Pthreads matrix multiplication error

Question

I want to use pthreads on my existing serial matrix multiplication code. My goal is to achieve better execution time using pthreads, simply to achieve speed-up. But at that point I'm stuck. My original serial code, works just fine, and I finish 1000x1000 square matrix multiplication in about 15 seconds. But when I execute my current pthreads program, I get a segmentation fault. Here is my code:

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

int SIZE, NTHREADS;
int **A, **B, **C;

void init()
{
    int i, j;

    A = (int**)malloc(SIZE * sizeof(int *));
    for(i = 0; i < SIZE; i++)
        A[i] = malloc(SIZE * sizeof(int));

    B = (int**)malloc(SIZE * sizeof(int *));
    for(i = 0; i < SIZE; i++)
        B[i] = malloc(SIZE * sizeof(int));

    C = (int**)malloc(SIZE * sizeof(int *));
    for(i = 0; i < SIZE; i++)
        C[i] = malloc(SIZE * sizeof(int));

    srand(time(NULL));

    for(i = 0; i < SIZE; i++) {
        for(j = 0; j < SIZE; j++) {
            A[i][j] = rand()%100;
            B[i][j] = rand()%100;
        }
    }
}

void mm(int tid)
{
    int i, j, k;
    int start = tid * SIZE/NTHREADS;
    int end = (tid+1) * (SIZE/NTHREADS) - 1;

    for(i = start; i <= end; i++) {
        for(j = 0; j < SIZE; j++) {
            C[i][j] = 0;
            for(k = 0; k < SIZE; k++) {
                C[i][j] += A[i][k] * B[k][j];
            }
        }
    }
}

void *worker(void *arg)
{
    int tid = *((int *) arg);
    mm(tid);
}

int main(int argc, char* argv[])
{
    pthread_t* threads;
    int rc, i;

    if(argc != 3)
    {
        printf("Usage: %s <size_of_square_matrix> <number_of_threads>\n", argv[0]);
        exit(1);
    }

    SIZE = atoi(argv[1]);
    NTHREADS = atoi(argv[2]);
    init();
    threads = (pthread_t*)malloc(NTHREADS * sizeof(pthread_t));

    clock_t begin, end;
    double time_spent;


    begin = clock();

    for(i = 0; i < NTHREADS; i++) {
        rc = pthread_create(&threads[i], NULL, worker, (void *)i);
        assert(rc == 0);
    }

    for(i = 0; i < NTHREADS; i++) {
        rc = pthread_join(threads[i], NULL);
        assert(rc == 0);
    } 

    end = clock();

    time_spent = (double)(end - begin) / CLOCKS_PER_SEC;
    printf("Elapsed time: %.2lf seconds.\n", time_spent);

    for(i = 0; i < SIZE; i++)
        free((void *)A[i]);
    free((void *)A);

    for(i = 0; i < SIZE; i++)
        free((void *)B[i]);
    free((void *)B);

    for(i = 0; i < SIZE; i++)
        free((void *)C[i]);
    free((void *)C);

    free(threads);

    return 0;
}

If someone could help me make my pthreads program run, and achieve some speed-up I would be glad.

simonc simonc · Accepted Answer · 2013-04-12T14:56:10

With your current code, you should retrieve the index using

int tid = (int)arg;

(Your code is effectively treating the loop counter as an address then dereferencing addresses at or around 0. These addresses may not be readable by your process and/or won't be suitably aligned, hence the seg fault)

The above change might get things working for you but note that passing an int as a void* isn't completely correct. It relies on sizeof(int) <= sizeof(void*) which is likely but not guaranteed to be true. If you cared about this, you could either allocate memory for the data you pass to each thread instead or pass the address of i and include synchronisation to ensure that you wait after each pthread_create call until the thread has been scheduled and has read its arg.

Pthreads matrix multiplication error

1 Answers