[petsc-dev] What's the point of D(A/M)GetGlobalVector?

[Aron Ahmadia]

Not to mention the various Get routines that are actually used to create
things, such as DAGetMatrix.  Still, the idea of a pool of work vectors
makes sense, I was just trying to wrap my head around the actual use for
those routines.

A

On Fri, Aug 27, 2010 at 4:06 PM, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:

> Except VecGetArray, etc, which operate a "pool" of one object.
> I think this may be the root cause of confusion.
>
> Dmitry.
>
> On Fri, Aug 27, 2010 at 6:42 AM, Matthew Knepley <knepley at gmail.com>
> wrote:
> > Simply, in PETSc, getFoo() and restoreFoo() operate an object pool.
> >    Matt
> >
> > On Fri, Aug 27, 2010 at 11:23 AM, Jed Brown <jed at 59a2.org> wrote:
> >>
> >> On Fri, 27 Aug 2010 14:13:01 +0300, Aron Ahmadia
> >> <aron.ahmadia at kaust.edu.sa> wrote:
> >> > What exactly is the purpose of these routines then?  Is there a global
> >> > Vector associated with a DA?  If so, why are the values uninitialized?
> >>
> >> It's common to need work vectors in places like residual evaluation and
> >> Jacobian assembly.  There is a little bit of setup cost to allocate a
> >> new vector each time, so usually we'd prefer that they be persistent and
> >> just reuse them.  One option would be to make the user manage this
> >> themselves, but that's error prone because it's easy to accidentally
> >> alias the work vectors, so instead the DA keeps a cache of vectors.  It
> >> starts out empty, and each time you call DAGetGlobalVector(), the cache
> >> is searched for an available vector.  If none are found, a new one is
> >> allocated and the cache grows by one.  DARestoreGlobalVector() checks a
> >> vector back in so it may be used elsewhere.  These vectors are destroyed
> >> in DADestroy().
> >>
> >> Jed
> >
> >
> >
> > --
> > What most experimenters take for granted before they begin their
> experiments
> > is infinitely more interesting than any results to which their
> experiments
> > lead.
> > -- Norbert Wiener
> >
>
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