多线程编程，为何用了多线程时间反而暴增？

void cal_pi(int intervals)
{
    unsigned int seed = time(NULL);

    int in_circle_points = 0;
    int in_square_points = 0;

    for (int i=0; i<intervals*intervals; ++i)
    {
        double x = (double)rand()/RAND_MAX;
        double y = (double)rand()/RAND_MAX;

        if (x*x + y*y <= 1) in_circle_points += 1;
        in_square_points += 1;
    }
   
    double pi = 4.0 * in_circle_points / in_square_points;
    printf("Total Points: %d, the points in circle: %d, Pi:%lf\n", in_circle_points, in_square_points, pi);
}

int main(int argc, const char *argv[])
{
    clock_t start, end;
    double time_used;

    srand(time(NULL));

    start = clock();
   
    for (int i=0; i<10; ++i)
    {
        cal_pi(100*(i+1));
    }

    end = clock();
    printf("Total Used Time: %f seconds\n", (double)(end-start)/CLOCKS_PER_SEC);
    return 0;
}

void* cal_pi(void* intervals)
{
    unsigned int seed = time(NULL);

    int in_circle_points = 0;
    int in_square_points = 0;

    int num = *((int*)intervals);
    for (int i=0; i<num*num; ++i)
    {
        double x = (double)rand()/RAND_MAX;
        double y = (double)rand()/RAND_MAX;

        if (x*x + y*y <= 1) in_circle_points += 1;
        in_square_points += 1;
    }
   
    double pi = 4.0 * in_circle_points / in_square_points;
    printf("Total Points: %d, the points in circle: %d, Pi:%lf\n", in_circle_points, in_square_points, pi);
    pthread_exit(0);
}

int main(int argc, const char *argv[])
{
    clock_t start, end;
    double time_used;

    srand(time(NULL));

    start = clock();
   
    pthread_t cal_thread[10];
    int args[10];
    for (int i=0; i<10; ++i)
    {
        args[i] = 100*(i+1);
        pthread_create(cal_thread+i, NULL, cal_pi, args+i);
    }

    for (int i=0; i<10; ++i)
    {
        pthread_join(cal_thread[i], NULL);
    }

    end = clock();
    printf("Total Used Time: %f seconds\n", (double)(end-start)/CLOCKS_PER_SEC);
    return 0;
}

Total Points: 7887, the points in circle: 10000, Pi:3.154800
Total Points: 31487, the points in circle: 40000, Pi:3.148700
Total Points: 70776, the points in circle: 90000, Pi:3.145600
Total Points: 125450, the points in circle: 160000, Pi:3.136250
Total Points: 196211, the points in circle: 250000, Pi:3.139376
Total Points: 282642, the points in circle: 360000, Pi:3.140467
Total Points: 384858, the points in circle: 490000, Pi:3.141698
Total Points: 502177, the points in circle: 640000, Pi:3.138606
Total Points: 636192, the points in circle: 810000, Pi:3.141689
Total Points: 785339, the points in circle: 1000000, Pi:3.141356
Total Used Time: 0.097244 seconds
./mul  0.10s user 0.00s system 99% cpu 0.099 total

Total Points: 7906, the points in circle: 10000, Pi:3.162400
Total Points: 31469, the points in circle: 40000, Pi:3.146900
Total Points: 70687, the points in circle: 90000, Pi:3.141644
Total Points: 125873, the points in circle: 160000, Pi:3.146825
Total Points: 195919, the points in circle: 250000, Pi:3.134704
Total Points: 282496, the points in circle: 360000, Pi:3.138844
Total Points: 384970, the points in circle: 490000, Pi:3.142612
Total Points: 502715, the points in circle: 640000, Pi:3.141969
Total Points: 635928, the points in circle: 810000, Pi:3.140385
Total Points: 785658, the points in circle: 1000000, Pi:3.142632
Total Used Time: 3.574630 seconds
./thread  0.75s user 2.83s system 485% cpu 0.737 total