> On Aug 29, 2017, at 9:49 PM, zakaryah . <[email protected]> wrote: > > 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?
-pc_type none is almost always a bad 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? Use -pc_type lu and send all the output with -snes_monitor -snes_linesearch_monitor -ksp_monitor Step one is to get Newton converging well, step 2 is to optimize the linear solver, but never do step 2 before step 1, hence stick to LU until Newton is converging well. > > Thanks for the continuing help! >
