Yeah, we'd like the implementation to stay in alpha2.c. There could be a new interface TSAlpha2SetPredictorType (with -ts_alpha2_predictor_type [none,same_velocity,...]) or TSAlpha2SetPredictorFunction.
David Kamensky <da...@coreform.com> writes: > Hi Jed, > > The current workaround I'm using is very minimal and basically just moves > the definition of `TS_Alpha` from `alpha2.c` up to `petsc/private/tsimpl.h` > (and renames it to avoid a conflict with `TS_Alpha` in `alpha1.c`), but I > gather that we're really still not "supposed to" include that header in > applications. So, I don't know whether something like that would be > welcomed upstream. The actual computation of the predictor is done on the > application side. > > Having options like `-ts_alpha_same_velocity` and > `-ts_alpha_same_acceleration` could probably be implemented by analogy to > `-ts_theta_initial_guess_extrapolate`, although they wouldn't quite cover > my specific use-case, where I'm only setting the predictor on part of the > solution vector. So, maybe something more general, like providing a > generalized-$\alpha$-specific option for a custom predictor callback that > takes `X0`, `V0`, and `A0` arguments would be the cleanest solution (and > some convenient shortcuts for full-solution same-velocity and > same-acceleration predictors could subsequently make use of that > infrastructure). I've been working quickly over the past week, but I might > be able to take some time to implement a more sustainable solution soon. > > Thanks again, > David > > On Fri, Aug 4, 2023 at 9:23 AM Jed Brown <j...@jedbrown.org> wrote: > >> Some other TS implementations have a concept of extrapolation as an >> initial guess. Such method-specific initial guesses sound like they fit >> that pattern and would be welcome to be included in alpha2.c. Would you be >> willing to make a merge request to bring your work upstream? >> >> David Kamensky <da...@coreform.com> writes: >> >> > Hi Jed, >> > >> > What I'm trying to compute is basically a standard same-velocity or >> > same-acceleration predictor (although slightly more complicated, since >> I'm >> > restricting it to a sub-system). I hadn't looked into >> > `SNESSetComputeInitialGuess` yet, although one difficulty is that it >> would >> > need access to the `X0`, `V0`, and `A0` members of the `TS_Alpha` struct, >> > which is only defined in `alpha2.c`, and thus not available through the >> > API. >> > >> > For now, we just worked around this by patching PETSc to move the >> > definition of `TS_Alpha` up into a header to make it accessible. >> > (Modifying the library obviously introduces a maintenance headache; I >> also >> > considered just casting the `ts->data` pointer to `(char*)`, calculating >> > memory offsets based on `sizeof` the struct members, and casting back to >> > `Vec`, but that relies on compiler-specific assumptions, and could also >> > break if the PETSc source code was updated.) We also shuffled the order >> of >> > some calls to `VecCopy` and `TSPreStage` in the routine >> `TSAlpha_Restart`, >> > so that `TSPreStage` can set the initial guess, although that sounds like >> > it would be unnecessary if we instead used a callback in >> > `SNESSetComputeInitialGuess` that had access to the internals of >> > `TS_Alpha`. >> > >> > Thanks, David >> > >> > On Thu, Aug 3, 2023 at 11:28 PM Jed Brown <j...@jedbrown.org> wrote: >> > >> >> I think you can use TSGetSNES() and SNESSetComputeInitialGuess() to >> modify >> >> the initial guess for SNES. Would that serve your needs? Is there >> anything >> >> else you can say about how you'd like to compute this initial guess? Is >> >> there a paper or something? >> >> >> >> David Kamensky <da...@coreform.com> writes: >> >> >> >> > Hi, >> >> > >> >> > My understanding is that the second-order generalized-alpha time >> stepper >> >> in >> >> > PETSc uses a same-displacement predictor as the initial guess for the >> >> > nonlinear solver that executes in each time step. I'd like to be >> able to >> >> > set this to something else, to improve convergence. However, my >> >> > (possibly-naive) attempts to use `TSSetPreStep` and `TSSetPreStage` >> >> haven't >> >> > worked out. Is there any way to set a custom predictor? >> >> > >> >> > Thanks, >> >> > David Kamensky >> >> >>
