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

[Matthew Knepley]

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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