[petsc-users] Setting a custom predictor in the generalized-alpha time stepper

[Matthew Knepley]

If you want to make a PR with your hack, we can help build out the
infrastructure for what Jed is recommending.

  Thanks,

     Matt

On Fri, Aug 4, 2023 at 2:56 PM Jed Brown <jed at jedbrown.org> wrote:

> 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 <david at 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 <jed at 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 <david at 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 <jed at 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 <david at 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
> >> >>
> >>
>


-- 
What most experimenters take for granted before they begin their
experiments is infinitely more interesting than any results to which their
experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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