[petsc-users] help needed for -snes_mf_operator

[Dominik Szczerba]

>> Now I want to reproduce the same results without providing the
>> Jacobian, preparing for future real life cases where it will not be
>> known. I am under the impression that Petsc can approximate the
>> Jacobian itself using some sort of secant method. How exactly am I
>> supposed to use it? If I change nothing in the code, just pass
>> -snes_mf_operator to the options, the solver stagnates. The same if I
>> fill the Jacobian with zeros. The same if only with A. What am I
>> supposed to do without the known Jacobian to trigger its
>> approximation?
>
>
> Run with -ksp_monitor_true_residual -ksp_converged_reason -snes_monitor
> -snes_converged_reason -snes_ls_monitor -snes_view and send the output.

I see the following stagnated output:

0 SNES Function norm 1.747536282885e+01
  0 KSP unpreconditioned resid norm 1.747536282885e+01 true resid norm
1.747536282885e+01 ||r(i)||/||b|| 1.000000000000e+00
  1 KSP unpreconditioned resid norm 6.112827145626e-01 true resid norm
1.751036369876e+01 ||r(i)||/||b|| 1.002002869425e+00
  2 KSP unpreconditioned resid norm 6.046520497681e-01 true resid norm
2.300357730584e+01 ||r(i)||/||b|| 1.316343330386e+00
  3 KSP unpreconditioned resid norm 5.994471388765e-01 true resid norm
5.655832090079e+01 ||r(i)||/||b|| 3.236460464638e+00
  4 KSP unpreconditioned resid norm 5.974054975332e-01 true resid norm
5.997203971117e+01 ||r(i)||/||b|| 3.431805124649e+00
  5 KSP unpreconditioned resid norm 5.956696306552e-01 true resid norm
9.280866979439e+01 ||r(i)||/||b|| 5.310829348914e+00
  6 KSP unpreconditioned resid norm 5.944323800377e-01 true resid norm
9.573083746155e+01 ||r(i)||/||b|| 5.478045772159e+00
  7 KSP unpreconditioned resid norm 5.933318368437e-01 true resid norm
1.283420019927e+02 ||r(i)||/||b|| 7.344168086790e+00
  8 KSP unpreconditioned resid norm 5.924396174526e-01 true resid norm
1.311457478032e+02 ||r(i)||/||b|| 7.504608006575e+00
  9 KSP unpreconditioned resid norm 5.916244555561e-01 true resid norm
1.636933991914e+02 ||r(i)||/||b|| 9.367095882044e+00
 10 KSP unpreconditioned resid norm 5.909266031948e-01 true resid norm
1.664685701658e+02 ||r(i)||/||b|| 9.525900652028e+00
 11 KSP unpreconditioned resid norm 5.902780068683e-01 true resid norm
1.989974529969e+02 ||r(i)||/||b|| 1.138731452650e+01
 12 KSP unpreconditioned resid norm 5.897052848025e-01 true resid norm
2.017662138832e+02 ||r(i)||/||b|| 1.154575248933e+01
 13 KSP unpreconditioned resid norm 5.891666923831e-01 true resid norm
2.342886435057e+02 ||r(i)||/||b|| 1.340679708915e+01

and so on, excessively long

> Your SNES callback gets two Mat arguments, into which are you assembling
> your approximate Jacobian?

I am assembling my (exact) Jacobian in the second (preconditioner) matrix.

> Are you still calling MatAssemblyBegin/End on the
> first argument, as in the examples?

Yes, I do.

The moment I remove -snes_mf_operator from the options I get the
expected results in the expected time.

Dominik