[petsc-users] Jacobian test by finite difference

[Zhang, Hong]

Can you post your code (at least some essential snippets) for the residual evaluation and the Jacobian evaluation?

Hong (Mr.)

On May 19, 2020, at 11:08 AM, zakaryah . <zakaryah at gmail.com<mailto:zakaryah at gmail.com>> wrote:

Thanks Matt. I should have said that the boundary is set to BOUNDARY_NONE. I am not sure what you mean about the ordering. I understand that the actual indexing, internal to PETSc, does not match the natural ordering, as explained in the manual. But, doesn't MatView always use the natural ordering? Also, my hand-coded Jacobian routine uses MatSetValuesStencil, and the corresponding output from snes_test_jacobian_display exactly matches what I expect to see - correct layout of nonzero terms according to natural ordering. It is only the finite difference Jacobian that contains the unexpected, off-stencil terms.

This is weird. I run the Jacobian test manually, by adding a small perturbation to the configuration vector at each index and calculating the function for the perturbed configuration, then the result looks like it should. This makes me think there is not a problem with my routine for calculating the function, but I can't explain why the Jacobian test by finite difference that PETSc calculates would be different, and have non-zero values outside the stencil.

On Tue, May 19, 2020 at 10:13 AM zakaryah . <zakaryah at gmail.com<mailto:zakaryah at gmail.com>> wrote:
Thanks Matt. I should have said that the boundary is set to BOUNDARY_NONE. I am not sure what you mean about the ordering. I understand that the actual indexing, internal to PETSc, does not match the natural ordering, as explained in the manual. But, doesn't MatView always use the natural ordering? Also, my hand-coded Jacobian routine uses MatSetValuesStencil, and the corresponding output from snes_test_jacobian_display exactly matches what I expect to see - correct layout of nonzero terms according to natural ordering. It is only the finite difference Jacobian that contains the unexpected, off-stencil terms.

On Tue, May 19, 2020, 5:56 AM Matthew Knepley <knepley at gmail.com<mailto:knepley at gmail.com>> wrote:
On Tue, May 19, 2020 at 1:57 AM zakaryah . <zakaryah at gmail.com<mailto:zakaryah at gmail.com>> wrote:
Hi all,

I'm debugging some convergence issues and I came across something strange. I am using a nonlinear solver on a structured grid. The DMDA is 3 dimensional, with 3 dof, and a box stencil of width 1. There are some small errors in the hand-coded Jacobian, which I am trying to sort out, but at least the fill pattern of the matrix is correct.

However, when I run with -snes_test_jacobian -snes_test_jacobian_display -snes_compare_explicit, I see something very strange. The finite difference Jacobian has large terms outside the stencil. For example, for x,y,z,c = 0,0,0,0 (row 0), the columns 6, 7, 8, 12, 13, and 14 (column 6 => x=2,y=0,z=0,c=0, etc.) have large values, while columns 9 through 20 are calculated but equal to zero. The "correct" values, i.e., in the stencil, are calculated as well, and nearly agree with my hand-coded Jacobian. This issue does NOT occur in serial, but occurs for any number of processors greater than 1.

I have checked the indexing by hand, carefully, and the memory access with valgrind, and the results were clean. Does anyone have an idea why the finite difference calculation of the Jacobian would produce large values outside the stencil? I am using PETSc 3.12.2 and openMPI 3.1.0. Thanks for your help.

Since it only appears in parallel, I am guessing that your calculation of global ordering does not take into account that we locally
reorder, rather than using lexicographic ordering, and you might have periodic boundary conditions.

  Thanks,

     Matt

--
What most experimenters take for granted before they begin their experiments is infinitely more interesting than any results to which their experiments lead.
-- Norbert Wiener

https://www.cse.buffalo.edu/~knepley/<http://www.cse.buffalo.edu/~knepley/>

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