Download MPI: Understanding the Message Passing Interface for Parallel Programming and more Slides Parallel Computing and Programming in PDF only on Docsity! MPI: the Message Passing Interface • MPI defines a standard library for message-passing that can be used to develop portable message- passing programs using either C or Fortran. • The MPI standard defines both the syntax as well as the semantics of a core set of library routines. • Vendor implementations of MPI are available on almost all commercial parallel computers. • It is possible to write (min) fully-functional message- passing programs by using only the six routines. docsity.com MPI: the Message Passing Interface The minimal set of MPI routines. MPI_Init Initializes MPI. MPI_Finalize Terminates MPI. MPI_Comm_size Determines the number of processes. MPI_Comm_rank Determines the label of calling process. MPI_Send Sends a message. MPI_Recv Receives a message. docsity.com Querying Information • The MPI_Comm_size & MPI_Comm_rank functions are used to determine the number of processes & the label of the calling process, respectively. • The calling sequences of these routines are as follows: int MPI_Comm_size(MPI_Comm comm, int *size) int MPI_Comm_rank(MPI_Comm comm, int *rank) • The rank of a process is an integer that ranges from zero up to the size of the communicator minus one. docsity.com Our First MPI Program #include <mpi.h> main(int argc, char *argv[]) { int npes, myrank; MPI_Init(&argc, &argv); MPI_Comm_size(MPI_COMM_WORLD, &npes); MPI_Comm_rank(MPI_COMM_WORLD, &myrank); printf("From process %d out of %d, Hello World!\n", myrank, npes); MPI_Finalize(); } docsity.com MPI Datatypes MPI Datatype C Datatype MPI_CHAR signed char MPI_SHORT signed short int MPI_INT signed int MPI_LONG signed long int MPI_UNSIGNED_CHAR unsigned char MPI_UNSIGNED_SHORT unsigned short int MPI_UNSIGNED unsigned int MPI_UNSIGNED_LONG unsigned long int MPI_FLOAT float MPI_DOUBLE double MPI_LONG_DOUBLE long double MPI_BYTE MPI_PACKED MPI provides equivalent datatypes for all C datatypes. This is done for portability reasons. The datatype MPI_BYTE corresponds to a byte (8 bits) and MPI_PACKED corresponds to a collection of data items that has been created by packing non-contiguous data. docsity.com Sending and Receiving Messages • On the receiving end, the status variable can be used to get information about the MPI_Recv operation. • The corresponding data structure contains: typedef struct MPI_Status { int MPI_SOURCE; int MPI_TAG; int MPI_ERROR; }; • The MPI_Get_count function returns the precise count of data items received. int MPI_Get_count(MPI_Status *status, MPI_Datatype datatype, int *count) docsity.com Example MPI-Program: Adding N-numbers using p-processors (N > p , N mod p = 0) docsity.com if (my rank == 0)
{
printf ("Enter N:" );
scanf ("“%d", &N) ;
for (i= 1; i < group size; i++)
MPI_Send(&N, 1, MPI_INT, i, i, MPI_COMM_WORLD) ;
for (1 = my_rank; i<N; 1=1+ group size)
sum = sum + x(i) ;
for (i = 1; 1< group size; i++)
(
MPI Recv(&tmp, 1, MPI_INT, i, i, MPI COMM WORLD, &status) ;
sum = sum + tmp;
printf (“The result = $d”, sum) ;
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