[petsc-users] About DIVERGED_ITS

[w_ang_temp]

    Thanks very much! I understand.
 
    I made a test. If the tolerance is large, PETSc says "converged" but the result is "incorrect" for my use
 
due to the tolerance. While the tolerance is tight, when the rtol reaches a certain value, the result is
 
not changing and it is not equal to the true value. And PETSc says "diverged". As Jed said, I need to either
 
increase the number of iterations or use a better preconditioner. And I think the latter is a better way in
 
my work.
 
    Thanks.




>At 2012-07-11 01:45:28,"Mark F. Adams" <mark.adams at columbia.edu> wrote:


>On Jul 10, 2012, at 10:51 AM, w_ang_temp wrote:


 
>    In my opinion, convergence in PETSc is decided by rtol, atol and dtol. The divergent hints just show >that
 
>in the solving process it does not satisfy the rule. The "right" result may be different from the true result >at
 
>the several back decimal places(I mean that they may be the same with four decimal places but may be >not
> 
>the same with more decimal places). 


>No, If you have an rtol=1.e-30 and atol=1.e-300 then your solution will probably be correct to all decimal places even >though PETSc will say you "diverged".  You can't be too sure about how many digits of accuracy you have with >simple linear algebra arguments unless your system is extremely well conditioned.


>As Jed said if you are doing 10000 iterations are happy with the solution then your tolerances are probably too >tight.  Look at the residual history and see if you are stagnating.


   
>    Is it right?




>At 2012-07-08 00:28:54,"Mark F. Adams" <mark.adams at columbia.edu> wrote:
>It sounds like your -ksp_rtol is too small.  Experiment with looser tolerances until your solution is >not "correct" to see >how much accuracy you want.


>On Jul 7, 2012, at 12:15 PM, w_ang_temp wrote:


>    Maybe it is a problem of mathematical concept. I compare the result with the true >result which is
 
>computed and validated by other tools. I think it is right if I get the same result.


>>在 2012-07-08 00:03:21，"Matthew Knepley" <knepley at gmail.com> 写道：
>>On Sat, Jul 7, 2012 at 10:00 AM, w_ang_temp <w_ang_temp at 163.com> wrote:

>>Hello,
 
>>    I am a little puzzled that I get the right result while the converged reason says that 'Linear solve >>did not
 
>>converge due to DIVERGED_ITS iterations 10000'. This infomation means that the >>iterations >reach >the maximum
 
>>iterations. But the result is right now. So why says 'did not converge'? Can I think >>that the result is >>right and
 
>>can be used?

>>Obviously, your definition of "right" is not the same as the convergence tolerances >>you are using.


>>    Matt
 
 
>>    Thanks.
   
>>                                            Jim








--
>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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