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

[Dmitry Karpeev]

Maybe Create/Destroy isn't the right solution, but there appears to be
some confusion
about the meaning of Get/Restore in this context.  It definitely
differs from VecGet/RestoreArray.
For example: there is no guarantee that subsequent DAGetXXXs,
punctuated by DARestoreXXXs,
will give one the same object.

Dmitry.

On Fri, Aug 27, 2010 at 10:02 AM, Jed Brown <jed at 59a2.org> wrote:
> On Fri, 27 Aug 2010 09:45:36 -0500, Dmitry Karpeev <karpeev at mcs.anl.gov> wrote:
>> I think DAGetXXX etc should really be thought of as constructors,
>> that under the hood manage a pool to amortize the construction time.
>> Perhaps more precisely, DA acts as a factory.
>> It would be natural to rename them DACreateXXX, except that
>> then XXXDestroy is the wrong thing to do (unless the destructor
>> is overloaded in the corresponding object).
>
> Hmm, I think there is an important distinction, in terms of overall
> memory use, between objects that live a long time and those that do not.
> I might allocate a bunch of memory in a preprocessing stage, release it,
> and then build solver objects.  If all that preprocessing memory stayed
> alive for the life of the program, I would run out of memory.
>
> You can't overload Destroy in place of Restore unless you maintain
> upward links or guarantee that the managing object will never need to do
> anything when you restore.  One way to do this would be to
> double-reference "gotten" objects and consider them to be checked in any
> time the reference count drops to 1.  But this excludes more elaborate
> data structures and extra consistency checks.
>
> I don't see any great nastiness of Get/Restore for managed objects and
> Create/Destroy for objects that the user wants to own.
>
> Jed
>