[petsc-dev] PETSC_COMM_WORLD not a PETSc communicator

[Barry Smith]

   Junchao,

    Thanks for checking this.  

    Could you use PetscRegisterFinalize()? 

 Barry


> On Aug 14, 2021, at 10:46 PM, Junchao Zhang <junchao.zhang at gmail.com> wrote:
> 
> 
> 
> On Thu, Aug 12, 2021 at 11:22 AM Barry Smith <bsmith at petsc.dev <mailto:bsmith at petsc.dev>> wrote:
> 
>  User visible communicators generally do not have a keyval attached. Rather the keyval is attached to the inner communicator; because we don't want both PETSc and the user doing MPI operations on the same communicator (to prevent mixing up tags).
> 
>   I think PetscShmCommGet() is wrong. I think it should not call MPI_Comm_get_attr(globcomm,Petsc_Counter_keyval,&counter,&flg); but should call PetscCommDuplicate() and use that communicator to stash the pshmcomm; then you would not have the problem you are having.
> Barry, I think it over and find the problem is:  if I PetscCommDuplicate() the outer comm in PetscShmCommGet(), then I can not find a place to destroy the inner communicator (note usually petsc inner communciators are destroyed with petsc objects. Doing what you said breaks the rule)
> 
> PetscShmCommGet() was designed to help doing OpenMP multithreading on a communicator that some petsc objects live in. So requiring the input communicator to be petsc comm is not totally nonsense.
> 
> I tried another approach: Don't check whether the input comm in PetscShmCommGet(globcomm,&pshmcomm) is an outer comm or an inner comm. We instead check its Petsc_ShmComm_keyval. If it does not have one, we just create one for it (along with a new shared memory communicator)
> With that, one is able to call PetscShmCommGet(PETSC_COMM_WORLD, ...).  The problem is we attached an attribute to PETSC_COMM_WORLD. It is deleted inside MPI_Finalize().   PETSc -malloc_dump complains of unfreed memory (since I used PetscMalloc inside PetscShmCommGet). I could
> bypass PetscMalloc and directly use malloc() to avoid this situation. Is it worthy?
>  
> 
> 
> Barry
> 
> 
> 
> > On Aug 12, 2021, at 11:05 AM, Pierre Jolivet <pierre at joliv.et <mailto:pierre at joliv.et>> wrote:
> > 
> > Hello,
> > Is there a specific reason why PETSC_COMM_WORLD is not a PETSc communicator, i.e., has no Petsc_Counter_keyval attached?
> > ierr = PetscOmpCtrlCreate(PETSC_COMM_WORLD,nthreads,&ctrl);CHKERRQ(ierr);
> > yields
> > [0]PETSC ERROR: Bad MPI communicator supplied must be a PETSc communicator
> > [0]PETSC ERROR: #1 PetscShmCommGet() at src/sys/utils/mpishm.c:60
> > [0]PETSC ERROR: #2 PetscOmpCtrlCreate() at src/sys/utils/mpishm.c:340
> > 
> > Thanks,
> > Pierre
> 

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