[petsc-users] non-convergence for SNES ex5 and ex20

[Jed Brown]

On Wed, Jun 22, 2011 at 17:32, Qian Zhu <qzhu at mcs.anl.gov> wrote:

> Thanks for the reply, Jed. I worked on ex27 so I know setting certain
> values for the input parameters can make the case converge/non-converge...I
> am just wondering whether this is also true with ex5 and ex20 (We picked
> them for other interesting characteristics).


Well, ex5 is a really simple problem which converges pretty easily when
lambda < 6.81 (the bifurcation point). I don't know how hard ex20 is, but
you could certainly increase the power beta.

What characteristics are you looking for? Asking for a solver that doesn't
converge just seems like an odd request considering that we spend lots of
time figuring out how to make them converge robustly.


> Which examples are solving the Stokers problem?


src/ksp/ksp/examples/tutorials/ex43.c

This will happily not converge:

$ ./ex43 -mx 40 -my 40 -c_str 2 -sinker_eta0 1e-8 -sinker_eta1 1
-stokes_ksp_monitor_true_residual -stokes_ksp_type fgmres

Run this to see some configurations that do work:

$ make -n runex43 runex43_2


The discretization above is not particularly robust, but is relatively easy
to solve. This one is harder [1]

src/ksp/ksp/examples/tests/ex11.c

This will pretend to converge, but the true residuals are nonsense.

./ex11 -f $DATAFILESPATH/matrices/underworld32.gz
-fc_ksp_monitor_true_residual

If you add -fc_ksp_type fgmres to the above, it will stop pretending (and
just not converge). To see some solver configurations that work, use

$ make -n runex11 runex11_2


[1] Follow the directions here to get the test matrix:
http://www.mcs.anl.gov/petsc/petsc-2/documentation/faq.html#datafiles
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