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Full Discussion: semaphore access speed
Top Forums Programming semaphore access speed Post 302238478 by otheus on Saturday 20th of September 2008 08:26:43 AM
Old 09-20-2008
Quote:
Originally Posted by migurus
Jim,
I never tried profiling before, so I ran into some problem here:
$ gcc -pg -o tstloop tstloop.c
$ tstloop
128205.13 semop/s [0,0]
$ gprof tstloop
gprof: gmon.out file is missing call-graph data
That's strange. Any chance you're running a different program that's elsewhere in the path? Try for instance running "./tstloop".

Also, for gprof, use the -l option (small L). I get:

Code:
$ gprof -l  ./semget
Flat profile:

Each sample counts as 0.01 seconds.
  %   cumulative   self              self     total
 time   seconds   seconds    calls  Ts/call  Ts/call  name
 57.10      0.13     0.13                             main (semget.c:21 @ 80485f0)
 25.12      0.18     0.06                             main (semget.c:18 @ 804861f)
  9.14      0.20     0.02                             main (semget.c:17 @ 8048614)
  6.85      0.22     0.02                             main (semget.c:16 @ 80485e4)

Line 21 is the semctl() function. Line 18 is the semget() call.

Now that we have concrete results, I'd cross-post this on the Linuxquestions.org com site.

The next step would be to use the debug/profiling version of libc (which I don't have floating around) and see if we can use gprof to find the bottleneck therein. Or, look at the source. It would seem to be of great benefit to have this improved.
 

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semget(2)							System Calls Manual							 semget(2)

Name
       semget - get set of semaphores

Syntax
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semget (key, nsems, semflg)
       key_t key;
       int nsems, semflg;

Description
       The  system  call  returns the semaphore identifier associated with key.  A semaphore identifier and associated data structure and set con-
       taining nsems semaphores are created for key, if one of the following is true:

       o   The key is equal to IPC_PRIVATE

       o   The key does not already have a semaphore identifier associated with it, and (semflg & IPC_CREAT ) is true.

       For further information, see

       Upon creation, the data structure associated with the new semaphore identifier is initialized as follows:

       o   The sem_perm.cuid, sem_perm.uid, sem_perm.cgid and sem_perm.gid are set equal to the effective user ID and effective group ID,  respec-
	   tively, of the calling process.

       o   The low-order nine bits of sem_perm.mode are set equal to the low-order nine bits of semflg.

       o   The sem_nsems is set equal to the value of nsems.

       o   The sem_otime is set equal to zero (0) and sem_ctime is set equal to the current time.

Return Values
       Upon  successful  completion,  a nonnegative integer, namely a semaphore identifier, is returned.  Otherwise, a value of -1 is returned and
       errno is set to indicate the error.

Diagnostics
       The semget system call fails if any of the following is true:

       [EINVAL]       The nsems is either less than or equal to zero or greater than the system-imposed limit

       [EACCES]       A semaphore identifier exists for key, but operation permission, as specified by the low-order nine bits of semflg would not
		      be granted.  For further information, see

       [EINVAL]       A  semaphore  identifier	exists	for key, but the number of semaphores in the set associated with it is less than nsems and
		      nsems is not equal to zero.

       [ENOENT]       A semaphore identifier does not exist for key and (semflg & IPC_CREAT ) is false.

       [ENOSPC]       A semaphore identifier is to be created but the system-imposed limit on the maximum number of allowed semaphore  identifiers
		      in the system would be exceeded.

       [EEXIST]       A semaphore identifier exists for key but ((semflg & IPC_CREAT ) & (semflg & IPC_EXCL )) is true.

See Also
       semctl(2), semop(2), ftok(3)

																	 semget(2)
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