[Discuss-gnuradio] Latency measurements with Unix-domain sockets

[Marcus D. Leech]

I used the following two little programs:


============ unixdomain.c ==============
#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <unistd.h>
#include <string.h>
#include <sys/time.h>
#include <time.h>

int
main (void)
{
  struct sockaddr_un address;
  int socket_fd, nbytes;
  char buffer[256];

  socket_fd = socket (PF_UNIX, SOCK_DGRAM, 0);
  if (socket_fd < 0)
    {
      printf ("socket() failed\n");
      return 1;
    }

  /* start with a clean address structure */
  memset (&address, 0, sizeof (struct sockaddr_un));

  address.sun_family = AF_UNIX;
  snprintf (address.sun_path, FILENAME_MAX, "./demo_socket");

  if (connect (socket_fd,
           (struct sockaddr *) &address,
           sizeof (struct sockaddr_un)) != 0)
    {
      printf ("connect() failed\n");
      return 1;
    }

  while (1)
    {
      struct timeval tv;

      gettimeofday (&tv, NULL);
      write (socket_fd, &tv, sizeof (tv));
      usleep (100000);
    }

  return 0;
}


===== unixdomain_server.c ========

#include <stdio.h>
#include <sys/socket.h>
#include <sys/un.h>
#include <sys/types.h>
#include <unistd.h>
#include <string.h>
#include <sys/time.h>
#include <time.h>


int
main (int argc, char **argv)
{
  struct sockaddr_un address;
  int socket_fd;
  socklen_t address_length;
  unsigned char buffer[512];
  struct timeval now, msg;
  int cnt = 0;
  long long diff = 0LL;

  socket_fd = socket (PF_UNIX, SOCK_DGRAM, 0);
  if (socket_fd < 0)
    {
      printf ("socket() failed\n");
      return 1;
    }

  /* start with a clean address structure */
  memset (&address, 0, sizeof (struct sockaddr_un));

  address.sun_family = AF_UNIX;
  snprintf (address.sun_path, FILENAME_MAX, "./demo_socket");
  unlink (address.sun_path);

  if (bind (socket_fd,
        (struct sockaddr *) &address, sizeof (struct sockaddr_un)) != 0)
    {
      perror ("bind() failed");
      return 1;
    }
  while (read (socket_fd, &msg, sizeof (msg)) >= sizeof (msg))
    {
      long long t1, t2;

      t1 = msg.tv_sec * 1000000;
      t1 += msg.tv_usec;

      gettimeofday (&now, NULL);
      t2 = now.tv_sec * 1000000;
      t2 += now.tv_usec;
      diff += (t2 - t1);

      cnt++;
      if ((cnt % atoi (argv[1])) == 0)
    {
      fprintf (stderr, "%f\n", (double) diff / (double) atoi (argv[1]));
      diff = 0LL;
    }
    }


  close (socket_fd);
  unlink ("./demo_socket");
  return 0;
}



And found no significant difference in peak and average latencies
between them.

The unixdomain_server takes a single command-line argument which tells
it how many samples
  to average over before producing a printed result.

So this confirms my earlier assertion that I would be surprised to find
a significant latency difference
 between Unix-domain sockets and FIFOs, since the interior kernel
mechanisms are broadly similar.
 Basically--some chunk of memory is copied from one place to another,
there's some housekeeping, and
  the system-call interface is traversed a couple of times.



-- 
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org



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