Yes, my starting state is 0, not such a good guess... what do you mean by constant +1 and constant -1? Is it the starting/user-defined state vector +1 and -1?
My problem should not be ill posed --- it's the Jacobian of the Navier Stokes operator, with good boundary conditions, at least theoretically --- but still I tried what you suggested. Here is the output using -mat_fd_type_ds -snes_type test: Testing hand-coded Jacobian, if the ratio is O(1.e-8), the hand-coded Jacobian is probably correct. Run with -snes_test_display to show difference of hand-coded and finite difference Jacobian. Norm of matrix ratio 9.17279e-09 difference 2.58883e-06 Norm of matrix ratio 8.79304e-08 difference 2.59233e-05 Norm of matrix ratio 8.87045e-08 difference 2.58875e-05 while using -ksp_rtol 1e-12 -pc_type lu -snes_monitor 0 SNES Function norm 2.981436587230e+00 1 SNES Function norm 1.487397936082e+00 2 SNES Function norm 3.490136703474e-01 3 SNES Function norm 1.303397000344e-01 4 SNES Function norm 3.181190030949e-02 5 SNES Function norm 5.280858553562e-03 6 SNES Function norm 3.168168082141e-04 7 SNES Function norm 1.217775965490e-06 8 SNES Function norm 1.559322624068e-11 do I have to deduce that it's ill posed? thanks gianluca On 18 March 2011 12:54, Jed Brown <jed at 59a2.org> wrote: > On Fri, Mar 18, 2011 at 12:41, Gianluca Meneghello <gianmail at gmail.com> > wrote: >> >> One more thing: what does the three lines stand for then? I was >> thinking it was three Newton steps, but I understand it's not... > > It is your Jacobian evaluated at user-defined state, constant -1.0, and > constant 1.0. I added this to the output for petsc-dev: > $ ./ex5 -snes_type test > Testing hand-coded Jacobian, if the ratio is > O(1.e-8), the hand-coded Jacobian is probably correct. > Run with -snes_test_display to show difference > of hand-coded and finite difference Jacobian. > Norm of matrix ratio 2.77166e-08 difference 2.17871e-07 (user-defined state) > Norm of matrix ratio 1.34123e-08 difference 1.23213e-07 (constant state > -1.0) > Norm of matrix ratio 1.31875e-07 difference 9.05494e-07 (constant state 1.0) > [0]PETSC ERROR: SNESSolve() line 2360 in > /home/jed/petsc/src/snes/interface/snes.c > [0]PETSC ERROR: main() line 220 in src/snes/examples/tutorials/ex5.c > >> >> The output I sent was actually the output of one of your examples. The >> one from my own code is >> >> Testing hand-coded Jacobian, if the ratio is >> O(1.e-8), the hand-coded Jacobian is probably correct. >> Run with -snes_test_display to show difference >> of hand-coded and finite difference Jacobian. >> Norm of matrix ratio 9.17257e-08 difference 2.58876e-05 > > Is your starting state zero? > >> >> Norm of matrix ratio 1.45865e-06 difference 0.000430034 >> Norm of matrix ratio 1.47148e-06 difference 0.000429436 > > If your problem is ill-conditioned, this might come from differencing error. > Try with -mat_fd_type ds to use a slightly more robust scheme. You can also > run with -ksp_rtol 1e-12 -pc_type lu -snes_monitor and check if you see > quadratic convergence. -- "[Je pense que] l'homme est un monde qui vaut des fois les mondes et que les plus ardentes ambitions sont celles qui ont eu l'orgueil de l'Anonymat" -- Non omnibus, sed mihi et tibi Amedeo Modigliani
