Question

I have two implementations of counting pi with Monte-Carlo method: with and without threads. Implementation without threads working just fine, but method with threads have problems with accuracy and perfomance. Here is code:

Without threads:

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


int main()
{
    srand(time(NULL));

    unsigned long N = 0, Nin = 0;
    float x,y;

    while(N < 2E+9)
    {
        x = rand()/((float)RAND_MAX + 1.0)*10.0 - 5.0;
        y = rand()/((float)RAND_MAX + 1.0)*10.0 - 5.0;

        if(x*x + y*y < 25.0) Nin += 1;
        N++;
    }
    long double pi = 4.0 * (long double)Nin / (long double)N;

    printf("\tPi1: %.20Lf\n\t%lu %lu\n", pi, Nin, N);

    return 0;
}

And with threads:

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


typedef struct 
{
    unsigned long Nin;
    unsigned long N;
} nums;


void pi_counter(nums* a)
{
    float x,y;
    unsigned int N = 0, Nin = 0;

    while(N < 1E+9)
    {
        x = rand()/((float)RAND_MAX + 1.0)*10.0 - 5.0;
        y = rand()/((float)RAND_MAX + 1.0)*10.0 - 5.0;


        if(x*x + y*y < 25.0) Nin++;
        N++;
    }

    a -> Nin += Nin;
    a -> N   += N;
}


int main()
{
    pthread_t thread1, thread2, thread3;
    nums a;

    srand(time(NULL));

    pthread_create( &thread1, NULL, pi_counter, &a );
    pthread_create( &thread2, NULL, pi_counter, &a );

    pthread_join( thread1, NULL );
    pthread_join( thread2, NULL ); 

    long double pi = 4.0 * (long double)a.Nin / (long double)a.N;


    printf("\tPi2: %.20Lf\n\t%lu %lu\n", pi, a.Nin, a.N);

    return 0;
}

Results:

$ time ./pi2
    Pi2: 3.14147154999999999995
    1570735775 2000000000

real    1m1.927s
user    1m23.624s
sys 0m0.139s



$ time ./pi
    Pi1: 3.14158868600000000006
    1570794343 2000000000

real    0m49.956s
user    0m49.887s
sys 0m0.022s

Where is my mistake?

Answer 1

rand is not thread-safe; simultaneously using it in multiple threads will result in undefined behavior. You can either wrap it with a function that acquires and holds a mutex while calling rand, or you can use rand_r or (better yet) write a decent PRNG to use in its place.