If a topology has been defined with one of the above functions, then the topology information can be looked up using inquiry functions. They all are local calls.

MPI_TOPO_TEST(comm, status) | |

IN comm | communicator (handle) |

OUT status | topology type of communicator comm (state) |

` int MPI_Topo_test(MPI_Comm comm, int *status) `

` MPI_Topo_test(comm, status, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(OUT) :: status INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, STATUS, IERROR

The function MPI_TOPO_TEST returns the type of topology that is assigned to a communicator.

The output value status is one of the following:

MPI_GRAPHDIMS_GET(comm, nnodes, nedges) | |

IN comm | communicator for group with graph structure (handle) |

OUT nnodes | number of nodes in graph (integer) (same as number of processes in the group) |

OUT nedges | number of edges in graph (integer) |

` int MPI_Graphdims_get(MPI_Comm comm, int *nnodes, int *nedges) `

` MPI_Graphdims_get(comm, nnodes, nedges, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(OUT) :: nnodes, nedges INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, NNODES, NEDGES, IERROR

Functions MPI_GRAPHDIMS_GET and MPI_GRAPH_GET retrieve the graph-topology information that was associated with a communicator by MPI_GRAPH_CREATE.

The information provided by MPI_GRAPHDIMS_GET can be used to dimension the vectors index and edges correctly for the following call to MPI_GRAPH_GET.

MPI_GRAPH_GET(comm, maxindex, maxedges, index, edges) | |

IN comm | communicator with graph structure (handle) |

IN maxindex | length of vector index in the calling program (integer) |

IN maxedges | length of vector edges in the calling program (integer) |

OUT index | array of integers containing the graph structure (for details see the definition of MPI_GRAPH_CREATE) |

OUT edges | array of integers containing the graph structure |

` int MPI_Graph_get(MPI_Comm comm, int maxindex, int maxedges, int index[], int edges[]) `

` MPI_Graph_get(comm, maxindex, maxedges, index, edges, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: maxindex, maxedges INTEGER, INTENT(OUT) :: index(maxindex), edges(maxedges) INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, MAXINDEX, MAXEDGES, INDEX(*), EDGES(*), IERROR

MPI_CARTDIM_GET(comm, ndims) | |

IN comm | communicator with Cartesian structure (handle) |

OUT ndims | number of dimensions of the Cartesian structure (integer) |

` int MPI_Cartdim_get(MPI_Comm comm, int *ndims) `

` MPI_Cartdim_get(comm, ndims, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(OUT) :: ndims INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, NDIMS, IERROR

The functions MPI_CARTDIM_GET and MPI_CART_GET return the Cartesian topology information that was associated with a communicator by MPI_CART_CREATE. If comm is associated with a zero-dimensional Cartesian topology, MPI_CARTDIM_GET returns ndims=0 and MPI_CART_GET will keep all output arguments unchanged.

MPI_CART_GET(comm, maxdims, dims, periods, coords) | |

IN comm | communicator with Cartesian structure (handle) |

IN maxdims | length of vectors dims, periods, and coords in the calling program (integer) |

OUT dims | number of processes for each Cartesian dimension (array of integer) |

OUT periods | periodicity ( true/ false) for each Cartesian dimension (array of logical) |

OUT coords | coordinates of calling process in Cartesian structure (array of integer) |

` int MPI_Cart_get(MPI_Comm comm, int maxdims, int dims[], int periods[], int coords[]) `

` MPI_Cart_get(comm, maxdims, dims, periods, coords, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: maxdims INTEGER, INTENT(OUT) :: dims(maxdims), coords(maxdims) LOGICAL, INTENT(OUT) :: periods(maxdims) INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, MAXDIMS, DIMS(*), COORDS(*), IERROR

LOGICAL PERIODS(*)

MPI_CART_RANK(comm, coords, rank) | |

IN comm | communicator with Cartesian structure (handle) |

IN coords | integer array (of size ndims) specifying the Cartesian coordinates of a process |

OUT rank | rank of specified process (integer) |

` int MPI_Cart_rank(MPI_Comm comm, const int coords[], int *rank) `

` MPI_Cart_rank(comm, coords, rank, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: coords(*) INTEGER, INTENT(OUT) :: rank INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, COORDS(*), RANK, IERROR

For a process group with Cartesian structure, the function MPI_CART_RANK translates the logical process coordinates to process ranks as they are used by the point-to-point routines.

For dimension i with periods(i) = true, if the coordinate,
coords(i), is out of range, that is, coords(i) *<* 0 or
coords(i)
dims(i), it is shifted back to the interval
0
coords(i) *<* dims(i) automatically. Out-of-range
coordinates are erroneous for non-periodic dimensions.

If comm is associated with a zero-dimensional Cartesian topology, coords is not significant and 0 is returned in rank.

MPI_CART_COORDS(comm, rank, maxdims, coords) | |

IN comm | communicator with Cartesian structure (handle) |

IN rank | rank of a process within group of comm (integer) |

IN maxdims | length of vector coords in the calling program (integer) |

OUT coords | integer array (of size ndims) containing the Cartesian coordinates of specified process (array of integers) |

` int MPI_Cart_coords(MPI_Comm comm, int rank, int maxdims, int coords[]) `

` MPI_Cart_coords(comm, rank, maxdims, coords, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: rank, maxdims INTEGER, INTENT(OUT) :: coords(maxdims) INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, RANK, MAXDIMS, COORDS(*), IERROR

The inverse mapping, rank-to-coordinates translation is provided by MPI_CART_COORDS. If comm is associated with a zero-dimensional Cartesian topology, coords will be unchanged.

MPI_GRAPH_NEIGHBORS_COUNT(comm, rank, nneighbors) | |

IN comm | communicator with graph topology (handle) |

IN rank | rank of process in group of comm (integer) |

OUT nneighbors | number of neighbors of specified process (integer) |

` int MPI_Graph_neighbors_count(MPI_Comm comm, int rank, int *nneighbors) `

` MPI_Graph_neighbors_count(comm, rank, nneighbors, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: rank INTEGER, INTENT(OUT) :: nneighbors INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, RANK, NNEIGHBORS, IERROR

MPI_GRAPH_NEIGHBORS(comm, rank, maxneighbors, neighbors) | |

IN comm | communicator with graph topology (handle) |

IN rank | rank of process in group of comm (integer) |

IN maxneighbors | size of array neighbors (integer) |

OUT neighbors | ranks of processes that are neighbors to specified process (array of integer) |

` int MPI_Graph_neighbors(MPI_Comm comm, int rank, int maxneighbors, int neighbors[]) `

` MPI_Graph_neighbors(comm, rank, maxneighbors, neighbors, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: rank, maxneighbors INTEGER, INTENT(OUT) :: neighbors(maxneighbors) INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, RANK, MAXNEIGHBORS, NEIGHBORS(*), IERROR

MPI_GRAPH_NEIGHBORS_COUNT and MPI_GRAPH_NEIGHBORS provide
adjacency information for a graph topology.
The returned count and array of neighbors for the queried rank will
both include * all* neighbors and reflect the same edge ordering as
was specified by the original call to MPI_GRAPH_CREATE.
Specifically, MPI_GRAPH_NEIGHBORS_COUNT and
MPI_GRAPH_NEIGHBORS will return values based on the original
index and edges array passed to MPI_GRAPH_CREATE
(for the purpose of this example, we assume that index[-1] is zero):

- The number of neighbors ( nneighbors) returned from MPI_GRAPH_NEIGHBORS_COUNT will be ( index[rank] - index[rank-1]).
- The neighbors array returned from MPI_GRAPH_NEIGHBORS will be edges[index[rank-1]] through edges[index[rank]-1].

process | neighbors |

0 | 1, 1, 3 |

1 | 0, 0 |

2 | 3 |

3 | 0, 2, 2 |

Thus, the input arguments to MPI_GRAPH_CREATE are:

nnodes = | 4 |

index = | 3, 5, 6, 9 |

edges = | 1, 1, 3, 0, 0, 3, 0, 2, 2 |

Therefore, calling MPI_GRAPH_NEIGHBORS_COUNT and MPI_GRAPH_NEIGHBORS for each of the 4 processes will return:

Input rank | Count | Neighbors | |||

0 | 3 | 1, 1, 3 | |||

1 | 2 | 0, 0 | |||

2 | 1 | 3 | |||

3 | 3 | 0, 2, 2 | |||

** Example**
Suppose that comm is a communicator with a
shuffle-exchange topology. The group has *2 ^{n}* members.
Each process is labeled by

node | exchange | shuffle | unshuffle | ||

neighbors(1) | neighbors(2) | neighbors(3) | |||

0 | (000) | 1 | 0 | 0 | |

1 | (001) | 0 | 2 | 4 | |

2 | (010) | 3 | 4 | 1 | |

3 | (011) | 2 | 6 | 5 | |

4 | (100) | 5 | 1 | 2 | |

5 | (101) | 4 | 3 | 6 | |

6 | (110) | 7 | 5 | 3 | |

7 | (111) | 6 | 7 | 7 | |

Suppose that the communicator comm has this topology associated with it. The following code fragment cycles through the three types of neighbors and performs an appropriate permutation for each.

! assume: each process has stored a real number A. ! extract neighborhood information CALL MPI_COMM_RANK(comm, myrank, ierr) CALL MPI_GRAPH_NEIGHBORS(comm, myrank, 3, neighbors, ierr) ! perform exchange permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(1), 0, & neighbors(1), 0, comm, status, ierr) ! perform shuffle permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(2), 0, & neighbors(3), 0, comm, status, ierr) ! perform unshuffle permutation CALL MPI_SENDRECV_REPLACE(A, 1, MPI_REAL, neighbors(3), 0, & neighbors(2), 0, comm, status, ierr)

MPI_DIST_GRAPH_NEIGHBORS_COUNT and MPI_DIST_GRAPH_NEIGHBORS provide adjacency information for a distributed graph topology.

MPI_DIST_GRAPH_NEIGHBORS_COUNT(comm, indegree, outdegree, weighted) | |

IN comm | communicator with distributed graph topology (handle) |

OUT indegree | number of edges into this process (non-negative integer) |

OUT outdegree | number of edges out of this process (non-negative integer) |

OUT weighted | false if MPI_UNWEIGHTED was supplied during creation, true otherwise (logical) |

` int MPI_Dist_graph_neighbors_count(MPI_Comm comm, int *indegree, int *outdegree, int *weighted) `

` MPI_Dist_graph_neighbors_count(comm, indegree, outdegree, weighted, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(OUT) :: indegree, outdegree LOGICAL, INTENT(OUT) :: weighted INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, INDEGREE, OUTDEGREE, IERROR

LOGICAL WEIGHTED

MPI_DIST_GRAPH_NEIGHBORS(comm, maxindegree, sources, sourceweights, maxoutdegree, destinations, destweights) | |

IN comm | communicator with distributed graph topology (handle) |

IN maxindegree | size of sources and sourceweights arrays (non-negative integer) |

OUT sources | processes for which the calling process is a destination (array of non-negative integers) |

OUT sourceweights | weights of the edges into the calling process (array of non-negative integers) |

IN maxoutdegree | size of destinations and destweights arrays (non-negative integer) |

OUT destinations | processes for which the calling process is a source (array of non-negative integers) |

OUT destweights | weights of the edges out of the calling process (array of non-negative integers) |

` int MPI_Dist_graph_neighbors(MPI_Comm comm, int maxindegree, int sources[], int sourceweights[], int maxoutdegree, int destinations[], int destweights[]) `

` MPI_Dist_graph_neighbors(comm, maxindegree, sources, sourceweights, maxoutdegree, destinations, destweights, ierror) TYPE(MPI_Comm), INTENT(IN) :: comm INTEGER, INTENT(IN) :: maxindegree, maxoutdegree INTEGER, INTENT(OUT) :: sources(maxindegree), destinations(maxoutdegree) INTEGER :: sourceweights(*), destweights(*) INTEGER, OPTIONAL, INTENT(OUT) :: ierror `

INTEGER COMM, MAXINDEGREE, SOURCES(*), SOURCEWEIGHTS(*), MAXOUTDEGREE,

DESTINATIONS(*), DESTWEIGHTS(*), IERROR

These calls are local. The number of edges into and out of the process returned by MPI_DIST_GRAPH_NEIGHBORS_COUNT are the total number of such edges given in the call to MPI_DIST_GRAPH_CREATE_ADJACENT or MPI_DIST_GRAPH_CREATE (potentially by processes other than the calling process in the case of MPI_DIST_GRAPH_CREATE). Multiply defined edges are all counted and returned by MPI_DIST_GRAPH_NEIGHBORS in some order. If MPI_UNWEIGHTED is supplied for sourceweights or destweights or both, or if MPI_UNWEIGHTED was supplied during the construction of the graph then no weight information is returned in that array or those arrays. If the communicator was created with MPI_DIST_GRAPH_CREATE_ADJACENT then for each rank in comm, the order of the values in sources and destinations is identical to the input that was used by the process with the same rank in comm_old in the creation call. If the communicator was created with MPI_DIST_GRAPH_CREATE then the only requirement on the order of values in sources and destinations is that two calls to the routine with same input argument comm will return the same sequence of edges. If maxindegree or maxoutdegree is smaller than the numbers returned by MPI_DIST_GRAPH_NEIGHBOR_COUNT, then only the first part of the full list is returned.

* Advice
to implementors.*

Since the query calls are defined to be local, each process needs to
store the list of its neighbors with incoming and outgoing
edges. Communication is required at the collective
MPI_DIST_GRAPH_CREATE call in order to compute the neighbor
lists for each process from the distributed graph specification.
(* End of advice to implementors.*)

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