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

[Barry Smith]

On Aug 27, 2010, at 8:13 AM, Aron Ahmadia wrote:

> 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.

   Aron,

    DAGetMatrix() is actually a bug and should be DACreateMatrix()   (or maybe better DACreateMat() while the DACreateGlobalVector() and friends should really be DACreateGlobalVec()). 

    Are there others beside DAGetMatrix() that are incorrect with gets that should be creates?

     Thanks

       Barry

     It would actually be nice if we made DACreateGlobal/LocalVector() so light-weight that it could be used for work vectors (instead of needing a different set of light weight get routines)  but then we would need DADestroyGlobal/Vector() to handle putting back in the free list or need to modify VecDestroy() to handle not actually destroying but managing a free list). And there is also the issue of zeroing or not zeroing the Vec initially.
This is why we still have the Create and Get versions.


> 
> 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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