[petsc-users] vec norm for local portion of a vector

[Xiangdong]

On Fri, Jul 29, 2016 at 10:41 AM, Barry Smith <bsmith at mcs.anl.gov> wrote:

>
> > On Jul 27, 2016, at 4:42 PM, Xiangdong <epscodes at gmail.com> wrote:
> >
> > Hello everyone,
> >
> > I have a global dmda vector vg. On each processor, if I want to know the
> norm of local portion of vg, which function should I call?
> >
> > So far I am thinking of using DMDAVecGetArray and then write a loop to
> compute the norm of this local array.
> >
> > Is there a simple function available to call? like
> *vg->ops->norm_local(vg,NORM_2, &normlocal)?
>
> There isn't a public interface to this call because it really isn't a
> mathematically well defined object; the subdomains in the decomposition of
> the array are arbitrary based on the number of processes used.
>
>    Anyways if you want it and it is the NON-overlapping portion then yes,
> you can write a little routine (basically just cut and paste VecNorm())
> call it say VecNormLocal() and have it call the function pointer you
> indicated above. Note for the 2 norm the norm_local() returns the square of
> the norm so you need to take the square root.
>

I am interested in this non-overlapping case. I found that this
norm_local() function returns the correct l2 norm, not the square of norm.
I am using old version 3.5. Are there changes in recent version such that
norm_local() returns the square of the norm?

Xiangdong


>
>    If you want the overlapping portion of the vector then you should just
> do the DMDAVecGetArray() as you already do.
>
>    Barry
>
>
>
> >
> > Thanks.
> >
> > Best,
> > Xiangdong
> >
>
>
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