[mpiwg-p2p] Ordering of P2P messages in multithreaded applications
Joachim Protze
protze at itc.rwth-aachen.de
Thu Nov 15 11:57:56 CST 2018
I guess the paragraph is even more confusing than I thought. As I
understand after reading some English grammar pages (oxford/cambridge),
"may not" can be prohibitive like "shall not"/"must not"/"cannot" or
optional like "might not".
Searching the MPI standard for use of "may not", it is most often used
in the optional sense.
From this perspective, I interpreted the paragraph as:
... if the process is multithreaded and the semantics of thread
execution do not define a relative order between two send operations
executed by two distinct threads, then the operations are logically
concurrent, even if one physically precedes the other.
So, probably replacing "may not" by "cannot" would better meet the
intention?
Best
Joachim
On 11/15/18 6:19 PM, Joachim Protze 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 at lists.mpi-forum.org <mailto: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
>> <mailto: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 <mailto: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
>>>> <mailto: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 <mailto:protze at itc.rwth-aachen.de>
>>>> www.itc.rwth-aachen.de <http://www.itc.rwth-aachen.de>
>>>>
>>>> _______________________________________________
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>>>
>>> The University of Edinburgh is a charitable body, registered in
>>> Scotland, with registration number SC005336.
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>>
>>
>> --
>> Jeff Hammond
>> jeff.science at gmail.com <mailto:jeff.science at 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 at itc.rwth-aachen.de
www.itc.rwth-aachen.de
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