Why make it difficult when an int in the mpi_init call seems to be sufficient? -----Original Message----- From: mpi3-subsetting-bounces_at_[hidden] [mailto:mpi3-subsetting-bounces_at_[hidden]] On Behalf Of Bronis R. de Supinski Sent: Friday, June 20, 2008 7:05 PM To: MPI 3.0 Sub-setting working group Subject: Re: [Mpi3-subsetting] MPI subsetting: charting the way forward atatelecon next week? Yes, but the best approach would be a query/subscribe interface, possibly with some set of standard ketwords that provide portability. On Fri, 20 Jun 2008, Supalov, Alexander wrote:

Hi,

Ignoring an assertion should be perfectly legal.

Best regards.

Alexander

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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:

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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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