[petsc-users] Why PetscDestroy global collective semantics?

[Alberto F. Martín]

Thanks all for your very insightful answers.

We are leveraging PETSc from Julia in a parallel distributed memory 
context (several MPI tasks running the Julia REPL each).

Julia uses Garbage Collection (GC), and we would like to destroy the 
PETSc objects automatically when the GC decides so along the simulation.

In this context, we cannot guarantee deterministic destruction on all 
MPI tasks as the GC decisions are local to each task, no global 
semantics guaranteed.

As far as I understand from your answers, there seems to be the 
possibility to defer the destruction of objects till points in the 
parallel program in which you can guarantee collective semantics, 
correct? If yes I guess that this may occur at any point in the 
simulation, not necessarily at shut down via PetscFinalize(), right?

Best regards,

  Alberto.


On 24/10/21 1:10 am, Jacob Faibussowitsch wrote:
> Depending on the use-case you may also find 
> PetscObjectRegisterDestroy() useful. If you can’t guarantee your 
> PetscObjectDestroy() calls are collective, but have some other 
> collective section you may call it then to punt the destruction of 
> your object to PetscFinalize() which is guaranteed to be collective.
>
> https://petsc.org/main/docs/manualpages/Sys/PetscObjectRegisterDestroy.html 
> <https://petsc.org/main/docs/manualpages/Sys/PetscObjectRegisterDestroy.html>
>
> Best regards,
>
> Jacob Faibussowitsch
> (Jacob Fai - booss - oh - vitch)
>
>> On Oct 22, 2021, at 23:33, Jed Brown <jed at jedbrown.org 
>> <mailto:jed at jedbrown.org>> wrote:
>>
>> Junchao Zhang <junchao.zhang at gmail.com 
>> <mailto:junchao.zhang at gmail.com>> writes:
>>
>>> On Fri, Oct 22, 2021 at 9:13 PM Barry Smith <bsmith at petsc.dev 
>>> <mailto:bsmith at petsc.dev>> wrote:
>>>
>>>>
>>>>  One technical reason is that PetscHeaderDestroy_Private() may call
>>>> PetscCommDestroy() which may call MPI_Comm_free() which is defined 
>>>> by the
>>>> standard to be collective. Though PETSc tries to limit its use of 
>>>> new MPI
>>>> communicators (for example generally many objects shared the same
>>>> communicator) if we did not free those we no longer need when 
>>>> destroying
>>>> objects we could run out.
>>>>
>>> PetscCommDestroy() might call MPI_Comm_free() , but it is very unlikely.
>>> Petsc uses reference counting on communicators, so in 
>>> PetscCommDestroy(),
>>> it likely just decreases the count. In other words, 
>>> PetscCommDestroy() is
>>> cheap and in effect not collective.
>>
>> Unless it's the last reference to a given communicator, which is a 
>> risky/difficult thing for a user to guarantee and the consequences 
>> are potentially dire (deadlock being way worse than a crash) when the 
>> user's intent is to relax ordering for destruction.
>>
>> Alberto, what is the use case in which deterministic destruction is 
>> problematic? If you relax it for individual objects, is there a place 
>> you can be collective to collect any stale communicators?
>
-- 
Alberto F. Martín-Huertas
Senior Researcher, PhD. Computational Science
Centre Internacional de Mètodes Numèrics a l'Enginyeria (CIMNE)
Parc Mediterrani de la Tecnologia, UPC
Esteve Terradas 5, Building C3, Office 215,
08860 Castelldefels (Barcelona, Spain)
Tel.: (+34) 9341 34223
e-mail:amartin at cimne.upc.edu

FEMPAR project co-founder
web: http://www.fempar.org

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