On Dec 14, 2011, at 4:53 PM, Matthew Knepley wrote: > On Wed, Dec 14, 2011 at 3:50 PM, Barry Smith <bsmith at mcs.anl.gov> wrote: > > On Dec 14, 2011, at 3:13 PM, Mark F. Adams wrote: > > > > > On Dec 14, 2011, at 3:44 PM, Barry Smith wrote: > > > >> > >> On Dec 14, 2011, at 12:45 PM, Mark F. Adams wrote: > >> > >>> I have a nonlinear problem and am finding the the convergence test for > >>> the interior PETSc linear solver is apparently testing against the norm > >>> of the first linear solve (see appended). > >> > >> No it is || r = b - Ax ||/|| b|| with a B in there if left > >> preconditioned > >> > >> It has nothing to do with the "first linear solve". Each linear solve is > >> self contained and it does not use anything from a previous linear solve > >> to determine convergence of a later linear solve. > >> > > > > Matts suggestion -ksp_converged_use_initial_residual_norm did the trick. > > > >>> This is clearly not what anyone would want, > >> > >> Why not? > >> > > > > OK, I should say I don't know why anyone would want this and if you pop a > > KSP into a nonlinear loop, which is a very common thing to do obviously, > > then you get behavior that is probably not what you want. Namely, an > > approximate Newton where the outer nonlinear tolerance is much smaller than > > the inner linear tolerance. > > I would think you would want exactly that. Why oversolve the linear system > since it is "wrong" anyways"? > > Actually with Newton it should really be setting the linear tolerances > based on the progress of the Newton method, this is done via -snes_ksp_ew I > would like to make that the default but this scheme is not robust and > occasionally strange things might happen. I'd like it if someone could > investigate using EW by default. > > I spent a ton of time on EW (Eisenstat-Walker) for my thesis. You are right > that its just not robust, but version 2 is more > robust than version 1 in my tests. EW is just two possible implementations > for the Dembo-Eisenstat-Steihaug theory. I > think Trond has some additional ones that are stuck down deep inside his > bizarre optimization code. I would love to turn on > version 2 by default, but we need a better nonlinear test set to see what > happens. Maybe some fluids thing? >
I've used EW and have had some difficulties. They are a bunch of heuristics and I recently found that my users problems were finding a "hole" and under solving the problems for months, Argh!! I am now more partial to simply using a fixed tolerance: its simple, less error prone and probably with a factor of say 2x of the perfect algorithm. Mark > Matt > > > Barry > > > > > > Just my perspective. > > > > Mark > > > >> Barry > >> > >> > >>> is this the intended semantics? > >> > >> > >>> Is there perhaps a method to reset rtol? > >>> > >>> Mark > >>> > >>> Picard iteration 0 max(resid) = 781020 > >>> 0 KSP Residual norm 2.499264167662e+07 > >>> 1 KSP Residual norm 6.330915810095e+06 > >>> 2 KSP Residual norm 1.856999435466e+06 > >>> 3 KSP Residual norm 3.967325725053e+05 > >>> 4 KSP Residual norm 9.037332775808e+04 > >>> 5 KSP Residual norm 1.826960320400e+04 > >>> Linear solve converged due to CONVERGED_RTOL iterations 5 > >>> Picard iteration 1 max(resid) = 727577 ------- Rate = 1.07345 > >>> 0 KSP Residual norm 6.885328126772e+06 > >>> 1 KSP Residual norm 2.150319244570e+06 > >>> 2 KSP Residual norm 2.297759417371e+05 > >>> 3 KSP Residual norm 3.321819214451e+04 > >>> 4 KSP Residual norm 4.155970071625e+03 > >>> Linear solve converged due to CONVERGED_RTOL iterations 4 > >>> Picard iteration 2 max(resid) = 486788 ------- Rate = 1.49465 > >>> 0 KSP Residual norm 2.987417661953e+06 > >>> 1 KSP Residual norm 5.046119985008e+05 > >>> 2 KSP Residual norm 3.882757664605e+04 > >>> 3 KSP Residual norm 4.106091678345e+03 > >>> Linear solve converged due to CONVERGED_RTOL iterations 3 > >>> Picard iteration 3 max(resid) = 137348 ------- Rate = 3.54418 > >>> 0 KSP Residual norm 7.285359985607e+05 > >>> 1 KSP Residual norm 9.132700545850e+04 > >>> 2 KSP Residual norm 6.834608381922e+03 > >>> Linear solve converged due to CONVERGED_RTOL iterations 2 > >>> Picard iteration 4 max(resid) = 25082.7 ------- Rate = 5.47582 > >>> 0 KSP Residual norm 1.207523847817e+05 > >>> 1 KSP Residual norm 1.286174706329e+04 > >>> Linear solve converged due to CONVERGED_RTOL iterations 1 > >>> Picard iteration 5 max(resid) = 5723.21 ------- Rate = 4.38264 > >>> 0 KSP Residual norm 2.211945822133e+04 > >>> Linear solve converged due to CONVERGED_RTOL iterations 0 > >>> > >> > >> > > > > > > > -- > 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 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.mcs.anl.gov/pipermail/petsc-dev/attachments/20111214/df783d43/attachment.html>
