, there should be better ways than using
> checkpoint-restart.
> >>
> >> Hong (Mr.)
> >>
> >>
> >>
> >> This would replace the current default approach, which starts from a
> single time tn, solution yn and uses lower order BDF steps as you build up
> to the selected order.
> >>
> >> I am not sure why, but an abrupt change in integration order or time
> step leads to unwanted numerical noise in my solution, which I blame on the
> high nonlinearity of the system (I have tested extensively to rule out
> bugs).
> >>
> >> Thank you and let me know if you have any questions,
> >>
> >> -Alfredo
> >>
> >> On Wed, Mar 9, 2022 at 4:49 PM Zhang, Hong <hongzhang at anl.gov<mailto:
> hongzhang at anl.gov>> wrote:
> >> TSTrajectory supports checkpointing for multistage methods and can
> certainly be extended to multistep methods. But I doubt it is the best
> solution to Alfredo’s problem. Alfredo, can you elaborate a bit on what you
> would like to do? TSBDF_Restart is already using the previous solution to
> restart the integration with first-order BDF.
> >>
> >> Hong(Mr.)
> >>
> >>> On Mar 9, 2022, at 4:24 PM, Jed Brown <jed at jedbrown.org<mailto:
> jed at jedbrown.org>> wrote:
> >>>
> >>> Can you restart using small low-order steps?
> >>>
> >>> Hong, does (or should) your trajectory stuff support an exact
> checkpointing scheme for BDF?
> >>>
> >>> I think we could add an interface to access the stored steps, but
> there are few things other than checkpointing that would make sense
> mathematically. Would you be up for making a merge request to add
> TSBDFGetStepVecs(TS ts, PetscInt *num_steps, const PetscReal **times, const
> Vec *vecs) and the respective setter?
> >>>
> >>> Alfredo J Duarte Gomez <aduarteg at utexas.edu<mailto:aduarteg at utexas.edu>>
> writes:
> >>>
> >>>> Good morning PETSC team,
> >>>>
> >>>> I am currently using a TSBDF object, which is working very well.
> >>>>
> >>>> However, I am running into trouble restarting higher order BDF
> methods.
> >>>>
> >>>> My problem is highly nonlinear, and when restarted for higher order
> BDF
> >>>> methods (using the TSBDF_Restart function), wiggles appear in a
> specific
> >>>> region of the solution.
> >>>>
> >>>> Is there any way I can initialize the higher order BDF restart loading
> >>>> previous solutions from a data file? I took a look at the code, but
> there
> >>>> is no obvious way to do this.
> >>>>
> >>>> Thanks,
> >>>>
> >>>> -Alfredo
> >>>>
> >>>> --
> >>>> Alfredo Duarte
> >>>> Graduate Research Assistant
> >>>> The University of Texas at Austin
> >>
> >>
> >>
> >> --
> >> Alfredo Duarte
> >> Graduate Research Assistant
> >> The University of Texas at Austin
> >>
> >>
> >>
> >> --
> >> Alfredo Duarte
> >> Graduate Research Assistant
> >> The University of Texas at Austin
>
>

-- 
Alfredo Duarte
Graduate Research Assistant
The University of Texas at Austin
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