[mpiwg-p2p] Ordering of P2P messages in multithreaded applications
HOLMES Daniel
d.holmes at epcc.ed.ac.uk
Thu Nov 15 12:56:42 CST 2018
Hi Joachim,
In more recent changes to MPI, the MPI Forum is trying to be consistent in its usage of terms like “shall” “can” “may” and so on. Specifically, we have adopted the meanings set out here:
https://www.iec.ch/standardsdev/resources/draftingpublications/directives/principles/verbal_forms.htm
For existing text, usage of such words is not as clear-cut.
Your re-wording is one way to interpret the text as written. It is also possible to read the “may not” sentence as one example (sufficient but not necessary) of when this might happen or become relevant. Unfortunately, neither of these interpretations state clearly what happens in the converse situation, i.e. when the thread execution *does* define a relative order (as in your example OpenMP code).
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 11:57, Joachim Protze via mpiwg-p2p <mpiwg-p2p at lists.mpi-forum.org<mailto:mpiwg-p2p at lists.mpi-forum.org>> wrote:
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> <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>
<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> <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>
<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> <mailto:protze at itc.rwth-aachen.de>
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--
Jeff Hammond
jeff.science at gmail.com<mailto: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<mailto:protze at itc.rwth-aachen.de>
www.itc.rwth-aachen.de
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