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#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <stdlib.h>
#include <math.h>
#include "mpi.h"
#define MAX_PARTICLES 1000
#define MAX_PROCS 128
#define EPSILON 1.0E-10
typedef struct {
double x, y, z;
double mass;
} Particle;
typedef struct {
double vx, vy, vz;
} ParticleV;
int main ( argc, argv )
int argc;
char **argv;
{
int pool_size, my_rank, source, node_name_length;
char char_buffer[BUFSIZ], my_node_name[BUFSIZ];
MPI_Status status;
extern double drand48();
extern void srand48();
Particle particles[MAX_PARTICLES]; /* Particles on all nodes */
ParticleV vector[MAX_PARTICLES]; /* Particle velocity */
int counts[MAX_PROCS]; /* Number of ptcls on each proc */
int displacements[MAX_PROCS]; /* Offsets into particles */
int offsets[MAX_PROCS]; /* Offsets used by the master */
int particle_number, i, j,
my_offset; /* Location of local particles */
int total_particles; /* Total number of particles */
int count; /* Number of times in loop */
double start_time, end_time;
MPI_Datatype particle_type;
MPI_Init ( &argc, &argv );
MPI_Comm_size ( MPI_COMM_WORLD, &pool_size );
MPI_Comm_rank ( MPI_COMM_WORLD, &my_rank );
MPI_Get_processor_name ( my_node_name, &node_name_length );
sprintf ( char_buffer, "processor %s, rank %d", my_node_name, my_rank );
MPI_Send ( char_buffer, strlen(char_buffer) + 1, MPI_CHAR, 0, 2002,
MPI_COMM_WORLD );
if ( my_rank == 0 ) {
for ( source = 0; source < pool_size; source++ ) {
MPI_Recv ( char_buffer, BUFSIZ, MPI_CHAR, source, 2002,
MPI_COMM_WORLD, &status );
printf ( "%s\n", char_buffer );
}
}
/* For simplicity we assume that every process is responsible for the same
number of particles. But the program can also handle the case when
particle numbers are different for different processes.
*/
particle_number = MAX_PARTICLES / pool_size;
if ( my_rank == 0 )
printf ("%d particles per processor\n", particle_number);
/* Define the new MPI data type "particle_type", which correponds to
structure "Particle".
*/
MPI_Type_contiguous ( 4, MPI_DOUBLE, &particle_type );
MPI_Type_commit ( &particle_type );
/* Distribute the number of particles each process is supposed to look after
in such a way that every process will find that number in
count[my_rank]. In other slots of that array each process finds
the number of particles other processes will work on.
*/
MPI_Allgather ( &particle_number, 1, MPI_INT, counts, 1, MPI_INT,
MPI_COMM_WORLD );
/* Data about all particles is stored on a large array called "particles".
Every process has a copy of that array. Here we evaluate, where the
segment of the array begins, which a given process will work on.
*/
displacements[0] = 0;
for (i = 1; i < pool_size; i++)
displacements[i] = displacements[i-1] + counts[i-1];
total_particles = displacements[pool_size - 1] + counts[pool_size - 1];
if ( my_rank == 0 )
printf ("total number of particles = %d\n", total_particles);
my_offset = displacements[my_rank];
/* Process rank 0 gathers information from all other processes about
offsets they calculated for the array "particles".
*/
MPI_Gather ( &my_offset, 1, MPI_INT, offsets, 1, MPI_INT, 0,
MPI_COMM_WORLD );
if ( my_rank == 0 ) {
printf ("offsets: ");
for (i = 0; i < pool_size; i++)
printf ("%d ", offsets[i]);
printf("\n");
fflush(stdout);
}
/* Every process seeds the random number routine drand48 with its own
process rank.
*/
srand48 ( (long) my_rank );
/* And now every process initialises positions and mass of particles
it is responsible for.
*/
for (i = 0; i < particle_number; i++) {
particles[my_offset + i].x = drand48();
particles[my_offset + i].y = drand48();
particles[my_offset + i].z = drand48();
particles[my_offset + i].mass = 1.0;
}
start_time = MPI_Wtime();
/* Here all processes exchange information about their particles with
one another. When this operation is finished every process will have
an identical fully initialised array "particles".
*/
MPI_Allgatherv ( particles + my_offset, particle_number,
particle_type,
particles, counts, displacements, particle_type,
MPI_COMM_WORLD );
end_time = MPI_Wtime();
if ( my_rank == 0 ) {
printf ("Communicating = %8.5f seconds\n", end_time - start_time);
printf ("particles[offsets[i]].x: ");
for (i = 0; i < pool_size; i++)
printf ("%8.5f ", particles[offsets[i]].x);
printf("\n"); fflush(stdout);
}
start_time = MPI_Wtime();
count = 0;
for (i = 0; i < particle_number; i++) {
vector[my_offset + i].vx = 0.0;
vector[my_offset + i].vy = 0.0;
vector[my_offset + i].vz = 0.0;
/* Knowing about positions of all particles in the system, every process
can evaluate gravitational accelerations acting on its own particles,
and write them on array "vector". Once accelerations are known,
particle velocities (not calculated here) can be updated, and
particles can be "pushed".
*/
for (j = 0; j < total_particles; j++) {
if (j != i) {
double dx, dy, dz, r2, r, mimj_by_r3;
dx = particles[my_offset + i].x - particles[j].x;
dy = particles[my_offset + i].y - particles[j].y;
dz = particles[my_offset + i].z - particles[j].z;
r2 = dx * dx + dy * dy + dz * dz; r = sqrt(r2);
if (r2 < EPSILON) mimj_by_r3 = 0.0;
else
mimj_by_r3 = particles[my_offset + i].mass
* particles[j].mass / (r2 * r);
vector[my_offset + i].vx = vector[my_offset + i].vx +
mimj_by_r3 * dx;
vector[my_offset + i].vy = vector[my_offset + i].vy +
mimj_by_r3 * dy;
vector[my_offset + i].vz = vector[my_offset + i].vz +
mimj_by_r3 * dz;
count = count + 1;
}
}
}
end_time = MPI_Wtime();
if ( my_rank == 0 )
printf ("done my job in %8.5f seconds, waiting for slow \
processes...\n", end_time - start_time);
MPI_Barrier (MPI_COMM_WORLD);
end_time = MPI_Wtime();
if ( my_rank == 0 )
printf ("evaluated %d 3D interactions in %8.5f seconds\n",
count * pool_size, end_time - start_time);
MPI_Finalize ();
}
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