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<div class="">On Nov 1, 2017, at 7:46 PM, Jed Brown <<a href="mailto:jed@jedbrown.org" class="">jed@jedbrown.org</a>> wrote:</div>
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<div class="">Matthew Knepley <<a href="mailto:knepley@gmail.com" class="">knepley@gmail.com</a>> writes:<br class="">
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<blockquote type="cite" class="">On Wed, Nov 1, 2017 at 6:50 PM, Blaise A Bourdin <<a href="mailto:bourdin@lsu.edu" class="">bourdin@lsu.edu</a>> wrote:<br class="">
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<blockquote type="cite" class="">Hi,<br class="">
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I have just spent 2h helping a student debugging a code, and I think that<br class="">
the problem is not ours…<br class="">
See the attached example: 1 create 1 DM and 2 SNES.<br class="">
If I assign the same DM to both SNES, the function and Jacobean of the<br class="">
second are ignored, and the first one is used.<br class="">
Replace the block l 138 - l. 149 with the one commented above, and the<br class="">
result is even weirder.<br class="">
<br class="">
Is this the intended behavior? If so, should there be a note of this<br class="">
behavior in the SNESSetDM man page?<br class="">
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You can DMClone() when solving the other problem.<br class="">
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<blockquote type="cite" class="">I am not sure I understand everything this is doing, but I want to make an<br class="">
upfront point:<br class="">
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SNESSetFunction() is intended for use without a DM<br class="">
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When the solver has a DM, we intend you to use<br class="">
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DMSNESSetFunctionLocal() and DMSNESSetJacobianLocal()<br class="">
</blockquote>
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This doesn't really matter -- SNESSetFunction just calls<br class="">
DMSNESSetFunction. Getting rid of SNESSetFunction would save the<br class="">
confusion but break every existing PETSc code.</div>
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<blockquote type="cite" class="">However, now I think I see what is happening. The DMSNES is a structure<br class="">
that is intended to mediate between the solver<br class="">
and mesh. When you do SNESSetDM(), it copies over the DMSNES context. This<br class="">
context is already holding the formfunction<br class="">
and formjacobian pointers. Yes, this is confusing.<br class="">
<br class="">
Jed, how should this be documented?<br class="">
</blockquote>
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I don't know of a simple rule to prevent this in code. You can have<br class="">
several SNES that are "related" in the sense that they help solve a<br class="">
given problem (nonlinear preconditioners, for example) and they can<br class="">
share a DM. If you are solving different problems, you need to create a<br class="">
different DM. Does this documentation help?<br class="">
<br class="">
<a href="https://bitbucket.org/petsc/petsc/commits/e03a659ce1e595e4412d70ada3603101a46e94e2" class="">https://bitbucket.org/petsc/petsc/commits/e03a659ce1e595e4412d70ada3603101a46e94e2</a><br class="">
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<div>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?</div>
<div>I should have thought about it.</div>
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<div>Blaise</div>
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-- </div>
<div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; " class="">
Department of Mathematics and Center for Computation & Technology</div>
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Louisiana State University, Baton Rouge, LA 70803, USA</div>
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Tel. +1 (225) 578 1612, Fax +1 (225) 578 4276 <a href="http://www.math.lsu.edu/~bourdin" class="">
http://www.math.lsu.edu/~bourdin</a></div>
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