[petsc-users] questions about the SNES Function Norm

[Xiangdong]

It turns out to a be a bug  in my FormFunctionLocal(DMDALocalInfo
*info,PetscScalar **x,PetscScalar **f,AppCtx *user). I forgot to initialize
the array f. Zero the array f solved the problem and gave consistent result.

Just curious, why does not petsc initialize the array f to zero by default
inside petsc when passing the f array to FormFunctionLocal?

I have another quick question about the array x passed to
FormFunctionLocal. If I want to know the which x is evaluated, how can I
output x in a vector format? Currently, I created a global vector vecx and
a local vector vecx_local, get the array of vecx_local_array, copy the x to
vecx_local_array,  scatter to global vecx and output vecx. Is there a quick
way to restore the array x to a vector and output?

Thank you.

Best,
Xiangdong



On Mon, Apr 28, 2014 at 10:28 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> On Apr 28, 2014, at 3:23 PM, Xiangdong <epscodes at gmail.com> wrote:
>
> > Hello everyone,
> >
> > When I run snes program,
>
>                ^^^^ what SNES program”?
>
> > it outputs "SNES Function norm 1.23456789e+10". It seems that this norm
> is different from residue norm (even if solving F(x)=0)
>
>    Please send the full output where you see this.
>
> > and also differ from norm of the Jacobian. What is the definition of
> this "SNES Function Norm”?
>
>    The SNES Function Norm as printed by PETSc is suppose to the 2-norm of
> F(x) - b (where b is usually zero) and this is also the same thing as the
> “residue norm”
>
>    Barry
>
> >
> > Thank you.
> >
> > Best,
> > Xiangdong
>
>
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