That is technically correct (although, as Jeff points out, not at all ideal). In practice, you will probably get the behaviour you expect (i.e. ordering) because of the debate over the meaning of this text - as demonstrated by the comment from Pavan.

Disambiguating this paragraph is on the to-do list for the point-to-point working group. Thanks for bringing this up again and thereby raising its priority for us.

Cheers,

Dan.

—
Dr Daniel Holmes PhD
Applications Consultant in HPC Research
d.holmes@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 11:19, Joachim Protze via mpiwg-p2p <mpiwg-p2p@lists.mpi-forum.org> wrote:

I think, I was once again confused by "may not" in the cited paragraph. As a non-native speaker this hits me from time to time.

So if I understand it right now, the paragraph says that even if the threading semantics provide an ordering, the operations are still logically concurrent and have no ordering.

Thanks
Joachim

On 11/15/18 6:09 PM, Jeff Hammond wrote:

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@lists.mpi-forum.org <mailto:mpiwg-p2p@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@lists.mpi-forum.org
   <mailto:mpiwg-p2p@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@epcc.ed.ac.uk <mailto:d.holmes@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@lists.mpi-forum.org
   <mailto:mpiwg-p2p@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@itc.rwth-aachen.de <mailto:protze@itc.rwth-aachen.de>
   www.itc.rwth-aachen.de <http://www.itc.rwth-aachen.de>

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jeff.science@gmail.com <mailto:jeff.science@gmail.com>
http://jeffhammond.github.io/

--
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@itc.rwth-aachen.de
www.itc.rwth-aachen.de

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