Ryan Yan vyan2000 at gmail.com
Tue Sep 15 22:14:40 CDT 2009

```Hi Barry,
Thank you very much for the explanation. It is good to know there are more

Yan

On Tue, Sep 15, 2009 at 8:32 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Sep 15, 2009, at 6:04 PM, Ryan Yan wrote:
>
> Thanks Jed.
>>
>> And in the term ||Ae||/||Ax||,  PETSc impilictly assume that we use zero
>> as initial guess and here the x really means exact solution, right?
>>
>
>   If you call KSPSetInitialGuessNonzero() then PETSc will use the initial
> value in x as the initial guess. The default convergence tests
> use the ratio of || r_current||/ || b ||. Thus if you start with a great
> initial guess it may not iterate much at all.  You can use also
> -ksp_converged_use_initial_residual_norm
> or KSPDefaultConvergedSetUIRNorm() to have it use the ratio of ||
> r_current||/|| r_initial|| for the relative convergence test.
>
>   You can also provide your own convergence test with
> KSPSetConvergenceTest().
>
>   Barry
>
>
>
>
>> Yan
>>
>>
>>
>>
>> On Tue, Sep 15, 2009 at 6:41 PM, Jed Brown <jed at 59a2.org> wrote:
>> Ryan Yan wrote:
>> > But can you say a little more about what happened in the first scenario,
>> I
>> > did not quite fellow you. Let's say, If I set -ksp_rtol 1e-2, what does
>> this
>> > mean in the left PC case. More specifically, which term will be
>> considered
>> > as a stop creteria for the KSP solve.
>>
>> Look at the first column (preconditioned residual norm).  Notice that
>> this is decreasing by 1e-2 and 1e-3 respectively in your examples.
>>
>> Jed
>>
>>
>>
>
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