Have fun with MPI in C

Group Management

MPI_GROUP_SIZE returns the size of a group.

MPI_GROUP_SIZE(group, size)

• IN group, group (handle)
• OUT size, number of processes in the group (integer)

C version

int MPI_Group_size(MPI_Group group, int *size)

MPI_GROUP_RANK returns the rank of this process in the given group.

MPI_GROUP_RANK(group, rank)

• IN group, group (handle)
• OUT rank, rank of the calling process in group, or MPI_UNDEFINED if the process is not a member (integer)

C version

int MPI_Group_rank(MPI_Group group, int *rank)

MPI_GROUP_TRANSLATE_RANKS translates the ranks of processes in one group to those in another group.

MPI_GROUP_TRANSLATE_RANKS(group1, n, ranks1, group2, ranks2)

• IN group1, group1 (handle)
• IN n, number of ranks in ranks1 and ranks2 arrays (integer)
• IN ranks1, array of zero or more valid ranks in group1
• IN group2, group2 (handle)
• OUT ranks2, array of corresponding ranks in group2, MPI_UNDEFINED when no correspondence exists C version

int MPI_Group_translate_ranks(MPI_Group group1, int n, const int ranks1[],MPI_Group group2, int ranks2[])

MPI_GROUP_COMPARE compares two groups.

MPI_GROUP_COMPARE(group1, group2, result)

• IN group1, first group (handle)
• IN group2, second group (handle)
• OUT result, result (integer) C version

int MPI_Group_compare(MPI_Group group1,MPI_Group group2, int *result)

Group Constructors

Group constructors are used to subset and superset existing groups. These constructors construct new groups from existing groups. These are local operations, and distinct groups may be defined on different processes; a process may also define a group that does notinclude itself. Consistent definitions are required when groups are used as arguments in communicator-building functions. MPI does not provide a mechanism to build a group from scratch, but only from other, previously defined groups.

MPI_COMM_GROUP accesses the group associated with given communicator.

MPI_COMM_GROUP(comm, group)

• IN comm, communicator (handle)
• OUT group, group corresponding to comm (handle)

C version

int MPI_Comm_group(MPI_Comm comm, MPI_Group *group)

Some useful functions are the set-like operations (definition -> c syntax):

• MPI_GROUP_UNION

  int MPI_Group_union(MPI_Group group1, MPI_Group group2,MPI_Group *newgroup)
  

• MPI_GROUP_INTERSECTION

  int MPI_Group_intersection(MPI_Group group1, MPI_Group group2,MPI_Group *newgroup)
  

• MPI_GROUP_DIFFERENCE

  int MPI_Group_difference(MPI_Group group1, MPI_Group group2,MPI_Group *newgroup)
  

• MPI_GROUP_INCL

  int MPI_Group_incl(MPI_Group group, int n, const int ranks[],MPI_Group *newgroup)
  

  The function MPI_GROUP_INCL creates a group newgroup that consists of then processes in group with ranks ranks[0],..., ranks[n-1]; the process with i in newgroup is the process with rank ranks[i] in group.

• MPI_GROUP_EXCL

  int MPI_Group_excl(MPI_Group group, int n, const int ranks[],MPI_Group *newgroup
  

  The function MPI_GROUP_EXCL creates a group of processes newgroup that is obtained by deleting from group those processes with ranks ranks[0] ,...ranks[n-1]. The ordering of processes in newgroup is identical to the ordering ingroup.

Group Destructors

MPI_GROUP_FREE marks the group object for deallocation.

MPI_GROUP_FREE( group)

• INOUT group, group (handle)

C version

int MPI_Group_free(MPI_Group *group)

Example: MPI groups

The following example uses the MPI_Comm_create_group, which is a function that returns a new communicator for a group of processes. This function is described in the next section.

The following example uses 14 processes.