Rich, yes and no: - Yes, any real change about fairness is going beyond MPI-3.0. - No, the current text is not clear enough because there is no reference between MPI_ANY_SOURCE and the "lack of fairness" statement. For this, I proposed the clarifications below. What does the Pt-to-Pt chapter team, i.e., Richard Graham(c), Anthony Skjellum, Fab Tillier, Brian Smith, Devendar Bureddy, Bill Gropp, Torsten Hoefler, Adam Moody, Martin Schulz, Brian Barrett, think about my proposal? - And all discussion on any changes of the current text should go through "Main MPI Forum mailing list" <[email protected]> otherwise the risk of problems popping up between Sep 12 and Sep 20 is to big. Best regards Rolf ----- Original Message -----

From: "Richard Graham" <[email protected]> To: "Main MPI Forum mailing list" <[email protected]> Sent: Thursday, August 9, 2012 11:23:57 AM Subject: Re: [Mpi-forum] Fairness of MPI_ANY_SOURCE - MPI-3.0 Draft2 comment by Sebastien Boisvert Rolf, Thanks a lot for the help here, but this is really not needed at this stage. I have started to farm off requests to the appropriate working groups. In this case, the response will be that we will need a specific proposal for something to happen beyond MPI 3.0, as this can be either an implementation issue, or a change in long-standing semantics, which are beyond the scope of the current work.

Thanks, Rich

-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Rolf Rabenseifner Sent: Thursday, August 09, 2012 12:20 PM To: Main MPI Forum mailing list Subject: [Mpi-forum] Fairness of MPI_ANY_SOURCE - MPI-3.0 Draft2 comment by Sebastien Boisvert

I try to test the proposed process for comments.

This comment is about the Point-to-Point chapter. Rich is the chapter author.

Here is the comment (see bottom of this email) together with my reply to the comment's author and a first proposal for solving it:

All references are related to http://meetings.mpi-forum.org/draft_standard/mpi3.0_draft_2.pdf

Summary about the comment: ==========================

The text in new MPI_IMPROBE about MPI_ANY_SOURCE does not say anything about fairness. The commentator asks for a round-robin fairness behavior.

Summary of my proposal to answer this comment: ==============================================

- Keep the unfair behavior as defined on p42:10-17 - Clarify this faiirness-paragraph to make clear that it also applies to MPI_ANY_SOURCE. - Add cross-references between p42:10-17, MPI_RECV and all MPI_PROBE routines to make clear that there is no fairness with MPI_ANY_SOURCE.

Proposed solution: ==================

All references are related to http://meetings.mpi-forum.org/draft_standard/mpi3.0_draft_2.pdf

mpi3.0_draft_2.pdf p29:25-31 read

The receiver may specify a wildcard MPI_ANY_SOURCE value for source, and/or a wildcard MPI_ANY_TAG value for tag, indicating that any source and/or tag are acceptable. It cannot specify a wildcard value for comm. Thus, a message can be received by a receive operation only if it is addressed to the receiving process, has a matching communicator, has matching source unless source=MPI_ANY_SOURCE in the pattern, and has a matching tag unless tag=MPI_ANY_TAG in the pattern.

and the following text should be added:

Note that MPI makes no guarantee of fairness in the handling of communication, especially when using MPI_ANY_SOURCE; for details see the section on {\em Fairness} on page 42.

mpi3.0_draft_2.pdf p42:10 read

Fairness[ ] MPI makes no guarantee of fairness in the handling of communication.

but should read

Fairness[ ] MPI makes no guarantee of fairness in the handling of communication, e.g., when using MPI_ANY_SOURCE, MPI_WAITANY or MPI_WAITSOME in a singlethreaded process, or using MPI_RECV or MPI_MPROBE by several threads in a multithreaded process.

mpi3.0_draft_2.pdf p65:16-18 (in the definition of MPI_IPROBE) read

The call matches the same message that would have been received by a call to MPI_RECV(..., source, tag, comm, status) executed at the same point in the program, and returns in status the same value that would have been returned by MPI_RECV().

but should read (only the reference on the last line is added):

The call matches the same message that would have been received by a call to MPI_RECV(..., source, tag, comm, status) executed at the same point in the program, and returns in status the same value that would have been returned by MPI_RECV(), see Section 3.2.4 on page 28.

mpi3.0_draft_2.pdf p68:33-35 (in the definition of MPI_IMPROBE) read

The call matches the same message that would have been received by a call to MPI_RECV(..., source, tag, comm, status) executed at the same point in the program and returns in status the same value that would have been returned by MPI_RECV.

but should read (only the reference on the last line is added):

The call matches the same message that would have been received by a call to MPI_RECV(..., source, tag, comm, status) executed at the same point in the program and returns in status the same value that would have been returned by MPI_RECV, see Section 3.2.4 on page 28.

Proposed answer to the commentator (together with the proposed solution): ======================================

MPI defines to be unfair, see page 42, lines 10-17. Cross-references were missing in the MPI standard, i.e., it was not easy to detect that this paragraph on fairness also applies to MPI_ANY_SOURCE in any call (MPI_RECV, MPI_IRECV, and all versions of MPI_PROBE) The proposed solution adds the missing cross-references.

For performance reasons, the MPI Forum decided not to change this "unfair" behavior.

You may use other mechanisms to implement some sort of fairness. Especially the tag can be used in a cyclic way (i.e. with values between 0 and 32767) to implement some sort of fairness, but this is outside of the scope of the MPI standard.

We did not add an advice to users about mechanisms to implement some sort of fairness within the application. Such an advice would go beyond the task of the MPI standard.

Background (not to be part of the answer to the commentator) ============================================================

The following part of the comment

...

Presently, the MPI standard contains nothing about which source should be probed when MPI_ANY_SOURCE is provided. is not fully true. The MPI standard clearly states in mpi3.0_draft_2.pdf in Section 3.5 Semantics of Point-to-Point Communication on page 42 lines 10-17

Fairness. MPI makes no guarantee of fairness in the handling of communication. Suppose that a send is posted. Then it is possible that the destination process repeatedly posts a receive that matches this send, yet the message is never received, because it is each time overtaken by another message, sent from another source. Similarly, suppose that a receive was posted by a multithreaded process. Then it is possible that messages that match this receive are repeatedly received, yet the receive is never satisfied, because it is overtaken by other receives posted at this node (by other executing threads). It is the programmer's responsibility to prevent starvation in such situations.

that there is no fairness. And I expect that the MPI Forum does not want to change this statement. This section does not mention MPI_ANY_SOURCE. My proposal adds here a note on MPI_ANY_SOURCE that readers can find the Fairness paragraph when the look at all locations of MPI_ANY_SOURCE.

MPI_ANY_SOURCE is defined in the text about the source rank of MPI_RECV without any reference to the Fairness paragraph, see page 29 lines 23-31. I added a reference to the Fairness paragraph to solve this lack of reference.

And MPI_IPROBE and MPI_IMPROBE have own text on MPI_ANY_SOURCE. Here I would propose to add only a reference to Section 3.2.4, which defines MPI_RECV and which should contain the reference to the fairness paragraph.

---------------------- My goal was not to change anything, i.e., only to add clarifying references. Any comments about this proposal? Corrections about my wording? Agreed?

Best regards Rolf

----- Forwarded Message ----- From: "Rolf Rabenseifner" <[email protected]> To: "Sébastien Boisvert" <[email protected]> Sent: Thursday, August 9, 2012 8:09:49 AM Subject: Re: [Mpi-comments] One comment on MPI-3.0 Draft 2, August 2012

Dear Mr. Boisvert,

the MPI Forum will discuss your comment and will return an answer before our meeting Sep. 20-21 in Vienna.

Best regards Rolf Rabenseifner

----- Original Message -----

...

From: "Sébastien Boisvert" <[email protected]> To: [email protected] Sent: Sunday, August 5, 2012 6:50:29 AM Subject: [Mpi-comments] One comment on MPI-3.0 Draft 2, August 2012 Dear MPI Forum committee members,

I would like to submit a comment on the MPI-3.0 Draft 2, August 2012 for your consideration.

Version: MPI-3.0 Draft 2, August 2012.

The URL of the version of the MPI standard: http://meetings.mpi-forum.org/draft_standard/mpi3.0_draft_2.pdf

Page: 65

Line number: 28

Section: 3.8.1

In:

3. Point-to-Point Communication 3.8 Probe and Cancel 3.8.1 Probe

Comment:

It says that the source argument of MPI_Iprobe can be MPI_ANY_SOURCE, but it does say anything about fairness. Therefore MPI_ANY_SOURCE can lead to resource starvation.

I think it would be better if probing would be done in a round-robin fashion when the source is MPI_ANY_SOURCE so that any MPI rank has an equal chance of having its message probed and received.

Presently, the MPI standard contains nothing about which source should be probed when MPI_ANY_SOURCE is provided.

I hope you will consider my comment.

Sincerely,

Sébastien Boisvert PhD student Université Laval

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-- Dr. Rolf Rabenseifner . . . . . . . . . .. email [email protected] High Performance Computing Center (HLRS) . phone ++49(0)711/685-65530 University of Stuttgart . . . . . . . . .. fax ++49(0)711 / 685-65832 Head of Dpmt Parallel Computing . . . www.hlrs.de/people/rabenseifner Nobelstr. 19, D-70550 Stuttgart, Germany . (Office: Allmandring 30)