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Full Discussion: semaphore access speed
Top Forums Programming semaphore access speed Post 302238329 by jim mcnamara on Friday 19th of September 2008 03:23:43 PM
Old 09-19-2008
Accessing a semaphore - assuming there is no deadlock - it is a direct memory access operation, adding a whole page or multiples to process memory has overhead. It can involve an expensive call: brk(), if no memory is already there.

It depends on what
Code:
size mycode

says in total, rounded up to a minimum of the PAGE_SIZE (mutiple) boundary (stack frame boundary usually), which can leave extra multiple pages of memory. Look at what is show to be allocated to heap. As a start. You can call sbrk(0) to find the end of process memory.

shmget allocates from heap in most implementations: /proc/sys/kernel/sh* proc directories have shared memory information. kernel settings control shared memory operations.

try this:
Code:
gcc -p -g -o otherus otheus.c
otheus.c
grpof otheus

That will display cum time spent + # calls for each of the function calls. You can see if/where there is a problem. If there is one with semaphores then semget will be your most likely problem - allocating pages of memory.
 

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sem_unlink(2)							System Calls Manual						     sem_unlink(2)

NAME
sem_unlink - unlink a named POSIX semaphore SYNOPSIS
DESCRIPTION
is used to unlink named semaphores. A successful call to marks the semaphore, specified by name, for removal. Calling does not affect processes, including the calling process, which currently have a descriptor, obtained from a call to Named semaphores are uniquely identi- fied by character strings. All character string names will be pre-processed to ensure variations of a pathname resolve to the same sema- phore name. If the semaphore is successfully marked for removal by a call to the semaphore will be removed when all processes remove their descriptors to the specified semaphore by calling Subsequent calls to using the string name will refer to a new semaphore. To use this function, link in the realtime library by specifying on the compiler or linker command line. EXAMPLES
The following call to will remove the named semaphore named by the string name. If the semaphore is currently referenced by one or more processes, the semaphore will be marked for removal and removed when there are no more processes referencing it. RETURN VALUE
If the semaphore was unlinked successfully, returns 0. If the semaphore could not be unlinked, the call returns -1 and sets to indicate the error. ERRORS
fails and does not perform the requested operation if any of the following conditions are encountered: [EACCES] The named semaphore exists and the process does not have the permissions to unlink the semaphore. [ENAMETOOLONG] The name string is longer than [ENOENT] The flag is not set in oflag (see sem_open(2)) and the named semaphore does not exist. SEE ALSO
sem_close(2), sem_open(2), privileges(5), <semaphore.h>. STANDARDS CONFORMANCE
sem_unlink(2)
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