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Full Discussion: semaphore access speed
Top Forums Programming semaphore access speed Post 302237417 by migurus on Wednesday 17th of September 2008 03:54:26 PM
Old 09-17-2008
semaphore access speed
I am investigating some locking scheme using semaphores. To evaluate basic system speed I run a loop of getting some semaphore info and display it:

while : ; do ./semshow; done > res.txt

I ran this on 3 boxes - two similar modern HP XEON boxes, one running SCO OpenServer 5, the other is Fedora 2.6.9, and one old PIII box under modern Linux (have no info).
The results are very counter intuitive:
H/W | OS | avg number of runs ber sec
---- | --- | -------------------------
XEON | SCO | 1700
XEON | Fedora | 500
PIII | Linux (recent distro, unknown) | 900
All three systems were pretty much idle at the test run time.

I would like to ask, what would be the factors that makes ancient Unix to outperform modern OSes, also, how come PIII box outperform modern XEON box under similar OS. Any ponters would be appreciated.

The semshow program is very basic, see listing below:
Code:
 
#include <stdio.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include  <sys/timeb.h>
#include <time.h>
#include "semlib.h"
#define MODE_CREATE     0
#define MODE_REMOVE     1
key_t   IPCKEY;
main(int argc, char *argv[])
{
  int   sid, i;
  pid_t last_rpid, last_wpid;
  char  dbuf[80];
  union semun           arg;
  unsigned short        vals[NSEMS];
  struct timeb tb;
  struct tm *tp;
        if((IPCKEY = get_ipc_key()) == -1)
        {
                errexit("Can Not Obtain IPC Key");
        }
        if((sid = semget(IPCKEY, NSEMS, 0)) == -1)
        {
                errexit("Can Not Get Semaphore ID");
        }
        memset(vals, 0, sizeof(vals));
        arg.array = &vals[0];
        if(semctl(sid, NSEMS, GETALL, arg) == -1)
        {
                errexit("Can Not Get Semaphore Values");
        }
        if((last_rpid = semctl(sid, RDLOCK, GETPID)) == -1)
        {
                errexit("Can Not Get Semaphore R-Pid");
        }
        if((last_wpid = semctl(sid, WRLOCK, GETPID)) == -1)
        {
                errexit("Can Not Get Semaphore W-Pid");
        }
        ftime(&tb);
        tp = localtime(&tb.time);
        strftime(dbuf, sizeof(dbuf) - 1, "%T", tp);
        printf("%12li.%03i %s  RD:[%i]  WR:[%i] %i/%i\n",
                tb.time, tb.millitm, dbuf, 
                vals[RDLOCK], vals[WRLOCK],
                last_rpid, last_wpid);
        exit(0);
}

semlib.h has these defs:
Code:
 
#define NSEMS   2
#define RDLOCK  0
#define WRLOCK  1
union   semun   {
        int             val;
        struct semid_ds *buf;
        unsigned short  *array;
};

 

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LEARN ABOUT SUNOS

sem.h

sem.h(3HEAD)							      Headers							      sem.h(3HEAD)

NAME

       sem.h, sem -  semaphore facility

SYNOPSIS

       #include <sys/sem.h>

DESCRIPTION

       The <sys/sem.h> header defines the following constants and structures.

       Semaphore operation flags:

       SEM_UNDO        Set up adjust on exit entry.

       Command definitions for the semctl() function are provided as listed below. See semctl(2).

       GETNCNT	       Get semncnt.

       GETPID	       Get sempid.

       GETVAL	       Get semval.

       GETALL	       Get all cases of semval.

       GETZCNT	       Get semzcnt.

       SETVAL	       Set semval.

       SETALL	       Set all cases of semval.

       The semid_ds structure contains the following members:

       struct ipc_perm sem_perm     /* operation permission structure */
       unsigned short  sem_nsems    /* number of semaphores in set */
       time_t	       sem_otime    /* last semop() time */
       time_t	       sem_ctime    /* last time changed by semctl() */

       The pid_t, time_t, key_t, and size_t types are defined as described in <sys/types.h>. See types.h(3HEAD).

       A semaphore is represented by an anonymous structure containing the following members:

       unsigned short semval	 /* semaphore value */
       pid_t	      sempid	 /* process ID of last operation */
       unsigned short semncnt	 /* number of processes waiting for semval
				 /* to become greater than current value */
       unsigned short semzcnt	 /* number of processes waiting for semval to become 0 */

       The sembuf structure contains the following members:

       unsigned short sem_num	  /* semaphore number */
       short	      sem_op	  /* semaphore operation */
       short	      sem_flg	  /* operation flags */

       All of the symbols from <sys/ipc.h> are defined when this header is included. See ipc.h(3HEAD).

ATTRIBUTES

       See attributes(5) for descriptions of the following attributes:

       +-----------------------------+-----------------------------+
       |      ATTRIBUTE TYPE	     |	    ATTRIBUTE VALUE	   |
       +-----------------------------+-----------------------------+
       |Interface Stability	     |Standard			   |
       +-----------------------------+-----------------------------+

SEE ALSO

       semctl(2), semget(2), semop(2), ipc.h(3HEAD), types.h(3HEAD), attributes(5), standards(5)

SunOS 5.10							    10 Sep 2004 						      sem.h(3HEAD)
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