[petsc-users] Why PetscDestroy global collective semantics?

[Barry Smith]

  Ahh, this makes perfect sense.

  The code for PetscObjectRegisterDestroy() and the actual destruction (called in PetscFinalize()) is very simply and can be found in src/sys/objects/destroy.c PetscObjectRegisterDestroy(), PetscObjectRegisterDestroyAll(). 

  You could easily maintain a new array like PetscObjectRegisterGCDestroy_Objects[] and add objects with PetscObjectRegisterGCDestroy() and then destroy them with PetscObjectRegisterDestroyGCAll(). The only tricky part is that you have to have, in the context of your Julia MPI, make sure that PetscObjectRegisterDestroyGCAll() is called collectively over all the MPI ranks (that is it has to be called where all the ranks have made the same progress on MPI communication) that have registered objects to destroy, generally PETSC_COMM_ALL.  We would be happy to incorporate such a system into the PETSc source with a merge request.

  Barry

> On Oct 23, 2021, at 10:40 PM, Alberto F. Martín <amartin at cimne.upc.edu> wrote:
> 
> 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 <mailto:e-mail:amartin at cimne.upc.edu>
> 
> FEMPAR project co-founder
> web: http://www.fempar.org <http://www.fempar.org/> 
> 
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