[petsc-dev] Semi-smooth VI solver and matrix-free SNES

[Dmitry Karpeyev]

Thanks, Jason.
Dmitry.
On Aug 1, 2014 9:52 AM, "Jason Sarich" <sarich at mcs.anl.gov> wrote:

> The ssfls and asfls solvers respect feasibility, ssils and asils do not
> (the 'f' stands for feasibility, 'i' for infeasibility)
>
> ssfls - semismooth, feasible, with line search
> asfls - active set semismooth, feasible, with line search
>
>
>
>
> On Fri, Aug 1, 2014 at 10:45 AM, Karpeyev, Dmitry Aleksandrovich <
> karpeev at mcs.anl.gov> wrote:
>
>>  Todd,
>>
>>  What's the semi-smooth version in TAO that respect the bounds at the
>> iterates?
>> Maybe that would be a good place to attempt a unification with SNESVI.
>>
>>  The current implementation could benefit from refactoring to enable
>> more composability:
>> could we use a VIRS method inside VISS and allow for a user-defined
>> active set definition/projection?
>> Then a bound-respecting VISS would be a combination of the current VISS
>> and VIRS, potentially
>> controllable from the command line.
>>
>>  The same goes for the linesearch: Armijo would be another
>>  SNESLineSearch type that could be turned on on demand.
>> Finally, perhaps we could enable composition of DM(SNES) objects?  Then
>> wrapping projection onto bounds around residual/Jacobian evaluation
>> routines would be a composition of two DM(SNES) objects.  Right now
>> something along those lines takes place in VIRS where an RS DM is
>> "composed" with the full space DM, although this composition is
>> currently ad hoc.
>>
>>  Dmitry.
>>
>>
>> On Fri, Aug 1, 2014 at 9:35 AM, Munson, Todd S. <tmunson at mcs.anl.gov>
>> wrote:
>>
>>>
>>> Possibly it can be taken out; don't have time to go through the details
>>> today though.
>>>
>>> Regarding feasibility, there are feasible versions that respect the
>>> bounds in TAO
>>> as well as infeasible versions that only respect the bounds at the
>>> solution.
>>>
>>> Todd.
>>>
>>> On Aug 1, 2014, at 10:02 AM, Dmitry Karpeyev <karpeev at mcs.anl.gov>
>>> wrote:
>>>
>>> > Currently the semi-smooth VI solver (VINEWTONSSLS) doesn't play well
>>> with the matrix-free SNES (aka JFNK).  This is partly because the
>>> calculation of the merit function gradient requires the action of the
>>> transpose of the Jacobian, which isn't implemented for MATMFFD.
>>>  Presumably, the merit function would be used in a fancy (Armijo?) line
>>> search as described here: http://arxiv.org/abs/math/0307305.  However,
>>> it appears that only a basic line search is performed and the merit
>>> function gradient isn't used.  Should the merit gradient calculation be
>>> taken out?
>>> >
>>> > This fix might not be enough to make the SS method work with JFNK,
>>> however: the line search
>>> > there isn't projected, so MatMFFD might have an infeasible base at
>>> some iterates.  This might break residual evaluation (e.g., due to negative
>>> densities). More importantly, however, even if the intermediate iterates
>>> were feasible, the increments used in the MatMFFD differencing algorithm
>>> might not be, and there is really no way around it with something like
>>> projection without breaking the semantics of MatMult.  Is VINEWTONSSLS
>>> unusable with JFNK in its current form?  In any event, should the Armijo
>>> line search be implemented?
>>> >
>>> > Dmitry.
>>> >
>>>
>>>
>>
>
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