[Mpi3-subsetting] MPI subsetting: charting the way forward atatelecon next week?

Fri Jun 20 12:10:47 CDT 2008

At 09:58 AM 6/20/2008, Supalov, Alexander wrote:
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>Hi,
>
>Ignoring an assertion should be perfectly legal.

I fully agree, ignoring should always be OK, which ensures
the portability of any application using assertions.

However, I do see Rich's point - how useful are assertions
if we have hundreds of them and each just works on a particular
MPI implementation (or even version)? Also, if these constants
are really implementation specific, does it make sense to have
them in the MPI standard? Each vender will want their own set
(and rightfully so) and the burden is then on the programmer to
know all of the different options and understand the subtle
differences (and we have to document them all in the standard).

Perhaps we should just define broad groups of assertions and
define those in the standard. The user can then query for all
available assertions in that group for a particular implementation.
This would have to coupled with an ability to uniquely identify
certain MPI implementations at runtime. Also, this does not
solve the problem for the end-user of how to select the correct
assertion.

Martin

Martin

>
>Best regards.
>
>Alexander
>
>
>----------
>From: mpi3-subsetting-bounces_at_[hidden] 
>[mailto:mpi3-subsetting-bounces_at_[hidden]] On Behalf Of 
>Richard Graham
>Sent: Friday, June 20, 2008 6:53 PM
>To: MPI 3.0 Sub-setting working group
>Subject: Re: [Mpi3-subsetting] MPI subsetting: charting the way 
>forward atatelecon next week?
>
>I think we need to be careful here when it comes to assertions, and 
>think hard about how
>  you want to handle these in a standard.  In some of the 
> implementations I am familiar with
>  a no-eager-throttle key word would be useless – it is vey 
> implementation specific.  I suppose
>  this is a big problem with trying to add implementation specific 
> keywords to a standard.
>  It is a given that this will also cause trouble when trying to 
> come up with an ABI, unless
>  one has a large set of defined constants, and are willing to have 
> these be no-ops in
>  certain implementations.
>
>Rich
>
>
>On 6/20/08 9:56 AM, "Richard Treumann" <treumann_at_[hidden]> wrote:
>
>Hi Alexander
>
>Comments imbedded below.
>
>I have no objections to someone providing a rationale for assertions 
>related to MPI-IO and MPI_1sided.  If the rationale is sound I have 
>no objection to putting them in the proposal.
>
>I feel the proposal should be evaluated by the following algorithm.
>
>If (this concept  is one that seems plausible) {
>  for each proposed assertion {
>  if (rationale not solid)
>  discard
>  if (deal breaker downside)
>  discard
>  }
>if ((concept makes sense) & (set of worthwhile assertions is not empty))
>  make this part of MPI 2.2
>
>I do not see much reason to get every assertion that eventually 
>gains traction into MPI 2.2.  MPI 3.0 is soon enough for any that do 
>not make the MPI 2.2 cut. I do not want to see the concept fall 
>because some particular assertion is controversial.
>
>I consider MPI_NO_EAGER_THROTTLE to be the single most valuable 
>assertion for MPI 2.2 because it is needed to allow MPI to scale to 
>the levels we are already seeing.
>
>
>Dick Treumann  -  MPI Team/TCEM
>IBM Systems & Technology Group
>Dept 0lva / MS P963 -- 2455 South Road -- Poughkeepsie, NY 12601
>Tele (845) 433-7846         Fax (845) 433-8363
>
>
>mpi3-subsetting-bounces_at_[hidden] wrote on 06/20/2008 02:58:41 AM:
>
> > Dear Dick,
> >
> > A couple of suggestions re your proposal:
> >
> > - If ASSERTIONS is put at the end of the MPI_INIT_ASSERTED argument
> > list, in C++ one can declare the last argument as having a zero
> > default value, and skip it if necessary. This might help with
> > deprecation of the earlier MPI_INIT_* calls.
>
>I have no objection. It seems reasonable to let C++ default the
>assertions parameter to "none"
>
> > - In non-Cray parts of the world, an MPI_INT followed by MPI_FLOAT
> > is likely to be a 4-byte int followed by a 4-byte float. This
> > sometimes depends on the compiler settings in effect, too.
>
>My rationale is not specific to any particular architecture.
>Some MPI datatypes are made entirely
>from the same base type. Some are mixtures of types. If libmpi knows
>at the moment a datatype is committed that the send side and receive
>side will always use the same internal representions then it does not
>need to keep track of the fact that one instance of {MPI_INT,MPI_FLOAT}
>has two distinct parts. The send side can gather and ship 8 bytes
>and the receive side can scatter the 8 bytes. If one side might use 4
>byte integers while the other side uses 8 byte integers then at
>least one side will need to know there is a conversion to be done for
>the MPI_INT part. If an MPI job does a spawn or join that links to a
>different architecture after the datatype has been committed, and
>the MPI_Type_commit has discarded the details, it is too late to get
>them back.  On the other hand, if it is known there will never be a
>different architecture added to the job, the extra information can be
>safely discarded.
>
> > - I don't think MPI_NO_THREAD_CONTENTION is really necessary. The
> > original thread level settings, in particular, the use of anything
> > but MPI_THREAD_MULTIPLE, seem to capture the semantics that you proposed.
>
>This one is kind of tricky and I also am not sure what it would mean. If
>we find a clear value we can keep it and if not we can remove it.
>
> > - I can't fully follow the motivation for MPI_NO_ANY_SOURCE
> > deprioritization. AFAIK, a rendezvous exchange usually starts with a
> > ready-to-send packet that contains the size of the message. In this
> > case the receiving side will normally reply with a ready-to-receive
> > regardless of the buffer space available, and flag MPI_ERR_TRUNCATED
> > on message arrival if necessary. In this case, neither
> > MPI_ANY_SOURCE not MPI_NO_ANY_SOURCE seem to get into way.
>
>My point is that MPI_NO_ANY_SOURCE might allow this round trip
>protocol to be replaced by a 1/2 rendezvous protocol. If it is known
>that MPI_ANY_SOURCE will not be used then the receive side can send
>an "envelop and ready for data" packet to the send side. As long as
>the send side knows it will receive the "envelop and ready for data"
>packet when the receive is posted, it does not need to do the first 1/2
>of the rendezvous. The message matching can be done at the send side.
>
>A send for which the receive was preposted has a
>good chance of finding the "envelop and ready for data" sitting in
>an early queue and the large send can avoid any rendezvous delay.
>Data begins to flow immediately vs waiting for a round trip of a
>full rendezvous. In many cases we cut the delay in half and best
>case we eliminate rendezvous delay completely. If the receive side
>is late in posting the receive we still save a packet traversal but
>do not save any time.
>
>If there may be an MPI_ANY_SOURCE then this does not work because the
>receive side that has an MPI_ANY_SOURCE cannot guess which sender to
>notify so the sender cannot count on getting a 1/2 rendezvous
>notification for a message that should match the MPI_ANY_SOURCE
>receive.
>
>The problem that made me lower the priority is that many MPIs use an
>eager protocol for small messages and a rendezvous protocol for large
>messages.  If the send side and receive side have the same size buffer
>then both sides can reach the same conclusion: eager vs 1/2 rendezvous.
>If both decide on eager, the receive side will not send an
>"envelop and ready for data" packet and the send side will not look
>for one. If both sides decide on 1/2 rendezvous then the receive side
>will send an "envelop and ready for data" packet and the send side will
>look for and consume the notice.  If the send side is for an 8 byte
>message and the receive uses a "big enough" receive buffer of 64KB
>then the two sides will probably not be able to reach the same
>conclusion about the protocol. The receive side will ship off an
>"envelop and ready for data" packet that the send side will not
>know what to do with.
>
>
> >
> > Best regards.
> >
> > Alexander
> >
> > From: Supalov, Alexander
> > Sent: Friday, June 20, 2008 8:29 AM
> > To: 'MPI 3.0 Sub-setting working group'
> > Subject: RE: [Mpi3-subsetting] MPI subsetting: charting the way
> > forward at atelecon next week?
>
> > Dear Dick,
> >
> > Thank you. I remember we exchanged a couple of emails about the
> > possible extensions to the set of assertions, like one-sided and
> > I/O, and in my recollection, almost reached an agreement that this
> > can improve performance and possibly memory footprint, as well as be
> > expressed thru assertions. Do you still feel favorable about this?
> >
> > Best regards.
> >
> > Alexander
> >
>
>
>
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_______________________________________________________________________
Martin Schulz, schulzm_at_[hidden], http://people.llnl.gov/schulz6
CASC @ Lawrence Livermore National Laboratory, Livermore, USA  

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