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

[Jed Brown]

They don't exist, but would be simple to implement. Is that something you might be able to contribute (we can advise) or that you want us to do?

Alfredo J Duarte Gomez <aduarteg at utexas.edu> writes:

> 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