Why is my OpemMP code performance worst than serial?

Question

I am doing a simple Pi calculation where I parallelize the loop in which random numbers are generated and count is incremented. The serial (non-OpenMP) code performs better than the OpenMP code. Here are some of measurements I took. Both codes are also provided below.

Compiled the serial code as: gcc pi.c -O3

Compiled the OpenMP code as: gcc pi_omp.c -O3 -fopenmp

What could be the problem?

# Iterations = 60000000

Serial Time = 0.893912

OpenMP 1 Threads Time = 0.876654
OpenMP 2 Threads Time = 23.8537
OpenMP 4 Threads Time = 7.72415

Serial Code:

/* Program to compute Pi using Monte Carlo methods */
/* from: http://www.dartmouth.edu/~rc/classes/soft_dev/C_simple_ex.html */

#include <stdlib.h>
#include <stdio.h>
#include <math.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#define SEED 35791246

int main(int argc, char* argv)
{
  int niter=0;
  double x,y;
  int i;
  long count=0; /* # of points in the 1st quadrant of unit circle */
  double z;
  double pi;

  printf("Enter the number of iterations used to estimate pi: ");
  scanf("%d",&niter);

  /* initialize random numbers */
  srand(SEED);
  count=0;
  struct timeval start, end;
  gettimeofday(&start, NULL);
  for ( i=0; i<niter; i++) {
    x = (double)rand()/RAND_MAX;
    y = (double)rand()/RAND_MAX;
    z = x*x+y*y;
    if (z<=1) count++;
  }
  pi=(double)count/niter*4;

  gettimeofday(&end, NULL);
  double t2 = end.tv_sec + (end.tv_usec/1000000.0);
  double t1 = start.tv_sec + (start.tv_usec/1000000.0);

  printf("Time: %lg\n", t2 - t1);

  printf("# of trials= %d , estimate of pi is %lg \n",niter,pi);
  return 0;
}

OpenMP Parallel Code:

#include <omp.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <time.h>
#include <sys/time.h>
#define SEED 35791246
/*
from: http://www.dartmouth.edu/~rc/classes/soft_dev/C_simple_ex.html
 */
#define CHUNKSIZE 500
int main(int argc, char *argv[]) {

  int chunk = CHUNKSIZE;
  int niter=0;
  double x,y;
  int i;
  long count=0; /* # of points in the 1st quadrant of unit circle */
  double z;
  double pi;

  int nthreads, tid;

  printf("Enter the number of iterations used to estimate pi: ");
  scanf("%d",&niter);

  /* initialize random numbers */
  srand(SEED);
  struct timeval start, end;

  gettimeofday(&start, NULL);
  #pragma omp parallel shared(chunk) private(tid,i,x,y,z) reduction(+:count)  
  {                                                                                                           
    /* Obtain and print thread id */
    tid = omp_get_thread_num();
    //printf("Hello World from thread = %d\n", tid);

    /* Only master thread does this */
    if (tid == 0)
    {
      nthreads = omp_get_num_threads();
      printf("Number of threads = %d\n", nthreads);
    }

    #pragma omp for schedule(dynamic,chunk)                                                                       
    for ( i=0; i<niter; i++) {                                                                              
      x = (double)rand()/RAND_MAX;                                                                          
      y = (double)rand()/RAND_MAX;                                                                          
      z = x*x+y*y;                                                                                          
      if (z<=1) count++;                                                                                    
    }                                                                                                       
  }                                                                                                           

  gettimeofday(&end, NULL);
  double t2 = end.tv_sec + (end.tv_usec/1000000.0);
  double t1 = start.tv_sec + (start.tv_usec/1000000.0);

  printf("Time: %lg\n", t2 - t1);

  pi=(double)count/niter*4;                                                                                   
  printf("# of trials= %d, threads used: %d, estimate of pi is %lg \n",niter,nthreads, pi);
  return 0;
}

Answer 1

In this particular case, there are many possibilities since openMP takes 10K - 100K cycles to start a loop, performance improvements with openMP are non trivial.

after this we have the additional problem that rand is not re-entrant http://man7.org/linux/man-pages/man3/rand.3.html

so most likely rand can only be called by one thread at a time, hence your open MP version is essentially single threaded since your loop does little else, with the additional contention overhead every time rand is called - hence the dramatic slowdown.

Answer 2

rand() is not re-entrant. It will either not work properly, crash, or only be possible to call from one thread at a time. Libraries like glibc will often serialize or use TLS for legacy non-re-entrant functions rather than have them randomly crash when they get used in multi-threaded code.

Try the re-entrant form, rand_r() :

tid = omp_get_thread_num();
unsigned int seed = tid;
...
x = (double)rand_r(&seed)/RAND_MAX;

I think you'll find it's much faster.

Notice how I set the seed to the tid. You might think, why not initialize the seed to SEED ? Given the same seed, rand_r() will produce the same sequence of numbers. If each thread uses the same series of pseudo-random numbers, it defeats the point of doing more iterations! You've got to get each thread to use different numbers.