[petsc-users] A matrix-free Jacobian for SNES using MatShell

[Jeremy Roberts]

To implement the safe method---i.e. copying the unknown vector u within
FOO---what is proper generic form of FOO?

I guess I'm not clear exactly how FOO and jac_shell (the J*x routine) are
linked.  It appears they aren't, given I can use an empty subroutine, but
how then is a local copy of u within FOO used in for J(u) in J(U)*x of
jac_shell?


On Tue, Aug 31, 2010 at 1:14 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Aug 31, 2010, at 1:03 PM, Jeremy Roberts wrote:
>
> >
> > Hi Barry,
> >
> > Thanks for the quick response.
> >
> > I have the unknown vector u globally accessible and so forgo use of the
> user-defined context (I tried to get it to work---can it be used in
> Fortran?).
> >
> > I used
> >
> > call SNESSetJacobian( snes, Jshell, Jshell, FOO, PETSC_NULL_OBJECT, ierr
> )
> >
> > where FOO is an empty function having the correct argument types.  It
> seems to work now, and I get the exact same answer using an explicit
> Jacobian matrix and the shell version.  Is there a way to get around needing
> FOO at all?
> >
>
>    If it works great. But I how you know what the "unknown vector u " that
> is "globally accessible"  in SNES is? How do you know it is the same each
> time a new Jacobian is needed? The only save way is to copy the vector
> passed into the FOO funcion.
>
>   Barry
>
>
>
> > Jeremy
> >
> >
>
>
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