[petsc-users] Reproducibility when restarting generalized-alpha

[David Kamensky]

>
> How are you currently restarting the simulation?


I just reviewed the code, and we're not currently using the `TSView/Load`
functions.  We're just manually (de)serializing displacement, velocity, and
acceleration data using a neutral format, populating PETSc `Vec`s with this
data, and associating them with a new `TS` object via `TS2SetSolution` (and
setting other relevant data, like time, time step size, etc.).  However,
`TS2SetSolution` only accepts displacement and velocity.

I think the correct way to handle this is to support storing/loading these
> extra vectors via TSView()/TSLoad().


I took a quick look at the implementations of `TSView/Load`, and it looks
like the "base class" (to borrow some OOP terminology) implementation in
`ts/interface/ts.c` only saves/loads the solution vector, while the
subclass-specific logic from `TSView_Alpha` in
`ts/impls/implicit/alpha/alpha2.c` only adds some additional output writing
the generalized-alpha parameters to ASCII viewers (and similar for BDF).
So, following the `TSView/Load` path, I don't see where it would even
save/load the velocity vector for 2nd-order-in-time integrators.  Is it the
case that this functionality is known to be incomplete, and you're
suggesting that the best path forward would be to update it?

Thanks, David


On Thu, Jan 2, 2025 at 10:41 AM Stefano Zampini <stefano.zampini at gmail.com>
wrote:

> Note that BDF has the same issue. I think the correct way to handle this
> is to support storing/loading these extra vectors via TSView()/TSLoad().
> How are you currently restarting the simulation?
>
> Il giorno gio 2 gen 2025 alle ore 19:25 David Kamensky via petsc-users <
> petsc-users at mcs.anl.gov> ha scritto:
>
>> Hi,
>>
>> I've recently been helping some co-workers with restarting PETSc time
>> integrators from saved solution data.
>>
>> It looks like the only supported path for restarting the
>> generalized-alpha integrator for 2nd-order-in-time systems (`TSALPHA2`) is
>> to follow the same procedure as initialization, in which two
>> first-order-accurate half-steps are used to estimate an acceleration from
>> the given displacement and velocity.  However, the resulting acceleration
>> is not exactly equivalent to the intermediate one that would have been used
>> by the integrator if the integration simply proceeded without restarting.
>> This prevents exact reproducibility of computations from saved intermediate
>> results.  (An analogous issue would also affect `TSALPHA` for
>> first-order-in-time problems, where velocity is estimated on
>> initialization/restart.)
>>
>> Am I misunderstanding this, or missing a better method of restarting the
>> 2nd-order generalized-alpha integrator?  If not, would there be interest in
>> adding an alternate initialization/restart option to the `TSALPHA2`
>> integrator that takes a user-provided `Vec` for the initial/intermediate
>> acceleration, and skips over the half-step estimation procedure?
>>
>> Thanks, David
>>
>
>
> --
> Stefano
>
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