clarification about convergence

[Matthew Knepley]

On Wed, Dec 9, 2009 at 12:49 PM, Dominik Szczerba <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>> 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
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