Jed Brown jedbrown at mcs.anl.gov
Tue Jul 10 12:12:13 CDT 2012

```Possibilities (not mutually exclusive)

1. the convergence tolerance is too tight to converge in double precision
arithmetic, so the solver is stagnating (you could try
--with-precision=__float128)

2. the tolerance is too tight to converge in 10000 iterations (either
increase the number of iterations or use a better preconditioner

3. the tolerance is tighter than necessary to get a solution that is "right
enough" for you (try loosening the tolerance)

On Tue, Jul 10, 2012 at 9:51 AM, w_ang_temp <w_ang_temp at 163.com> 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).
>
>     Is it right?
>
>
>
> >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
> >-- Norbert Wiener
>
>
>
>
>
>
>
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