/* Spike for testing filesystem log polling.

How much of a load does filesystem polling for updates (as opposed to
waiting on inotify) impose on current hardware?  This program stats a
file 60 times a second to find out.

I tried it on my Ryzen 5 3500U laptop, plugged in, running at 2700MHz
and got these results (edited transcript):

    $ time ./60hz-logpoll tmp.dumb
    tmp.dumb is now 357888 bytes.
    tmp.dumb is now 357893 bytes.
    tmp.dumb is now 357898 bytes.
    tmp.dumb is now 357903 bytes.
    ^C

    real    0m10.084s
    user    0m0.021s
    sys     0m0.001s
    $ time ./60hz-logpoll tmp.dumb
    tmp.dumb is now 357903 bytes.
    tmp.dumb is now 357908 bytes.
    tmp.dumb is now 357920 bytes.
    tmp.dumb is now 357927 bytes.
    tmp.dumb is now 357939 bytes.
    tmp.dumb is now 357951 bytes.
    ^C

    real    2m16.818s
    user    0m0.066s
    sys     0m0.213s

We can’t *really* trust the user and system numbers these days, but in
the first case we used 22ms over 10 seconds, which is 0.22% of a core
(or 0.05% of the whole CPU), and in the second case we used 279ms over
137 seconds, which is 0.204% of a core.

*/


#include <fcntl.h>
#include <sys/stat.h>
#include <sys/time.h>
#include <stdio.h>
#include <unistd.h>

double
now()
{
  struct timeval tv;
  gettimeofday(&tv, 0);
  return tv.tv_sec + 1e-6 * tv.tv_usec;
}

void
clock_tick(double hz, double *target)
{
  *target += 1.0/hz;
  double wait = *target - now();
  if (wait > 0) usleep(wait * 1e6);
}

int
main(int argc, char **argv)
{
  if (argc != 2) {
    fprintf(stderr, "Usage: %s logfile\n"
            "Prints a message every time `logfile` grows.\n",
            argv[0]);
    return 1;
  }

  int fd = open(argv[1], O_RDONLY);
  if (fd < 0) {
    perror(argv[1]);
    return 2;
  }

  double target = now();

  off_t size = 0;
  struct stat st;
  for (;;) {
    if (fstat(fd, &st)) {
      perror("fstat");
      return 3;
    }

    if (st.st_size != size) {
      size = st.st_size;
      printf("%s is now %ld bytes.\n", argv[1], (long)size);
      fflush(stdout);
    }

    clock_tick(60, &target);
  }
}