ITS Meccatronico - Soluzione esercizio 3 lezione 6

its-l6-es3-mt-dot-product.c

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

// Data la definizione di prodotto scalare
// tra vettori enunciata qui sopra,
// scrivere un programma single-threaded
// che calcoli il prodotto scalare tra
// due vettori di double di grande
// dimensione (es. 10k elementi).

// Scrivere una versione del programma
// multi-threaded con NUM_THREADS
// threads che divida il lavoro tra
// i vari thread. Verificare che i
// risultati siano identici alla
// versione single-threaded.
// Confrontare le performance con la
// versione single-threaded.

#define NUM_THREADS 4

double dot(double u[], double v[], int size) {
    double r = 0.0;
    for (int i = 0; i < size; i++)
        r += u[i] * v[i];
    return r;
}

void fillrand(double u[], int size) {
    for (int i = 0; i < size; i++) {
        u[i] = ((double)rand() / (double)RAND_MAX);
    }
}

void printvec(double u[], int size) {
    printf("[");
    for (int i = 0; i < size; i++) {
        printf("%g", u[i]);
        if (i + 1 < size)
            printf(", ");
    }
    printf("]\n");
}

struct subvecs {
    double *u;
    double *v;
    double r;
    int size;
};

void *threadfunc(void *param) {
    struct subvecs *work = (struct subvecs *)param;
    
    work->r = dot(work->u, work->v, work->size);
    return NULL;
}

double mt_dot(double u[], double v[], int size) {
    int size_per_thread = size / NUM_THREADS;
    pthread_t tid[NUM_THREADS];
    struct subvecs work[NUM_THREADS];

    for (int i = 0; i < NUM_THREADS; i++) {
        work[i].u = &u[i * size_per_thread];
        work[i].v = &v[i * size_per_thread];
        work[i].size = size_per_thread;
        if (pthread_create(&tid[i], NULL, threadfunc, &work[i]) != 0) {
            perror("pthread_create");
            exit(EXIT_FAILURE);
        }
    }
    
    double r = 0.0;
    for (int i = 0; i < NUM_THREADS; i++) {
        pthread_join(tid[i], NULL);
        
        r += work[i].r;
    }
    
    return r;
}

#define BIGVECSIZE 800000
double g_u[BIGVECSIZE];
double g_v[BIGVECSIZE];

int main() {
    fillrand(g_u, BIGVECSIZE);
    fillrand(g_v, BIGVECSIZE);

    // printf("u = "); printvec(g_u, BIGVECSIZE);
    // printf("v = "); printvec(g_v, BIGVECSIZE);
    time_t t1 = time(NULL);
    double r;
    for (int i = 0; i < 500; i++)
        r = dot(g_u, g_v, BIGVECSIZE);
    time_t t2 = time(NULL);
    printf("[S] u . v = %g (%lds)\n", r, (long)(t2-t1));

    time_t t3 = time(NULL);
    double r2;
    for (int i = 0; i < 500; i++)
        r2 = mt_dot(g_u, g_v, BIGVECSIZE);
    time_t t4 = time(NULL);
    printf("[T] u . v = %g (%lds)\n", r2, (long)(t4-t3));

    return 0;
}