I wish I could give you an definitive reason why, but I can't. Before TS gets into this situtation where it needs to take a tiny time step, TS takes time steps of ~1.0, and the shift in TSSetIJacobian is ~1.0. Then it gets really close to the max time and the time step drops to ~1e-12 and the shift in TSSetIJacobian becomes ~1e12.
When this happens, my ILU-preconditioned gmres linear solve converges in a single interation, and the backtracking line search in my newton solver fails. Thank you,Greg On Thu, 2022-04-21 at 00:57 +0000, Zhang, Hong wrote: > APL external email warning: Verify sender hongzh...@anl.gov before > clicking links or attachments > > > > > On Apr 20, 2022, at 3:58 PM, Phlipot, Greg <greg.phli...@jhuapl.edu > > > wrote: > > > > Hello, > > > > When using TS with the option TS_EXACT_FINALTIME_MATCHSTEP to force > > TS > > to stop at the final time, I'm seeing the adaptive step controller > > choose smaller time steps than the minimum time step that is set > > with > > TSAdaptGetStepLimits. For my problems, this leads to solutions that > > won't converge due to numerical issues. > > Can you please elaborate a bit on why the solution diverges when > using a small step size? > > Thanks, > Hong (Mr.) > > > It seems like this might be due to the fact that TSAdaptChoose ( > > https://petsc.org/main/src/ts/adapt/interface/tsadapt.c.html#TSAdaptChoose > > ) uses a tolerance of 10*PETSC_MACHINE_EPSILON to compare whether > > the > > candidate time step is close to the final time. For me, > > 10*PETSC_MACHINE_EPSILON is much smaller than my smallest time step > > so > > the adaptation scheme occasionally chooses a time step that results > > in > > a time much closer to the max time than the minimum time step. The > > subsequent time step is therefore forced to be smaller than the > > minimum > > time step. > > > > Does anyone have an idea of a workaround (without changing the > > source > > code) where I would be able to modify this logic to use the minimum > > time step instead of 10*PETSC_MACHINE_EPSILON? E.g. with Pre/Post > > stage > > callbacks and manually modifying the time step? > > > > Thank you, > > Greg
