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Microwulf: A Personal, Portable Beowulf Cluster



MPI Homework Project 1: Getting Started With MPI

Threads Homework Project 1: Threads in C

/* pi.c - parallel C code to demonstrate Linux thread interface 
 * Original Source: www.tldp.org/HOWTP/Parallel_processing-HOWTO 
 * Since PI == 4 * arctan(1), and arctan(x) is the 
 *  integral from 0 to x of (1/(1+x*x),
 *  the for loop below approximates that integration.
 * Generalized by: Joel Adams
 * Usage: ./a.out <numIntervals> <numThreads>
 */

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h> 

/* global variables (shared by all threads */
volatile long double pi = 0.0; /* the approximation, to 31 sigfigs */
pthread_mutex_t piLock;        /* how we synchronize writes to 'pi' */
long double intervals;         /* how finely we chop the integration */
int numThreads;                /* how many threads we use */

/* the function a thread executes
 * Parameters: arg, a void* storing the address of the thread ID.
 */
void *computePI(void *id)
{
    long double x,
                width,
                localSum = 0;
    int i,
        threadID = *((int*)id);

    width = 1.0 / intervals;

    for(i = threadID ; i < intervals; i += numThreads) {
        x = (i+0.5) * width;
        localSum += 4.0 / (1.0 + x*x);
    }

    localSum *= width; 

    pthread_mutex_lock(&piLock);
    pi += localSum;
    pthread_mutex_unlock(&piLock); 

    return NULL;
} 

int main(int argc, char **argv)
{
    pthread_t *threads;        /* dynarray of threads */
    void *retval;              /* unused; required for join() */
    int *threadID;             /* dynarray of thread id #s */
    int i;                     /* loop control variable */

   if (argc == 3) {
      intervals = atoi(argv[1]); 
      numThreads = atoi(argv[2]); 
      threads = malloc(numThreads*sizeof(pthread_t));
      threadID = malloc(numThreads*sizeof(int));
      pthread_mutex_init(&piLock, NULL);

      for (i = 0; i < numThreads; i++) {
        threadID[i] = i;
        pthread_create(&threads[i], NULL, computePI, threadID+i);
      }

      for (i = 0; i < numThreads; i++) {
         pthread_join(threads[i], &retval);
      }
      printf("Estimation of pi is %32.30Lf \n", pi);
      printf("(actual pi value is 3.141592653589793238462643383279...)\n");
    } else {
      printf("Usage: ./a.out <numIntervals> <numThreads>");    
    }

    return 0;
}