Parallel Trapezoidal Rule; uses a hand-coded * tree-structured broadcast.

/* get_data1.c -- Parallel Trapezoidal Rule; uses a hand-coded
* tree-structured broadcast.
*
* Input:
* a, b: limits of integration.
* n: number of trapezoids.
* Output: Estimate of the integral from a to b of f(x)
* using the trapezoidal rule and n trapezoids.
*
* Notes:
* 1. f(x) is hardwired.
* 2. the number of processes (p) should evenly divide
* the number of trapezoids (n).
*
* See Chap. 5, pp. 65 & ff. in PPMPI.
*/
#include

/* We'll be using MPI routines, definitions, etc. */
#include "mpi.h"

main(int argc, char** argv) {
int my_rank; /* My process rank */
int p; /* The number of processes */
float a; /* Left endpoint */
float b; /* Right endpoint */
int n; /* Number of trapezoids */
float h; /* Trapezoid base length */
float local_a; /* Left endpoint my process */
float local_b; /* Right endpoint my process */
int local_n; /* Number of trapezoids for */
/* my calculation */
float integral; /* Integral over my interval */
float total; /* Total integral */
int source; /* Process sending integral */
int dest = 0; /* All messages go to 0 */
int tag = 0;
MPI_Status status;

void Get_data1(float* a_ptr, float* b_ptr, int* n_ptr, int my_rank,
int p);
float Trap(float local_a, float local_b, int local_n,
float h); /* Calculate local integral */

/* Let the system do what it needs to start up MPI */
MPI_Init(&argc, &argv);

/* Get my process rank */
MPI_Comm_rank(MPI_COMM_WORLD, &my_rank);

/* Find out how many processes are being used */
MPI_Comm_size(MPI_COMM_WORLD, &p);

Get_data1(&a, &b, &n, my_rank, p);

h = (b-a)/n; /* h is the same for all processes */
local_n = n/p; /* So is the number of trapezoids */

/* Length of each process' interval of
* integration = local_n*h. So my interval
* starts at: */
local_a = a + my_rank*local_n*h;
local_b = local_a + local_n*h;
integral = Trap(local_a, local_b, local_n, h);

/* Add up the integrals calculated by each process */
if (my_rank == 0) {
total = integral;
for (source = 1; source < p; source++) {
MPI_Recv(&integral, 1, MPI_FLOAT, source, tag,
MPI_COMM_WORLD, &status);
total = total + integral;
}
} else {
MPI_Send(&integral, 1, MPI_FLOAT, dest,
tag, MPI_COMM_WORLD);
}

/* Print the result */
if (my_rank == 0) {
printf("With n = %d trapezoids, our estimate\n",
n);
printf("of the integral from %f to %f = %f\n",
a, b, total);
}

/* Shut down MPI */
MPI_Finalize();
} /* main */


/********************************************************************/
/* Ceiling of log_2(x) is just the number of times
* times x-1 can be divided by 2 until the quotient
* is 0. Dividing by 2 is the same as right shift.
*/
int Ceiling_log2(int x /* in */) {
/* Use unsigned so that right shift will fill
* leftmost bit with 0
*/
unsigned temp = (unsigned) x - 1;
int result = 0;

while (temp != 0) {
temp = temp >> 1;
result = result + 1 ;
}
return result;
} /* Ceiling_log2 */


/********************************************************************/
int I_receive(
int stage /* in */,
int my_rank /* in */,
int* source_ptr /* out */) {
int power_2_stage;

/* 2^stage = 1 << stage */
power_2_stage = 1 << stage;
if ((power_2_stage <= my_rank) &&
(my_rank < 2*power_2_stage)){
*source_ptr = my_rank - power_2_stage;
return 1;
} else return 0;
} /* I_receive */

/********************************************************************/
int I_send(
int stage /* in */,
int my_rank /* in */,
int p /* in */,
int* dest_ptr /* out */) {
int power_2_stage;

/* 2^stage = 1 << stage */
power_2_stage = 1 << stage;
if (my_rank < power_2_stage){
*dest_ptr = my_rank + power_2_stage;
if (*dest_ptr >= p) return 0;
else return 1;
} else return 0;
} /* I_send */

/********************************************************************/
void Send(
float a /* in */,
float b /* in */,
int n /* in */,
int dest /* in */) {

MPI_Send(&a, 1, MPI_FLOAT, dest, 0, MPI_COMM_WORLD);
MPI_Send(&b, 1, MPI_FLOAT, dest, 1, MPI_COMM_WORLD);
MPI_Send(&n, 1, MPI_INT, dest, 2, MPI_COMM_WORLD);
} /* Send */


/********************************************************************/
void Receive(
float* a_ptr /* out */,
float* b_ptr /* out */,
int* n_ptr /* out */,
int source /* in */) {

MPI_Status status;

MPI_Recv(a_ptr, 1, MPI_FLOAT, source, 0,
MPI_COMM_WORLD, &status);
MPI_Recv(b_ptr, 1, MPI_FLOAT, source, 1,
MPI_COMM_WORLD, &status);
MPI_Recv(n_ptr, 1, MPI_INT, source, 2,
MPI_COMM_WORLD, &status);
} /* Receive */


/********************************************************************/
/* Function Get_data1
* Reads in the user input a, b, and n.
* Input parameters:
* 1. int my_rank: rank of current process.
* 2. int p: number of processes.
* Output parameters:
* 1. float* a_ptr: pointer to left endpoint a.
* 2. float* b_ptr: pointer to right endpoint b.
* 3. int* n_ptr: pointer to number of trapezoids.
* Algorithm:
* 1. Process 0 prompts user for input and
* reads in the values.
* 2. Process 0 sends input values to other
* processes using hand-coded tree-structured
* broadcast.
*/
void Get_data1(
float* a_ptr /* out */,
float* b_ptr /* out */,
int* n_ptr /* out */,
int my_rank /* in */,
int p /* in */) {

int source;
int dest;
int stage;

int Ceiling_log2(int x);
int I_receive( int stage, int my_rank, int* source_ptr);
int I_send(int stage, int my_rank, int p, int* dest_ptr);
void Send(float a, float b, int n, int dest);
void Receive(float* a_ptr, float* b_ptr, int* n_ptr, int source);

if (my_rank == 0){
printf("Enter a, b, and n\n");
scanf("%f %f %d", a_ptr, b_ptr, n_ptr);
}
for (stage = 0; stage < Ceiling_log2(p); stage++)
if (I_receive(stage, my_rank, &source))
Receive(a_ptr, b_ptr, n_ptr, source);
else if (I_send(stage, my_rank, p, &dest))
Send(*a_ptr, *b_ptr, *n_ptr, dest);
} /* Get_data1*/


/********************************************************************/
float Trap(
float local_a /* in */,
float local_b /* in */,
int local_n /* in */,
float h /* in */) {

float integral; /* Store result in integral */
float x;
int i;

float f(float x); /* function we're integrating */

integral = (f(local_a) + f(local_b))/2.0;
x = local_a;
for (i = 1; i <= local_n-1; i++) {
x = x + h;
integral = integral + f(x);
}
integral = integral*h;
return integral;
} /* Trap */


/********************************************************************/
float f(float x) {
float return_val;
/* Calculate f(x). */
/* Store calculation in return_val. */
return_val = x*x;
return return_val;
} /* f */