[petsc-users] A number of questions about DMDA with SNES and Quasi-Newton methods

[zakaryah .]

Thanks Jed, that seems like it will work.

I still have a problem, which I suspect is pretty simple to solve.  To
initialize the solve, I need to run the SNES once without the control
parameter.  My composite DM is dac, the DMDA inside dac is dah, the
redundant field is dab, and I have the vectors set up for the solve and the
residual.  But I don't know how to setup the matrices for the Jacobian, in
the main loop, i.e. in the scope from which SNESSolve is called.  I think I
have the matrix set up properly for the composite, and the evaluation
routine for the composite system extracts the submatrices as we discussed.
But I'm not sure how to setup a global matrix for the first solve, which is
in a smaller space.  I've gotten a lot out of SNES ex28 but there the
smaller solve and the full solve never run in the same launch.  I guess I
can create two matrices for the Jacobians, one like DMCreateMatrix(dac,&J)
for the full system and DMCreateMatrix(dah,&J_hh) for the initial solve.
Is there a more efficient way - can I just create J and somehow extract the
global submatrix J_hh?

On Tue, Oct 24, 2017 at 11:01 PM, Jed Brown <jed at jedbrown.org> wrote:

> Hmm, this is a use case we would like to support, but I'm not sure how
> to do it.  I think at this point that your local submatrix will only
> have a stencil set if you were using MatNest, which I don't recommend
> (whatever you do, don't depend on it).  But since you have the DMDA for
> your J_hh block, you can call MatSetStencil() yourself.
>
>   ierr = DMDAGetGhostCorners(da,&starts[0],&starts[1],&starts[
> 2],&dims[0],&dims[1],&dims[2]);CHKERRQ(ierr);
>   ierr = MatSetStencil(A,dim,dims,starts,dof);CHKERRQ(ierr);
>
> I don't know of code you could use to translate the stencil coordinates
> to local indices (it's just a lexicographic ordering so simple for you
> to write), but you can use MatSetValuesLocal on the J_hr/J_rh blocks.
>
> "zakaryah ." <zakaryah at gmail.com> writes:
>
> > Well I made a little progress by considering SNES ex28.c.  In the
> Jacobian
> > routine, I call DMCompositeGetLocalISs, then use the IS to call
> > MatGetLocalSubMatrix.  I call these J_rr, J_rh, J_hr, and J_hh, where r
> > represents the redundant variables and h represents the displacements.  I
> > assume I can call MatSetValuesStencil on J_hh, as before, and MatSetValue
> > on J_rr (which is 1x1).  I'm guessing that J_rr, J_rh, and J_hr can only
> be
> > set on the processor which owns the redundant variable - is this correct?
> > How do I determine the ordering for J_hr and J_rh?
> >
> > On Tue, Oct 24, 2017 at 2:45 PM, zakaryah . <zakaryah at gmail.com> wrote:
> >
> >> I see - I use a local variable to add up the terms on each process, then
> >> call MPI_Reduce within the function on process 0, which owns the
> redundant
> >> variable.
> >>
> >> I have one more question - for the calculation of the Jacobian, my life
> is
> >> made much much easier by using MatSetValuesStencil.  However, the matrix
> >> which the SNES Jacobian receives as argument is the "full" matrix,
> >> containing the DMDA variables (displacements), plus the redundant
> >> variable.  How do I access the submatrix corresponding just to the DMDA?
> >> If I can do that, then I can call MatSetValuesStencil on the
> submatrix.  Is
> >> this the right approach?  I'm not sure how to set the elements of the
> >> Jacobian which correspond to the redundant variable, either - i.e., how
> do
> >> I get the ordering?
> >>
> >>
>
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