[petsc-dev] controlling vector values while doing matrix free operations

[Dmitry Karpeev]

On Wed, Dec 14, 2011 at 9:31 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Dec 14, 2011, at 9:22 PM, Jed Brown wrote:
>
> > On Wed, Dec 14, 2011 at 17:55, Barry Smith <bsmith at mcs.anl.gov> wrote:
> >  What Matt is trying to tell you is that changing values at this level
> to prevent function evaluations outside of the function domain inside
> Newton is a total hack that rarely works well and should be avoided. You
> really don't want to do this.
> >
> > It sounds like for the problems Bobby is asking about, the continuum
> solutions never produce values at a "constraint". In gas dynamics, the
> continuum equations are PDEs, not variational inequalities, but the
> pressure and temperature are still never negative. If a bound was
> available, MatMFFD could potentially choose the sign of the differencing to
> stay within the feasible domain. This does seem quite limited, however,
> because the positive and negative versions could cause violations in
> different parts of the domain.
>
>    If you are truly near 0 in temperature or pressure and differencing can
> push you over the limit then I submit it is perfectly reasonable to solve
> them as VI.
>
>   Though actually our VI solvers are currently not really matrix-free so
> this is a bit academic.
>

We could support a matrix-free SNESVI if extracting a submatrix from a MF
matrix was implemented by pre-/post-multiplying it by a suitable P, no?

Dmitry.

>
>   Barry
>
>
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