Dear all, after Torsten's positive review, I stored all (solution + review) in the new ticket #419: https://svn.mpi-forum.org/trac/mpi-forum-web/ticket/419 If technical or English corrections are needed, or if all is okay, I would be grateful if you could write your review as soon as possible to the ticket. Who is at the next meeting and could read the ticket at the meeting? As one-reading-one-vote MPI-3.0 errata ticket, it should still fit to the MPI-3.1 schedule. Best regards Rolf ----- Original Message -----

From: "Torsten Hoefler" <[email protected]> To: "Rolf Rabenseifner" <[email protected]> Cc: "MPI-3 Collective Subgroup Discussions" <[email protected]>, "David Henty" <[email protected]>, "Martin Schulz" <[email protected]>, "Purushotham V. Bangalore" <[email protected]>, "Shinji Sumimoto" <[email protected]>, "Dave Goodell" <[email protected]>, "Adam Moody" <[email protected]> Sent: Sunday, March 30, 2014 10:10:17 PM Subject: Re: Fwd: Question re neighbourhood collectives and graph topologies

Hi Rolf,

On Tue, Mar 25, 2014 at 12:25:51PM +0100, Rolf Rabenseifner wrote:

...

Based on David Henty's question, my analysis seems to imply that the definition of the neighborhood collectives for MPI_GRAPH_CREATE general graph topologies is fully broken, i.e., undefined.

I CC'ed the MPI-3.0 topology chapter committee and send this to the MPI-next collective working group.

Reason for my analysis:

MPI-3.0 p295:25-26 allows multiple edges and a non-symmetric adjacency matrix.

MPI-3.0 p295:26-27 tells that an adjacency edge from "process" to "neighbor" does not imply a communication direction, neither process=source nor process=destination. Yes, this is from the original MPI-1 text and is probably confusing.

...

MPI-3.0 p306:46-p307:3 clearly defines that MPI_GRAPH_NEIGHBORS return exactly the same adjacency information as defined in MPI_GRAPH_CREATE.

MPI-305 p307:11-34 (Example 7.5) uses a non-symmetric adjacency Matrix between process 2 and 3:

The three edges between nodes 2 and 3 are: 1. process 2 -- neighbor 3 2. process 3 -- neighbor 2 3. process 3 -- neighbor 2 This is truly horrible!

...

MPI-3.0 p315:11-13 defines:

For a general graph topology, created with MPI_GRAPH_CREATE, the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_GRAPH_NEIGHBORS.

For process rank 2, the relevant edges returned by MPI_GRAPH_NEIGHBORS is one edge to neighbor 3 and therefore one send buffer and one receive buffer will be defined.

For process rank 3, the relevant edges returned by MPI_GRAPH_NEIGHBORS are two edges to neighbor 2 and therefore two send buffers and two receive buffers will be defined.

This does not match and will cause broken communication! Yes, we did not check the non-scalable MPI-1 interface for additional brokenness. We wanted to deprecate it but didn't for various reasons. Instead, we put strong wording advising to never use it.

I mean: DO NOT USE MPI_GRAPH_CREATE EVER! :-)

Yet, we should fix this inconsistency with an erratum.

...

Please do not think that three buffers will be defined, because you have to look at the total example, i.e. - in process 2 -- one send buffer for communication with rank 3 and -- one receive buffer for communication with rank 3 are used, and - in process 3 -- three send buffers for communication with ranks 0, 3, 3 and -- three receive buffers for communication with rank 0, 3, 3 ares used.

The number of buffers between the processes 2 and 3 do not match. Yes

...

Proposal: We should restrict the current definition of collective neighbor communication on general graph topologies on those topologies that define for each pair of processes on both processes the same amount of edges between this pair of processes. (This implies a symmetric adjacency matrix, but the adjacency information, which also includes the sequence of the edges, needs not to be symmetric.) Well, this seems a bit like a hack, but since nobody should use this interface anyway, I have no strong feelings.

...

Proposed solution: ------------------

MPI-3.0 page 315 lines 11-14 read

For a general graph topology, created with MPI_GRAPH_CREATE, the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_GRAPH_NEIGHBORS. Note that general graph topologies should generally be replaced by the distributed graph topologies.

but should read

For a general graph topology, created with MPI_GRAPH_CREATE, | the use of neighborhood collective communication is | restricted to adjacency matrices with the number of edges | between any two processes is defined to be the same for both | processes (i.e., with a symmetric adjacency matrix). | In this case, the order of neighbors in the send and receive buffers is defined as the sequence of neighbors as returned by MPI_GRAPH_NEIGHBORS. Note that general graph topologies should generally be replaced by the distributed graph topologies. That is ok with me. Rolf, feel free to push this erratum forward, I'll support it and we can discuss it in the collectives WG.

Thanks & All the Best, Torsten

-- ### qreharg rug ebs fv crryF ----------- http://htor.inf.ethz.ch/ ----- Torsten Hoefler | Assistant Professor Dept. of Computer Science | ETH Zürich Universitätsstrasse 6 | Zurich-8092, Switzerland CAB E 64.1 | Phone: +41 76 309 79 29

-- 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: Room 1.307)