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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 <[email protected]> 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 > >> >
