[petsc-users] Cannot iterate well when using Newton iteration of SNES

[Jed Brown]

You should add VecZeroEntries(f) near the top of your FormFunction (it's currently accumulating into whatever was there last) and MatZeroEntries(B) to FormJacobian.

I reduced to nElem = 5 for ease of viewing. With these changes, I see quadratic convergence but the problem is still nonlinear. To explore further, consider using these diagnostics

./SNES_heat -{snes,ksp}_monitor -{snes,ksp}_converged_reason -snes_linesearch_monitor -ksp_view_mat

with and without -snes_fd.

For readability, I would suggest consistency in "u" vs "x".

"David Jiawei LUO LIANG"	<12431140 at mail.sustech.edu.cn> writes:

> I am using the Newton iteration to solve a nonlinear 1D heat equation problem by using FEM.
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> I attached my source code named "SNES_heat.cpp" 
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> when I run the code
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>   0 SNES Function norm 1.206289245288e+01
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>   1 SNES Function norm 7.128802192789e+00
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>   2 SNES Function norm 6.608812909525e+00
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> you can find that it only iterate 3 steps, and then do all the function evaluation and finally just stop the program. 
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> I think it is not reasonble. I check my code, it is correct if I set it as a linear problem. it means my Jacobian and Residual function is correct.
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> But when I set it as a nonlinear, the residual seems reduces as not expected. 
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> I doubt that whether my understanding of the newton iteration is different from SNES's newton iteration process.
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> David Jiawei LUO LIANG
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> 南方科技大学/学生/研究生/2024
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> 广东省深圳市南山区学苑大道1088号
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