1	Basic Message Passing
2	Processes Process – a set of tasks for a processor to execute Distinct from processor Process created either statically or dynamically during execution of a program
3	Processes source executable executable processor 0 processor 1
4	Processes
5	Basic Messages
6	MPI Messages
7	MPI Messages
8	MPI Messages
9	Hello World #include <stdio.h> #include "mpi.h“ main(int argc, char** argv) { int my_rank; /* Rank of process / int p; / Number of processes / int source; / Rank of sender / int dest; / Rank of receiver / int tag = 50; / Tag for messages / char message[100]; / Storage for the message / MPI_Status status; / Return status for receive */ MPI_Init(&argc, &argv); MPI_Comm_rank(MPI_COMM_WORLD, &my_rank); MPI_Comm_size(MPI_COMM_WORLD, &p);
10	Hello World if (my_rank != 0) { sprintf(message, “Hello from process %d!", my_rank); dest = 0; /* Use strlen(message)+1 to include endofline / MPI_Send(message, strlen(message)+1, MPI_CHAR, dest, tag, MPI_COMM_WORLD); } else / my_rank == 0 / { for (source = 1; source < p; source++) { MPI_Recv(message, 100, MPI_CHAR, source, tag, MPI_COMM_WORLD, &status); printf("%s＼n", message); } } MPI_Finalize(); } / main */
11	MPI Messages
12	MPI Messages
13	MPI Messages
14	MPI Messages
15	MPI Messages
16	MPI Messages
17	MPI Messages An alternative approach to grouping data is provided by the MPI functions MPI_Pack and MPI_Unpack. MPI Pack allows one to explicitly store noncon- tiguous data in contiguous memory locations, and MPI Unpack can be used to copy data from a contiguous buer into noncontiguous memory locations.
18	MPI Messages void Get_data4(int my_rank, float* a_ptr, float* b_ptr, int* n_ptr) { int root = 0; /* Argument to MPI_Bcast / char buffer[100]; / Arguments to MPI_Pack/Unpack / int position; / and MPI_Bcast/ if (my_rank == 0) { printf("Enter a, b, and n"); scanf("%f %f %d", a_ptr, b_ptr, n_ptr); / Now pack the data into buffer / position = 0; / Start at beginning of buffer / MPI_Pack(a_ptr, 1, MPI_FLOAT, buffer, 100,&position, MPI_COMM_WORLD); / Position has been incremented by / / sizeof(float) bytes */ MPI_Pack(b_ptr, 1, MPI_FLOAT, buffer, 100, &position, MPI_COMM_WORLD);
19	MPI Messages MPI_Pack(n_ptr, 1, MPI_INT, buffer, 100, &position, MPI_COMM_WORLD); /* Now broadcast contents of buffer / MPI_Bcast(buffer, 100, MPI_PACKED, root, MPI_COMM_WORLD); } else { MPI_Bcast(buffer, 100, MPI_PACKED, root, MPI_COMM_WORLD); / Now unpack the contents of buffer / position = 0; MPI_Unpack(buffer, 100, &position, a_ptr, 1, MPI_FLOAT, MPI_COMM_WORLD); / Again position incremented by sizeof(float) bytes */ MPI_Unpack(buffer, 100, &position, b_ptr, 1, MPI_FLOAT, MPI_COMM_WORLD); MPI_Unpack(buffer, 100, &position, n_ptr, 1, MPI_INT, MPI_COMM_WORLD); } }
20	MPI Messages In this code process 0 uses MPI_Pack to copy a to buffer and then append b and n. After the broadcast of buffer, the remaining processes use MPI_Unpack to extract a, b, and n from the buffer. The datatype for the calls to MPI_Bcast is MPI_PACKED. The syntax is int MPI_Pack(void* pack_data, int in_count, MPI_Datatype datatype, void* buffer, int size, int* position_ptr, MPI_Comm comm) int MPI_Unpack(void* buffer, int size, int* position_ptr, void* unpack_data, int count, MPI_Datatype datatype, MPI_comm comm)
21	MPI Messages MPI_COMM_WORLD Ability to define communicators, treat certain subsets of processes as a communication universe MPI_Group MPI_GROUP_WORLD MPI_Group first_row_group /* Create the new communicator */ MPI_Comm_create(MPI_COMM_WORLD, first_row_group, &first_row_comm);