Dear all, please make a straw-vote-decision TODAY about the two variants in this email. I myself prefer variant A. The current text does not include a precise definition of the sequence of the neighbors. I recommend to add such a definition before the Rationale in Sect. 7.6. New text: The sequence of the neighbors is defined - in a Cartesian topology by the order of the dimensions, and in each dimension, first the neighbor in the negative direction and then in the positive direction with displacement 1. If such a neighbor does not exist, i.e., at the border in the case of a non-periodic virtual grid dimension (i.e., periods[...]==false), then (VARIANT A) this neighbor is MPI_PROC_NULL and it is still part of the sequence of neighbors, but the corresponding buffer portion is not communicated. The values of indegree and outdegree in the communication routines are 2*ndims with ndims defined defined in MPI_CART_CREATE. (VARIANT B) this neighbor is not part of the sequence of neighbors. The values of indegree and outdegree in the communication routines are 2*ndims (with ndims defined defined in MPI_CART_CREATE), reduced by the number of non-existing neighbors. (END OF VARIANTS) The sequence of incoming and outgoing neighbors is identical. - in a general graph topology, the sequence of neighbors in each process is defined through the list of neighbors for this process as defined in the arguments index and edges in MPI_GRAPH_CREATE, which is identical to the sequence returned by MPI_GRAPH_NEIGHBORS. The values of indegree and outdegree in the communication routines are identical to nneighbors returned by MPI_GRAPH_NEIGHBOR_COUNT in each process for its own rank. The sequence of incoming and outgoing neighbors is identical. - in a distributed graph topology, the sequence of neighbors in each process is returned by MPI_DIST_GRAPH_NEIGHBORS. The values of indegree and outdegree in the communication routines are identical to the values returned by MPI_DIST_GRAPH_NEIGHBORS_COUNT. If the distributed graph topology was created by MPI_DIST_GRAPH_CREATE_ADJACENT, then the sequence of incoming and outgoing neighbors is identical to the sequence that was defined in MPI_DIST_GRAPH_CREATE_ADJACENT. With this detailled definition, the following remark in parentheses in 7.6.1 and 7.6.2 can be removed: (the order of neighbors is returned by the neighborhood query functions or for Cartesian communicators, it is in the order of the dimensions, in each dimension first the negative and then the positive direction) Torsten, please return the decision, Variant A or B. Best regards Rolf -- 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)