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

[Barry Smith]

On Aug 31, 2010, at 2:17 PM, Jeremy Roberts wrote:

> 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?

  Don't ask me. You wrote the code, I have no idea how you are accessing u.  But you need to compute J(u)*x so you need to use u.

  subroutine yourfoo(snes,u,A,B,flg,ierr)
  common /mycommonblock/ v 

  call veccopy(u,v,ierr)
  return

   and create the v upfront in your main program. Of course, you don't need to use common blocks but that is one way of maintaining this global variables.

   Barry

   

> 
> 
> 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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