[petsc-dev] Swarm tag error

[Dave May]

On Wed, 23 Nov 2022 at 08:57, Junchao Zhang <junchao.zhang at gmail.com> wrote:

> From my reading, the code actually does not need multiple tags. You can
> just let _get_tags() return a constant (say 0), or use your modulo
> MPI_TAG_UB approach.
>

Yes I believe that is correct.



>
> 541 for (i = 0; i < np; ++i) PetscCallMPI(MPI_Isend(&de->
> messages_to_be_sent[i], 1, MPIU_INT, de->neighbour_procs[i], de->send_tags[i],
> de->comm, &de->_requests[i]));
> 542 for (i = 0; i < np; ++i) PetscCallMPI(MPI_Irecv(&de->
> messages_to_be_recvieved[i], 1, MPIU_INT, de->neighbour_procs[i], de->
> recv_tags[i], de->comm, &de->_requests[np + i]));
>
> --Junchao Zhang
>
>
> On Tue, Nov 22, 2022 at 11:59 PM Matthew Knepley <knepley at gmail.com>
> wrote:
>
>> On Tue, Nov 22, 2022 at 11:23 PM Junchao Zhang <junchao.zhang at gmail.com>
>> wrote:
>>
>>> I don't understand why you need so many tags.  Is the
>>> communication pattern actually MPI_Alltoallv, but you implemented it in
>>> MPI_Send/Recv?
>>>
>>
>> I am preserving the original design from Dave until we do a more thorough
>> rewrite. I think he is using a different tag for each pair of processes to
>> make debugging easier.
>>
>> I don't think Alltoallv is appropriate most of the time. If you had a lot
>> of particles with a huge spread of velocities then you could get that, but
>> most
>> scenarios I think look close to nearest neighbor.
>>
>>   Thanks,
>>
>>       Matt
>>
>>
>>> --Junchao Zhang
>>>
>>>
>>> On Mon, Nov 21, 2022 at 2:37 PM Matthew Knepley <knepley at gmail.com>
>>> wrote:
>>>
>>>> In data_ex.c, Swarm uses a distinct tag for each pair of processes. If
>>>> the number of processes exceeds 1024, there are > 1024^2 tags which exceeds
>>>> MPI_TAG_UB on Intel MPI.
>>>>
>>>> My solution is going to be to use that process pair number modulo
>>>> MPI_TAG_UB. Does anyone have a slicker suggestion?
>>>>
>>>>   Thanks,
>>>>
>>>>       Matt
>>>>
>>>> --
>>>> What most experimenters take for granted before they begin their
>>>> experiments is infinitely more interesting than any results to which their
>>>> experiments lead.
>>>> -- Norbert Wiener
>>>>
>>>> https://www.cse.buffalo.edu/~knepley/
>>>> <http://www.cse.buffalo.edu/~knepley/>
>>>>
>>>
>>
>> --
>> What most experimenters take for granted before they begin their
>> experiments is infinitely more interesting than any results to which their
>> experiments lead.
>> -- Norbert Wiener
>>
>> https://www.cse.buffalo.edu/~knepley/
>> <http://www.cse.buffalo.edu/~knepley/>
>>
>
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