problem with -snes_mf_operator option

[Vijay S. Mahadevan]

Barry,

Perfect. Yes, that did the trick. 

I spent hours trying to figure out what was going on and never thought of
the following, since the Jacobian MFFD matrix for -snes_mf_operator is
created and handled by SNESSetup and SNESSolve. But now I understand that
for -snes_mf option, the PETSc wrapper function for FormJacobian is called
instead of the user function, and hence, it makes a difference in
-snes_mf_operator alone.

Thanks a lot for the help Barry. Also, appreciate updating the manual pages.

Cheers,
Vijay

> -----Original Message-----
> From: owner-petsc-users at mcs.anl.gov [mailto:owner-petsc-users at mcs.anl.gov]
> On Behalf Of Barry Smith
> Sent: Thursday, March 27, 2008 6:56 AM
> To: petsc-users at mcs.anl.gov
> Subject: Re: problem with -snes_mf_operator option
> 
> 
>      When you are providing the function with SNESetJacobian() and the
> A matrix and the B matrix are DIFFERENT they must BOTH have
> MatAssemblyBegin/End()
> called on them.  For example in snes/examples/tutorials/ex1.c we
> use
>    if (*jac != *B){
>      ierr = MatAssemblyBegin(*jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>      ierr = MatAssemblyEnd(*jac,MAT_FINAL_ASSEMBLY);CHKERRQ(ierr);
>    }
> 
> Note: in your original code both matrices are the same but with the
> -snes_mf_operator option the A matrix is changed to a MFFD matrix so is
> now different then the B matrix.
> 
> Please let us know if this resolves the problem,
> 
>    Barry
> 
> I have added this information to the manual page for SNESSetJacobian()
> 
> 
> 
> On Mar 26, 2008, at 4:14 PM, Vijay S. Mahadevan wrote:
> > Hi all,
> >
> > I have a nonlinear solver test code that can use a user specified
> > Jacobian function directly, set using SNESSetJacobian. Now if I use
> > -snes_mf option, the solution scheme depends only on the residual and
> > the convergence is equivalent to the case when using the real Jacobian
> > matrix itself. But if I try the -snes_mf_operator option, with the
> > Jacobian=NULL to start with and the preconditioner being correctly
> > allocated the memory needed, I end up with a NULL argument exception.
> > I also alternately set the preconditioner to Identity matrix in order
> > to avoid any problems from user code to trickling in, but to no avail.
> >
> > Here's the call stack.
> >
> > [0]PETSC ERROR: Null argument, when expecting valid pointer!
> > [0]PETSC ERROR: Null Object: Parameter # 1!
> > [0]PETSC ERROR:
> > ------------------------------------------------------------------------
> > [0]PETSC ERROR: Petsc Release Version 2.3.3, Patch 7, Fri Oct 26
> > 14:21:35 CDT 2007 HG revision:
> > 2e223033ba960114833e1f9713ab393ec78c056f
> > [0]PETSC ERROR: See docs/changes/index.html for recent updates.
> > [0]PETSC ERROR: See docs/faq.html for hints about trouble shooting.
> > [0]PETSC ERROR: See docs/index.html for manual pages.
> > [0]PETSC ERROR:
> > ------------------------------------------------------------------------
> > [0]PETSC ERROR: bin/test-dbg on a linux-gnu named grove by vijaysm Wed
> > Mar 26 16:06:45 2008
> > [0]PETSC ERROR: Libraries linked from
> > /state/partition1/local/petsc-2.3.3-p7/lib/linux-gnu-c-debug
> > [0]PETSC ERROR: Configure run at Fri Nov  9 15:02:46 2007
> > [0]PETSC ERROR: Configure options --with-cc=gcc --with-fc=ifort
> > --with-cxx=g++ --download-f-blas-lapack=1 --download-mpich=1
> > --with-shared=1
> > [0]PETSC ERROR:
> > ------------------------------------------------------------------------
> > [0]PETSC ERROR: PetscObjectGetComm() line 34 in src/sys/objects/
> > gcomm.c
> > [0]PETSC ERROR: VecNormBegin() line 495 in src/vec/vec/utils/comb.c
> > [0]PETSC ERROR: MatMFFDCompute_WP() line 73 in src/mat/impls/mffd/wp.c
> > [0]PETSC ERROR: MatMult_MFFD() line 294 in src/mat/impls/mffd/mffd.c
> > [0]PETSC ERROR: MatMult() line 1632 in src/mat/interface/matrix.c
> > [0]PETSC ERROR: PCApplyBAorAB() line 584 in src/ksp/pc/interface/
> > precon.c
> > [0]PETSC ERROR: GMREScycle() line 159 in src/ksp/ksp/impls/gmres/
> > gmres.c
> > [0]PETSC ERROR: KSPSolve_GMRES() line 241 in src/ksp/ksp/impls/gmres/
> > gmres.c
> > [0]PETSC ERROR: KSPSolve() line 379 in src/ksp/ksp/interface/itfunc.c
> > [0]PETSC ERROR: SNES_KSPSolve() line 2578 in src/snes/interface/snes.c
> > [0]PETSC ERROR: SNESSolve_LS() line 184 in src/snes/impls/ls/ls.c
> > [0]PETSC ERROR: SNESSolve() line 1871 in src/snes/interface/snes.c
> >
> > From what I can understand from the PETSc code (snes.c, mffd.c), the
> > current_u for MFFD context is NULL here. And that is the reason for
> > the final exception. But I do not understand how this can be since it
> > works perfectly fine with -snes_mf option with no changes in the code.
> >
> > Please do let me know if you think I should specifically be looking
> > for something to tackle this issue. Any help will be appreciated.
> >
> > Thanks,
> > Vijay
> >
> 
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