How Much Do Each Cost?

• Create-delete costs.

• "Costs" mean execution time.

The Contenders

• pid_t pid;
  timeit(s, 
    switch (pid = fork()) {
      case 0: 
        exit(EXIT_SUCCESS);
      case -1:
        perror("fork() error:");
        exit(EXIT_FAILURE);
      default:
        assert(wait(NULL) == pid);
      }
    );
  

• pthread_t tid;
  int e;
  timeit(s, 
    if ((e = pthread_create(&tid, NULL, f, NULL))) {
      printf("pthread_create() failed with error %d.\n", e);
      return EXIT_FAILURE;
      }
    if ((e = pthread_join(tid, NULL))) {
      printf("pthread_join() failed with error %d.\n", e);
      return EXIT_FAILURE;
      }
    );
  

The details

#define iters 2000

static void * f(void *) {
  return NULL;
  }

int main(int argc, char * argv[]) {

  int i;
  time_t s, oh;

  timeit(oh,);
  printf("%d usec loop overhead for %d iterations, %g usec/iter.\n",
	 oh, iters, ((double) oh)/((double) iters));

  // fork timing code

  printf("%g usec per fork() call for %d calls.\n", 
	 ((double) (s - oh))/((double) iters), iters);


  // thread timing code

  printf("%g usec per pthread_create() call for %d calls.\n",
	 ((double) (s - oh))/((double) iters),  iters);


  return EXIT_SUCCESS;
  }

The Results

cl date
Thu Mar 15 14:42:55 EST 2001

cl uname -a
SunOS clayton 5.7 Generic_106541-11 sun4u sparc SUNW,Ultra-5_10

cl g++ -o forktime forktime.cc -lpthread

cl forktime
Loop overhead is 79 usec for 2000 iterations, 0.0395 usec/iteration.
Time per fork() call for 2000 calls: 7817.5 usec.
Time per pthread_create() call for 2000 calls: 104.193 usec.

cl forktime
Loop overhead is 79 usec for 2000 iterations, 0.0395 usec/iteration.
Time per fork() call for 2000 calls: 7864.8 usec.
Time per pthread_create() call for 2000 calls: 109.993 usec.

cl CC -o forktime forktime.cc -lpthread

cl forktime
Loop overhead is 19 usec for 2000 iterations, 0.0095 usec/iteration.
Time per fork() call for 2000 calls: 7803.37 usec.
Time per pthread_create() call for 2000 calls: 82.0435 usec.

cl forktime
Loop overhead is 19 usec for 2000 iterations, 0.0095 usec/iteration.
Time per fork() call for 2000 calls: 7825.22 usec.
Time per pthread_create() call for 2000 calls: 103.33 usec.

cl 

Other Systems

• ro date
  Thu Mar 15 14:43:42 EST 2001
  
  ro uname -a
  Linux rockhopper.monmouth.edu 2.2.16-22smp #1 SMP 
  Tue Aug 22 16:39:21 EDT 2000 i686 unknown
  
  ro g++ -o forktime forktime.cc -lpthread
  
  ro forktime
  Loop overhead is 22 usec for 2000 iterations, 0.011 usec/iteration.
  Time per fork() call for 2000 calls: 497.743 usec.
  Time per pthread_create() call for 2000 calls: 102.731 usec.
  
  ro forktime
  Loop overhead is 22 usec for 2000 iterations, 0.011 usec/iteration.
  Time per fork() call for 2000 calls: 515.471 usec.
  Time per pthread_create() call for 2000 calls: 103.155 usec.
  

• mo date
  Thu Mar 15 14:46:00 EST 2001
  
  mo uname -a
  OSF1 moncol.MONMOUTH.EDU V5.1 732 alpha
  
  mo cxx -o forktime forktime.cc -lpthread
  
  mo forktime
  Loop overhead is 0 usec for 2000 iterations, 0 usec/iteration.
  Time per fork() call for 2000 calls: 40317.3 usec.
  Time per vfork() call for 2000 calls: 29895.1 usec.
  Time per pthread_create() call for 2000 calls: 766.591 usec.
  
  mo forktime
  Loop overhead is 0 usec for 2000 iterations, 0 usec/iteration.
  Time per fork() call for 2000 calls: 40083.4 usec.
  Time per vfork() call for 2000 calls: 30122.1 usec.
  Time per pthread_create() call for 2000 calls: 902.331 usec.
  

Conclusions

• Thread create and delete is cheap.

• Process create and delete is not - sometimes really not.

• Why not do threads always?

  • Process simplicity, support, and security.

• Amortize create-delete costs over many computations.

  • Inter-computation communication costs.

  • Can be tricky to do.

  • What about the connection?

This page last modified on 10 April 2001.