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

[Barry Smith]

  Yes, definitely not easy to debug. If you have concerns in your code about running out and then producing bugs you can fix it  by just keeping an array of returned tags and use these tags when needed. You'll also have to add something like PetscObject/CommRestoreTag() to return ones no longer needed.

  Barry

> On Jan 13, 2021, at 12:04 PM, Fande Kong <fdkong.jd at gmail.com> wrote:
> 
> 
> 
> On Tue, Jan 12, 2021 at 6:49 PM Barry Smith <bsmith at petsc.dev <mailto: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 <mailto: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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