Beispiellösung

#include <stdio.h>
#include <stdlib.h>
#include <sys/times.h>
#include <sys/wait.h>
#include <unistd.h>

#define N 1000 /* number of child processes */

/* return real time in seconds since some arbitrary point in the past */
double walltime() {
   static int ticks_per_second = 0;
   if (!ticks_per_second) {
      ticks_per_second = sysconf(_SC_CLK_TCK);
   }
   struct tms timebuf;
   /* times returns the number of real time ticks passed since some
      arbitrary point in the past */
   return (double) times(&timebuf) / ticks_per_second;
}

volatile char global[1048576];

int main() {
   int pagesize = getpagesize();
   for (size_t count = 0; count < sizeof(global) / pagesize; ++count) {
      double t0 = walltime();
      for (int i = 1; i <= N; ++i) {
	 pid_t child = fork();
	 if (child == -1) {
	    perror("unable to fork"); exit(1);
	 }
	 if (child == 0) {
	    /* child process */
	    for (size_t index = 0; index < count; ++index) {
	       global[index*pagesize] = i;
	    }
	    _exit(i);
	 }
      }

      /* parent process */
      pid_t child; int stat;
      while ((child = wait(&stat)) >= 0) {
	 /* nothing to be done */
      }
      double t1 = walltime() - t0;
      printf("%d %.4lf\n", (int) count, t1*1000*1000 / N);
   }
}

set terminal svg size 900, 500
set output "copy-on-write.svg"
set title "Real time of fork and wait in dependence of updated pages"
set xlabel "number of updated pages"
set ylabel "avg time in us"
set key outside
set pointsize 0.5
plot "copy-on-write.out" using 1:2 with linespoints lt 2 \
   title "Linux 4.9.0-8-amd64 on Intel i5-3470 3.20GHz"

#include <stdio.h>
#include <unistd.h>

int main() {
   printf("%ld\n", sysconf(_SC_CLK_TCK));
}