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 >>
