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1 Process Groups & Communicators Communicator is a group of processes that can communicate with one another. Can create sub-groups of processes, or sub- communicators Cannot create process groups or sub- communicators from scratch Need to create sub-groups or sub-communicators from some existing groups or communicators i.e. need to start from MPI_COMM_WORLD (or MPI_COMM_SELF)

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2 Sub-communicators Communicator Process Group Process sub-group Sub-communicator MPI_Comm MPI_Group

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3 Communicator Process Group MPI_Group is a handle representing a process group in C; in Fortran it is an integer MPI_GROUP_EMPTY, predefined, no member in group MPI_GROUP_NULL, predefined, an invalid handle MPI_COMM_GROUP gets group associated with communicator int MPI_Comm_group(MPI_Comm comm, MPI_Group *group) MPI_COMM_GROUP(COMM, GROUP, IERROR) integer COMM, GROUP, IERROR... MPI_Group group; MPI_Comm_group(MPI_COMM_WORLD, &group);...

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4 Process Groups MPI_Group_size (): number of processes in group MPI_Group_rank (): rank of calling process in group; if does not belong to group, return MPI_UNDEFINED int MPI_Group_size(MPI_Group group, int *size) int MPI_Group_rank(MPI_Group group, int *rank) int ncpus, rank; MPI_Group MPI_GROUP_WORLD;... MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD); MPI_Group_size(MPI_GROUP_WORLD, &ncpus); MPI_Group_rank(MPI_GROUP_WORLD, &rank);...

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5 Group Constructor/Destructor MPI_Group_incl : create a new group newgroup consisting of n processes in group whose ranks are specified in *ranks ; MPI_Group_excl : create a new group newgroup consisting of processes from group excluding those n processes specified in *ranks ; int MPI_Group_incl(MPI_Group group, int n, int *ranks, MPI_Group *newgroup) int MPI_Group_excl(MPI_Group group, int n, int *ranks, MPI_Group *newgroup) int MPI_Group_free(MPI_Group *group); MPI_Group MPI_GROUP_WORLD, slave; int ranks[1]; ranks[0] = 0; MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD); MPI_Group_excl(MPI_GROUP_WORLD, 1, &ranks, &slave);... MPI_Group_free(&slave);

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6 Process Group Communicator Only communicators can be used in communication routines MPI_Comm_create (): create a communicator from a process group group is a sub-group of that associated with comm Collective operation int MPI_Comm_create(MPI_Comm comm, MPI_Group group, MPI_Comm *newcomm) MPI_Group MPI_GROUP_WORLD, slave; MPI_Comm slave_comm; int ranks[1]; Ranks[0] = 0; MPI_Comm_group(MPI_COMM_WORLD, &MPI_GROUP_WORLD); MPI_Group_excl(MPI_GROUP_WORLD, 1, &ranks, &slave); MPI_Comm_create(MPI_COMM_WORLD, slave, &slave_comm);... MPI_Group_free(&slave); MPI_Comm_free(&slave_comm);

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7 Communicator Constructors MPI_Comm_dup (): duplicate communicator Same group of processes, but different communication context Communications sent using duplicated comm cannot be received with original comm. Used for writing parallel libraries; will not interfere with user’s communications Collective routine; All processes must call same function int MPI_Comm_dup(MPI_Comm comm, MPI_Comm *newcomm)

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8 Communicator Constructors Partition comm into disjoint sub-communicators, one for each value color Processes providing same color values will belong to the same new communicator newcomm Within newcomm, processes ranked based on key values that are provided color must be non-negative. If MPI_UNDEFINED is provided in color, MPI_COMM_NULL will be returned in newcomm. int MPI_Comm_split(MPI_Comm comm, int color, int key, MPI_Comm *newcomm) int rank, color; MPI_Comm newcomm; MPI_Comm_rank(MPI_COMM_WORLD, &rank); color = rank%3; MPI_Comm_split(MPI_COMM_WORLD, color, rank, &newcomm);...

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9 Process Virtual Topology Linear ranking (0, 1, …, N-1) often does not reflect logical communication patterns of processes Desirable to arrange processes logically to reflect the topological patterns underlying the problem geometry or numerical algorithm, e.g. 2D or 3D grids Virtual topology: this logical process arrangement Virtual topology does not reflect machine’s physical topology Can build virtual topology on communicators

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10 Virtual Topology Communication patterns can be described by a graph in general Nodes stand for processes Edges connect processes that communicate with each other Many applications use process topologies like rings, 2D or high-D grids or tori. MPI provides 2 topology constructors Cartesian topology General graph topology

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11 Cartesian Topology n-dimensional Cartesian topology (m1, m2, m3, …, mn) grid; mi is the number of processes in i-th direction A process can be represented by its coordinate (j1, j2, j3, …, jn), where 0<=js<=ms Constructor: MPI_Cart_create() Translation: Process rank coordinate Coordinate process rank

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12 Cartesian Constructor comm_old : existing communicator ndims : dimension of Cartesian topology dims : vector, dimension [ ndims ], number of processes in each direction periods : vector, dimension [ ndims ], true or false, priodic or not in each direction reorder : true may reorder rank; false, no re-ordering of ranks comm_cart : new communicator with Cartesian topology Int MPI_Cart_create(MPI_Comm comm_old, int ndims, int *dims, int *periods, int reorder, MPI_Comm *comm_cart) MPI_Comm comm_new; int dims[2], periods[2], ncpus; MPI_Comm_size(MPI_COMM_WORLD, &ncpus); dims[0] = 2; dims[1] = ncpus/2; // assume ncpus dividable by 2 periods[0] = periods[1] = 1; MPI_Cart_create(MPI_COMM_WORLD, 2, dims, periods, 0, &comm_new);...

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13 Topology Inquiry Functions MPI_Cartdim_get returns the dimension of Cartesian topology MPI_Cart_coords returns the coordinates of a process of rank MPI_Cart_rank returns of the rank of a process with coordinate *coords int MPI_Cartdim_get(MPI_Comm comm, int *ndims) int MPI_Cart_coords(MPI_Comm comm, int rank, int maxdims, int *coords) int MPI_Cart_rank(MPI_Comm, int *coords, int *rank) MPI_Comm comm_cart; int ndims, rank, *coords;... // create Cartesian topology on comm_cart MPI_Comm_rank(comm_cart, &rank); MPI_Cartdim_get(comm_cart, &ndims); coords = new int[ndims]; MPI_Cart_coords(comm_cart, rank, ndims, coords); // coords contains coord... for(int i=0;i

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