Would the order be inferred by the number of vectors in TSBDFSetStepVecs(TS ts, PetscInt num_steps, const PetscReal *times, const Vec *vecs)?
"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
