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Hi Marc-Andre,<br>
<br>
The mpi_assert_allow_overtaking INFO key essentially makes *all*
send operations that have identical matching criteria logically
concurrent and *all* receive operations that have overlapping
matching criteria logically concurrent, irrespective of which thread
issued them. So, using the INFO key can be seen as a special case of
handling the full MPI_THREAD_MULTIPLE case.<br>
<br>
In the existing MPI Standard, any program that relies on the tool
being able to recover the particular matching order that actually
happened from a set of possible matching orders that all satisfy the
MPI definitions (for example by relying on the tool using some
externally imposed mechanism, such as piggy-backed sequence numbers)
is relying on a particular sequentialisation of a race-condition
between logically concurrent MPI messages, which is specifically
called out as being an erroneous program.<br>
<br>
From MPI Forum's point-of-view, therefore, any behaviour in this
situation is allowed, including setting the data-centre on fire, and
our work here is done.<br>
<br>
From a tool's perspective, however, the existence and consequences
of this situation should be discovered and presented to the user so
that they can fix their erroneous program. This could be done by
recognising when multiple send operations are logically concurrent
and when multiple receive operations are logically concurrent. If
two send operations are issued with the same matching criteria but
by different threads then these will be logically concurrent (edit:
unless an MPI synchronisation point exists such that the two cannot
conflict). The same is true of receive operations.<br>
<br>
The existence of this situation could be recorded during measurement
(at the cost of searching the entire trace so far for the OS process
(edit: since the last MPI synchronisation point) each time a new
send or receive is issued). Alternatively, the existence of this
situation could be discovered by examination of the trace during
post-processing or analysis.<br>
<br>
The consequences of this situation could be presented to the user by
showing all possible matching orders, as indicated by the
information in the trace concerning which messages were logically
concurrent during the actual run of the application. The tool would
not be able to tell the user which matching order actually occurred
during the measurement run but it would be able to identify that
there was a race-condition, display all the possible outcomes, and
simulate the effects of each possible route through the program.<br>
<br>
Does this make sense and is this sufficient?<br>
<br>
Cheers,<br>
Dan.<br>
<br>
<div class="moz-cite-prefix">On 17/12/2015 14:32, Marc-Andre
Hermanns wrote:<br>
</div>
<blockquote cite="mid:5672C792.60606@jara.rwth-aachen.de"
type="cite">
<pre wrap="">Hi Jeff,
at the moment we don't handle MPI_THREAD_MULTIPLE at all. But we want
to get there ;-)
Here is a short recollection of what we do/need. Sorry for the folks
who know/read this already in other context:
We use, what we call "parallel replay" to analyze large event traces
in parallel. Each thread has its own stream of events, such as enter
and exit for tracking the calling context as well as send and receive
for communication among ranks.
We analyze on the same scale as the measurement, thus we have one
thread per thread-local trace. Each thread processes its own
thread-local trace. When encountering a communication event, it
re-enacts this communication using the recorded communication
parameters (rank, tag, comm). A send event leads to an issued send, a
receive event leads to an issued receive.
It is critical that during the analysis, the message matching is
identical to the original application. However, we do not re-enact any
computational time, that is the temporal distance between sends and
receives is certainly different from the original application. As a
consequence, while two sends may have some significant temporal
distance in the original measurement, they could be issued right after
each other during the analysis.
Markus Geimer and I believe that creating some sort of a sequence
number during measurement could help matching the right messages
during the analysis, as a process could detect that it got mismatched
messages and communicate with other threads to get the correct one.
It is unclear, however, how to achieve this:
a) Sending an additional message within the MPI wrapper at measurement
time may lead to invalid matchings, as the additional message may be
received by a different thread.
b) Creating a derived datatype on the fly to add tool-level data to
the original payload may induce a large overhead in practically
_every_ send & receive operation and perturb the measurement.
The best idea in the room so far is some callback mechanism into the
MPI implementation that generates matching information both on sender
and receiver side to generate some form of a sequence number that can
then be saved during measurement. If available on both sender and
receiver this information could then be used to fix incorrect matching
during the analysis.
Cheers,
Marc-Andre
On 16.12.15 16:40, Jeff Hammond wrote:
</pre>
<blockquote type="cite">
<pre wrap="">How do you handle MPI_THREAD_MULTIPLE? Understanding what your tool
does there is a good starting point for this discussion.
Jeff
On Wed, Dec 16, 2015 at 1:37 AM, Marc-Andre Hermanns
<<a class="moz-txt-link-abbreviated" href="mailto:hermanns@jara.rwth-aachen.de">hermanns@jara.rwth-aachen.de</a> <a class="moz-txt-link-rfc2396E" href="mailto:hermanns@jara.rwth-aachen.de"><mailto:hermanns@jara.rwth-aachen.de></a>>
wrote:
Hi all,
CC: Tools-WG, Markus Geimer (not on either list)
sorry for starting a new thread and being so verbose, but I subscribed
just now. I quoted Dan, Jeff, and Jim from the archive as appropriate.
First, let me state that we do not want to prevent this assertion in
any way. For us as tools provider it is just quite a brain tickler on
how to support this in our tool and in general.
Dan wrote:
>>> [...] The basic problem is that message matching would be
>>> non-deterministic and it would be impossible for a tool to show
>>> the user which receive operation satisfied which send operation
>>> without internally using some sort of sequence number for each
>>> send/receive operation. [...]
>>>
>>> My responses were:
>>> 1) the user asked for this behaviour so the tool could simply
>>> gracefully give up the linking function and just state the
>>> information it knows
>
Giving up can only be a temporary solution for tools. The user wants
to use this advanced feature, thus just saying: "Hey, what you're
doing is too sophisticated for us. You are on your own now." is not a
viable long-term strategy.
>>> 2) the tool could hook all send and receive operations and
>>> piggy-back a sequence number into the message header
We discussed piggy-backing within the tools group some time in the
past, but never came to a satisfying way of how to implement this. If,
in the process of reviving the discussion on a piggy-backing
interface, we come to a viable solution, it would certainly help with
the our issues with message matching in general.
Scalasca's problem here is that we need to detect (and partly
recreate) the exact order of message matching to have the correct
message reach the right receivers.
>>> 3) the tool could hook all send and receive operations and
>>> serialise them to prevent overtaking
This is not an option for me. A "performance tool" should strive to
measure as close to the original behavior as possible. Changing
communication semantics just to make a tool "work" would have too
great of an impact on application behavior. After all, if it would
have only little impact, why should the user choose this option in the
first place.
Jeff wrote:
>> Remember that one of the use cases of allow_overtaking is
applications that
>> have exact matching, in which case allow_overtaking is a way of
turning off
>> a feature that isn't used, in order to get a high-performing
message queue
>> implementation. In the exact matching case, tools will have no
problem
>> matching up sends and recvs.
This is true. If the tools can identify this scenario, it could be
supported by current tools without significant change. However, as it
is not generally forbidden to have inexact matching (right?), it is
unclear on how the tools would detect this.
What about an additional info key a user can set in this respect:
exact_matching => true/false
in which the user can state whether it is indeed a scenario of exact
matching or not. The tool could check this, and issue a warning.
>> If tools cannot handle MPI_THREAD_MULTIPLE already, then I
don't really
>> care if they can't support this assertion either.
Not handling MPI_THREAD_MULTIPLE generally is not carved in stone. ;-)
As I said, we (Markus and I) see this as a trigger to come to a viable
solution for tools like ours to support either situation.
>> And in any case, such tools can just intercept the info
operations and
>> strip this key if they can't support it.
As I wrote above in reply to Dan, stripping options that influence
behavior is not a good option. I, personally, would rather bail out
than (silently) change messaging semantics. I can't say what Markus'
take on this is.
Jim wrote:
> I don't really see any necessary fix to the proposal. We could
add an
> advice to users to remind them that they should ensure tools are
compatible
> with the info keys. And the reverse advice to tools writers that
they
> should check info keys for compatibility.
I would second this idea, while emphasizing the burden to be on the
tool to check for this info key (and potentially others) and warn the
user of "undersupport".
Cheers,
Marc-Andre
--
Marc-Andre Hermanns
Jülich Aachen Research Alliance,
High Performance Computing (JARA-HPC)
Jülich Supercomputing Centre (JSC)
Schinkelstrasse 2
52062 Aachen
Germany
Phone: +49 2461 61 2509 | +49 241 80 24381
Fax: +49 2461 80 6 99753
<a class="moz-txt-link-abbreviated" href="http://www.jara.org/jara-hpc">www.jara.org/jara-hpc</a> <a class="moz-txt-link-rfc2396E" href="http://www.jara.org/jara-hpc"><http://www.jara.org/jara-hpc></a>
email: <a class="moz-txt-link-abbreviated" href="mailto:hermanns@jara.rwth-aachen.de">hermanns@jara.rwth-aachen.de</a>
<a class="moz-txt-link-rfc2396E" href="mailto:hermanns@jara.rwth-aachen.de"><mailto:hermanns@jara.rwth-aachen.de></a>
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--
Jeff Hammond
<a class="moz-txt-link-abbreviated" href="mailto:jeff.science@gmail.com">jeff.science@gmail.com</a> <a class="moz-txt-link-rfc2396E" href="mailto:jeff.science@gmail.com"><mailto:jeff.science@gmail.com></a>
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</pre>
</blockquote>
<pre wrap="">
</pre>
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<br>
<pre class="moz-signature" cols="72">--
Dan Holmes
Applications Consultant in HPC Research
EPCC, The University of Edinburgh
James Clerk Maxwell Building
The Kings Buildings
Peter Guthrie Tait Road
Edinburgh
EH9 3FD
T: +44(0)131 651 3465
E: <a class="moz-txt-link-abbreviated" href="mailto:dholmes@epcc.ed.ac.uk">dholmes@epcc.ed.ac.uk</a>
*Please consider the environment before printing this email.*</pre>
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