Re: [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  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  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  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  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  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  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/ 

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

[Jed Brown]

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  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  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  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  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  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
>> >>
>>

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

[David Kamensky]

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  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  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  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  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
> >>
>

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

[Jed Brown]

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  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  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  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
>>

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

[David Kamensky]

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  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  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
>

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

[Jed Brown]

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  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

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

[David Kamensky]

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