155. Overview of the Functions

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The functions MPI_GRAPH_CREATE and MPI_CART_CREATE are used to create general (graph) virtual topologies and Cartesian topologies, respectively. These topology creation functions are collective. As with other collective calls, the program must be written to work correctly, whether the call synchronizes or not.

The topology creation functions take as input an existing communicator comm_old, which defines the set of processes on which the topology is to be mapped. All input arguments must have identical values on all processes of the group of comm_old. A new communicator comm_topol is created that carries the topological structure as cached information (see Chapter Groups, Contexts, Communicators, and Caching ). In analogy to function MPI_COMM_CREATE, no cached information propagates from comm_old to comm_topol.

MPI_CART_CREATE can be used to describe Cartesian structures of arbitrary dimension. For each coordinate direction one specifies whether the process structure is periodic or not. Note that an n-dimensional hypercube is an n-dimensional torus with 2 processes per coordinate direction. Thus, special support for hypercube structures is not necessary. The local auxiliary function MPI_DIMS_CREATE can be used to compute a balanced distribution of processes among a given number of dimensions.

Rationale. Similar functions are contained in EXPRESS [12] and PARMACS. ( End of rationale.)
The function MPI_TOPO_TEST can be used to inquire about the topology associated with a communicator. The topological information can be extracted from the communicator using the functions MPI_GRAPHDIMS_GET and MPI_GRAPH_GET, for general graphs, and MPI_CARTDIM_GET and MPI_CART_GET, for Cartesian topologies. Several additional functions are provided to manipulate Cartesian topologies: the functions MPI_CART_RANK and MPI_CART_COORDS translate Cartesian coordinates into a group rank, and vice-versa; the function MPI_CART_SUB can be used to extract a Cartesian subspace (analogous to MPI_COMM_SPLIT). The function MPI_CART_SHIFT provides the information needed to communicate with neighbors in a Cartesian dimension. The two functions
MPI_GRAPH_NEIGHBORS_COUNT and MPI_GRAPH_NEIGHBORS can be used to extract the neighbors of a node in a graph. The function MPI_CART_SUB is collective over the input communicator's group; all other functions are local.

Two additional functions, MPI_GRAPH_MAP and MPI_CART_MAP are presented in the last section. In general these functions are not called by the user directly. However, together with the communicator manipulation functions presented in Chapter Groups, Contexts, Communicators, and Caching , they are sufficient to implement all other topology functions. Section Low-Level Topology Functions outlines such an implementation.

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