clarification about convergence

[Dominik Szczerba]

Great hint, I found the tolerances not as I was setting them...

Which brings me to the problem how to consistently set them. I am 
calling first:

PetscOptionsInsertString()

where I obtain the string from a user config file, and then am calling 
several times later:

KSPSetFromOptions()

I found experimentally that only the options ABSENT in the config string 
are eventually accepted from the command line. Those that were PRESENT 
will not be overwriten from the commandline - behavior not quite 
intuitive. Or am I doing something wrong?

Is there a way to completely disable processing of the options from the 
command line? Or at least to specify what has higher priority? I prefer 
to set the options only in one way not to confuse the users.

Many thanks,
Dominik

Matthew Knepley wrote:
> On Wed, Dec 9, 2009 at 12:49 PM, Dominik Szczerba <dominik at itis.ethz.ch 
> <mailto:dominik at itis.ethz.ch>> wrote:
> 
>     Excellent point. Where it broke the reason was:
> 
>     Starting KSPSolve
>      0 KSP Residual norm 7.841852910113e-04
>      1 KSP Residual norm 8.927576540999e-05
>      2 KSP Residual norm 1.694401713084e-05
>      3 KSP Residual norm 5.069425875318e-06
>     Linear solve converged due to CONVERGED_RTOL iterations 3
> 
>     which I assume means that the norm became exactly 0
> 
> 
> I don't think so. It would have printed that for the residual.
>  
> 
>     HOWEVER: putting -ksp_rtol -1 still does not eliminate the problem.
>     Any further suggestions?
> 
> 
> You -ksp_view to see the tolerances used.
> 
>   Matt
>  
> 
>     Dominik
> 
>     Matthew Knepley wrote:
> 
>         Did you use -ksp_converged_reason to determine why PETSc stopped
>         iterating?
> 
>          Matt
> 
>         On Wed, Dec 9, 2009 at 11:45 AM, Dominik Szczerba
>         <dominik at itis.ethz.ch <mailto:dominik at itis.ethz.ch>
>         <mailto:dominik at itis.ethz.ch <mailto:dominik at itis.ethz.ch>>> wrote:
> 
>            I am using
> 
>            -ksp_rtol 0 -ksp_atol 1e-6 (defaults for all the rest)
> 
>            and observe for most of the times convergence, as expected.
> 
>            Starting KSPSolve
>             0 KSP Residual norm 6.315132950112e-04
>             1 KSP Residual norm 9.139539245601e-05
>             2 KSP Residual norm 1.869750068013e-05
>             3 KSP Residual norm 6.788145356909e-06
>             4 KSP Residual norm 1.729115712274e-06
>             5 KSP Residual norm 7.214445966641e-07
>            solved in 2.328146e+00/4.144644e+00 s
>            solution converged in 5 iterations
>            ||Ax-b|| = 7.21445e-07
> 
>            with ||...|| being residue computed on my own for a check.
> 
>            However, SOMETIMES, I do observe:
> 
>            Starting KSPSolve
>             0 KSP Residual norm 1.228562818225e-02
>             1 KSP Residual norm 4.045833318123e-04
>             2 KSP Residual norm 6.589999324657e-05
>             3 KSP Residual norm 1.494849372638e-05
>             4 KSP Residual norm 4.311592172896e-06
>            solved in 1.368086e+00/2.235830e+00 s
>            solution converged in 4 iterations
>            ||Ax-b|| = 4.31159e-06
> 
>            which means that the iterations break sooner than they should.
>            Again, this is not often, but worries me why this should
>         happen...
>            Using or not -ksp_converged_use_initial_residual_norm does
>         not make
>            any difference to this behavior: the last output Residual norm is
>            sometimes higher than the requested atol.
> 
>            Any clarifications are highly appreciated.
> 
>            Dominik
> 
> 
> 
> 
>         -- 
>         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
> 
> 
> 
> 
> 
> -- 
> 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