[petsc-users] SNES norm control

[Dave May]

On 12 January 2016 at 14:33, Gideon Simpson <gideon.simpson at gmail.com>
wrote:

> I’m just a bit confused by the documentation
> for SNESConvergenceTestFunction.  the arguments for the xnorm, gnorm, and f
> are passed in, at the current iterate, correct?
>

Yes, but nothing requires you to use them :D


>  I interpreted this as though I had to build by convergence test based on
> those values.
>

This is a misinterpretation. You can ignore all of xnorm, gnorm and fnorm
and define any crazy stopping condition you like.

xnorm, gnorm and fnorm are commonly required for many stopping conditions
and are computed by the snes methods. As such, are readily available and
for efficiency and convenience they are provided to the user (e.g. to avoid
you having to re-compute norms).

Cheers,
  Dave


>
> -gideon
>
> On Jan 12, 2016, at 8:24 AM, Dave May <dave.mayhem23 at gmail.com> wrote:
>
>
>
> On 12 January 2016 at 14:14, Gideon Simpson <gideon.simpson at gmail.com>
> wrote:
>
>> That seems to to allow for me to cook up a convergence test in terms of
>> the 2 norm.
>>
>
> While you are only provided the 2 norm of F, you are also given access to
> the SNES object. Thus inside your user convergence test function, you can
> call SNESGetFunction() and SNESGetSolution(), then you can compute your
> convergence criteria and set the converged reason to what ever you want.
>
> See
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESGetFunction.html
>
>
> http://www.mcs.anl.gov/petsc/petsc-current/docs/manualpages/SNES/SNESGetSolution.html
>
> Cheers,
>   Dave
>
>
>
>
>
>> What I’m really looking for is the ability to change things to be
>> something like the 2 norm of the vector with elements
>>
>> F_i/|x_i|
>>
>> where I am looking for a root of F(x).  I can just build that scaling
>> into the form function, but is there a way to do it without rewriting that
>> piece of the code?
>>
>>
>> -gideon
>>
>> On Jan 12, 2016, at 12:14 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>
>>
>>   You can use SNESSetConvergenceTest() to use whatever test you want to
>> decide on convergence.
>>
>> Barry
>>
>> On Jan 11, 2016, at 3:26 PM, Gideon Simpson <gideon.simpson at gmail.com>
>> wrote:
>>
>> I’m solving nonlinear problem for a complex valued function which is
>> decomposed into real and imaginary parts, Q = u + i v.  What I’m finding is
>> that where |Q| is small, the numerical phase errors tend to be larger.  I
>> suspect this is because it’s using the 2-norm for convergence in the SNES,
>> so, where the solution is already, the phase errors are seen as small too.
>> Is there a way to use something more like an infinity norm with SNES, to
>> get more point wise control?
>>
>> -gideon
>>
>>
>>
>>
>
>
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