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. 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 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! 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. 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.) 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. Best regards Rolf ----- Forwarded Message ----- From: "David Henty" <[email protected]> To: "Rolf Rabenseifner" <[email protected]> Sent: Monday, March 24, 2014 7:09:00 PM Subject: Question re neighbourhood collectives and graph topologies Rolf, I see you were involved in the definition for the new scalable graph creation functions in MPI 3.0 so I wondered of you could answer a question re their use in neighbourhood collectives. Page 295 of the standard says for MPI_Graph_create that "The definition of a node-neighbor edge does not imply a direction of the communication." This made sense to me when graphs could only be used to look up the ranks of your neighbours: it was presumably up to the user how these were interpreted, e.g. if they implied a send or a receive. However, the new neighbourhood collective operations infer the communications from the graph and do the sends and receives automatically. For the new distributed creation routines this seems clear, e.g. they talk explicitly about "source and destination" or "indegree and outdegree". However, it isn't clear to me how this works with the old graph creation routine. Is it correct to assume that the explicit "neighbours" of each rank are the "out" ones, and the "in" ones are the links from all other processes who specify my rank as a neighbour? This seems the obvious definition to me, but it seems to conflict with the statement that "The definition of a node-neighbor edge does not imply a direction of the communication.". I see you have some pseudocode on how to translate between the different representations in the appendix of your 2010 paper, but I am being lazy and thinking it is quicker (for me if not for you) to you ask you rather than understand the pseudocode! Thanks in advance, David -- The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336. -- 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)