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 <[email protected]> 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 <[email protected]> wrote: > >> >> > On Mar 10, 2022, at 2:51 PM, Jed Brown <[email protected]> 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" <[email protected]> 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 < >> [email protected]<mailto:[email protected]>> 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 <[email protected]<mailto: >> [email protected]>> wrote: >> >> >> >> >> >> On Mar 10, 2022, at 10:05 AM, Alfredo J Duarte Gomez < >> [email protected]<mailto:[email protected]>> 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 <[email protected]<mailto: >> [email protected]>> 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 <[email protected]<mailto: >> [email protected]>> 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 <[email protected]<mailto:[email protected]>> >> 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
