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

[Fande Kong]

On Tue, Jan 12, 2021 at 6:49 PM Barry Smith <bsmith at petsc.dev> wrote:

>
>    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".
>

I agreed that it is "good enough" for most people for most cases. However,
I was worried that there is almost no way to debug once we reuse active
tags. At least it is not easy to debug.

Thanks,

Fande


>
>   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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