[mpiwg-p2p] Ordering of P2P messages in multithreaded applications

[Jeff Hammond]

Dan has convinced me that the MPI standard is terrible and, while my
original interpretation is what we want and which is consistent with the
principle of least surprise, it is not guaranteed by the following text.

Per our discussion, there are a few options:
1) make all MPI_Send logically concurrent, even on a single thread.  this
will break stuff and make people sad.
2) force MPI to order injection <somehow>, which might for some
implementations to add more memory ordering on the send path than they
want, particularly if they do not have a TSO memory model.
3) add something like MPI_Win_sync that logically orders sends from
multiple threads explicitly.
4) add MPI_THREAD_SERIALIZED_WITH_EXTRA_SAUCE that does the equivalent of 2
or 3 and thus doesn't cause a performance regression in
MPI_THREAD_SERIALIZED.

Jeff

*If a process has a single thread of execution, then any two communications
executed by this process are ordered. On the other hand, if the process is
multithreaded, then the semantics of thread execution may not define a
relative order between two send operations executed by two distinct
threads. The operations are logically concurrent, even if one physically
precedes the other. In such a case, the two messages sent can be received
in any order. Similarly, if two receive operations that are logically
concurrent receive two successively sent messages, then the two messages
can match the two receives in either order. *

On Thu, Nov 15, 2018 at 10:55 AM Balaji, Pavan via mpiwg-p2p <
mpiwg-p2p at lists.mpi-forum.org> wrote:

> Dan,
>
> The matching *is* ordered in this case.  So the program will print 0
> followed by 1.
>
> MPI does not order delivery of the actual data, but the first message is
> guaranteed to go into the first buffer.  If the second message ends up
> going first, the MPI implementation will need to buffer it.
>
>   — Pavan
>
> Sent from my iPhone
>
> On Nov 15, 2018, at 7:56 AM, HOLMES Daniel via mpiwg-p2p <
> mpiwg-p2p at lists.mpi-forum.org> wrote:
>
> Hi Joachim,
>
> There is no guarantee of ordering between the two sends because they are
> logically concurrent. If they were issued on the same thread then MPI
> guarantees delivery order will be identical to the sequential issuing order.
>
> Many MPI libraries are very likely to deliver these messages "in order”,
> that is, the first one to be called chronologically at the sender process
> is likely to leave first and therefore likely to arrive first. Interleaving
> execution of the sending threads may change the issuing order on the
> network and out-of-order networks may change the order of arrival.
>
> On the other hand, if an MPI implementation is internally using sequence
> numbers (or a similar mechanism) to enforce ordering for the same-thread
> case, then it may also (incidentally) reconstruct the issuing order for
> this case. However, you cannot rely on this behaviour being portable from
> system to system or from MPI library to MPI library.
>
> If you wish to enforce a particular ordering of these messages, then you
> can use tags to differentiate each from the other. There is an argument for
> always using tags in this type of situation to increase program readability.
>
> Cheers,
> Dan.
> —
> Dr Daniel Holmes PhD
> Applications Consultant in HPC Research
> d.holmes at epcc.ed.ac.uk
> Phone: +44 (0) 131 651 3465
> Mobile: +44 (0) 7940 524 088
> Address: Room 2.09, Bayes Centre, 47 Potterrow, Central Area, Edinburgh,
> EH8 9BT
> —
> The University of Edinburgh is a charitable body, registered in Scotland,
> with registration number SC005336.
> —
>
> On 15 Nov 2018, at 04:16, Joachim Protze via mpiwg-p2p <
> mpiwg-p2p at lists.mpi-forum.org> wrote:
>
> Hi all,
>
> I have a question on the "Semantics of Point-to-Point Communication" in a
> multithreaded context.
>
> For me the situation for the code below is not clear, especially with
> respect to the paragraph in MPI-3.1 p.41, l.10-17 :
>
>
> void test(int rank) {
>  int msg = 0;
>  if (rank == 0) {
> #pragma omp parallel num_threads(2)
> #pragma omp critical
>    {
>      MPI_Send(&msg, 1, MPI_INT, 1, 42, MPI_COMM_WORLD);
>      msg++;
>    }
>  } else if (rank == 1) {
>    MPI_Recv(&msg, 1, MPI_INT, 0, 42, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
>    printf("Received %i\n", msg);
>    MPI_Recv(&msg, 1, MPI_INT, 0, 42, MPI_COMM_WORLD, MPI_STATUS_IGNORE);
>    printf("Received %i\n", msg);
>  }
> }
>
> Two threads on the first process send a message, the first thread sends 0,
> the second thread send 1. From OpenMP semantics, the first send happens
> before the second send.
>
> Is there a guarantee, that the other process receives the 0 first?
>
> Thanks,
> Joachim
>
>
> --
> Dipl.-Inf. Joachim Protze
>
> IT Center
> Group: High Performance Computing
> Division: Computational Science and Engineering
> RWTH Aachen University
> Seffenter Weg 23
> D 52074  Aachen (Germany)
> Tel: +49 241 80- 24765
> Fax: +49 241 80-624765
> protze at itc.rwth-aachen.de
> www.itc.rwth-aachen.de
>
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-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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