[petsc-users] counter->tag = *maxval - 128

[Barry Smith]

   Fande,

   /* hope that any still active tags were issued right at the beginning of the run */

   PETSc actually starts with *maxval (see line 130). It is only when it runs out that it does this silly thing for the reason indicated in the comment. 

   PETSc should actually keep track which of which tags have been "returned" and if the counter gets to zero use those returned tags instead of starting again at the top which could clash with the same value used for another reason. In other words the current code is buggy,  but it has always been "good enough".

  Barry



> On Jan 12, 2021, at 10:41 AM, Fande Kong <fdkong.jd at gmail.com> wrote:
> 
> Hi All,
> 
> I am curious about why we subtract 128 from the max value of tag? Can we directly use the max tag value?
> 
> Thanks,
> 
> Fande,
> 
> 
> PetscErrorCode  PetscCommGetNewTag(MPI_Comm comm,PetscMPIInt *tag)
> {
>   PetscErrorCode   ierr;
>   PetscCommCounter *counter;
>    PetscMPIInt      *maxval,flg;
> 
> 
>   MPI_Comm_get_attr(comm,Petsc_Counter_keyval,&counter,&flg);
>   if (!flg) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_ARG_CORRUPT,"Bad MPI communicator supplied; must be a PETSc communicator");
> 
>  if (counter->tag < 1) {
>   PetscInfo1(NULL,"Out of tags for object, starting to recycle. Comm reference count %d\n",counter->refcount);
>   MPI_Comm_get_attr(MPI_COMM_WORLD,MPI_TAG_UB,&maxval,&flg);
>     if (!flg) SETERRQ(PETSC_COMM_SELF,PETSC_ERR_LIB,"MPI error: MPI_Comm_get_attr() is not returning a MPI_TAG_UB");
>     counter->tag = *maxval - 128; /* hope that any still active tags were issued right at the beginning of the run */
>   }
> 
>   *tag = counter->tag--;
>    if (PetscDefined(USE_DEBUG)) {
>      /*
>      Hanging here means that some processes have called PetscCommGetNewTag() and others have not.
>       */
>     MPI_Barrier(comm);
>   }
>   return(0);
> }

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