[petsc-dev] One DM two SNES

[Blaise A Bourdin]

On Nov 1, 2017, at 7:46 PM, Jed Brown <jed at jedbrown.org<mailto:jed at jedbrown.org>> wrote:

Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>> writes:

On Wed, Nov 1, 2017 at 6:50 PM, Blaise A Bourdin <bourdin at lsu.edu<mailto:bourdin at lsu.edu>> wrote:

Hi,

I have just spent 2h helping a student debugging a code, and I think that
the problem is not ours…
See the attached example: 1 create 1 DM and 2 SNES.
If I assign the same DM to both SNES, the function and Jacobean of the
second are ignored, and the first one is used.
Replace the block l 138 - l. 149 with the one commented above, and the
result is even weirder.

Is this the intended behavior? If so, should there be a note of this
behavior in the SNESSetDM man page?

You can DMClone() when solving the other problem.

I am not sure I understand everything this is doing, but I want to make an
upfront point:

 SNESSetFunction() is intended for use without a DM

When the solver has a DM, we intend you to use

 DMSNESSetFunctionLocal() and DMSNESSetJacobianLocal()

This doesn't really matter -- SNESSetFunction just calls
DMSNESSetFunction.  Getting rid of SNESSetFunction would save the
confusion but break every existing PETSc code.

However, now I think I see what is happening. The DMSNES is a structure
that is intended to mediate between the solver
and mesh. When you do SNESSetDM(), it copies over the DMSNES context. This
context is already holding the formfunction
and formjacobian pointers. Yes, this is confusing.

Jed, how should this be documented?

I don't know of a simple rule to prevent this in code.  You can have
several SNES that are "related" in the sense that they help solve a
given problem (nonlinear preconditioners, for example) and they can
share a DM.  If you are solving different problems, you need to create a
different DM.  Does this documentation help?

https://bitbucket.org/petsc/petsc/commits/e03a659ce1e595e4412d70ada3603101a46e94e2

Yes, I think it does. How about adding a  statement in the main DM man page stating that each field should get its SNES and its DM, possibly through cloning a main DM?
I should have thought about it.

Blaise




--
Department of Mathematics and Center for Computation & Technology
Louisiana State University, Baton Rouge, LA 70803, USA
Tel. +1 (225) 578 1612, Fax  +1 (225) 578 4276 http://www.math.lsu.edu/~bourdin







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