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

[zakaryah .]

I figured out the cause of my problems with the debugger and the Jacobian
evaluation - I had a silly memory corruption issue which is now fixed.

I am still having trouble with convergence.  I am using a test problem of
intermediate size, which has nearly identical Jacobians when comparing
finite difference to hand-coded.  If I run with -snes_type newtonls
-snes_monitor -ksp_monitor -ksp_ksp_monitor -ksp_monitor_true_residual
-ksp_converged_reason -snes_converged_reason, the very first SNES iteration
fails to converge, due to the KSP failing to converge after 10,000
iterations.

Following the "Why is my iterative solver not converging?" FAQ, I ran the
problem on an even smaller grid, with -snes_type newtonls -snes_monitor
-ksp_monitor -ksp_ksp_monitor -ksp_monitor_true_residual
-ksp_converged_reason -snes_converged_reason -pc_type svd -pc_svd_monitor,
and the linear system was not close to singular.  I don't have any reason
to suspect that the linear system becomes more singular as the grid size
increases.  Next, I ran with -snes_type newtonls -snes_monitor -ksp_monitor
-ksp_ksp_monitor -ksp_monitor_true_residual -ksp_converged_reason
-snes_converged_reason -ksp_gmres_restart 1000 -pc_type none, and the KSP
seemed to converge better, but the SNES only ran for a few iterations
before it stopped with "Nonlinear solve did not converge due to
DIVERGED_LINE_SEARCH iterations 7".

​Can I conclude from this that running the KSP without a pre-conditioner
​is a good idea?  At that point, should I go back to asking why the SNES
doesn't converge, ala the "Why is Newton's method not converging?" FAQ?

Thanks for the continuing help!
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