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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_GET(3)								 1								SEM_GET(3)

sem_get - Get a semaphore id

SYNOPSIS
resource sem_get (int $key, [int $max_acquire = 1], [int $perm = 0666], [int $auto_release = 1]) DESCRIPTION
sem_get(3) returns an id that can be used to access the System V semaphore with the given $key. A second call to sem_get(3) for the same key will return a different semaphore identifier, but both identifiers access the same underlying semaphore. PARAMETERS
o $key - o $max_acquire - The number of processes that can acquire the semaphore simultaneously is set to $max_acquire. o $perm - The semaphore permissions. Actually this value is set only if the process finds it is the only process currently attached to the semaphore. o $auto_release - Specifies if the semaphore should be automatically released on request shutdown. RETURN VALUES
Returns a positive semaphore identifier on success, or FALSE on error. CHANGELOG
+--------+-----------------------------------------+ |Version | | | | | | | Description | | | | +--------+-----------------------------------------+ | 4.3.0 | | | | | | | The $auto_release parameter was added. | | | | +--------+-----------------------------------------+ NOTES
Warning When using sem_get(3) to access a semaphore created outside PHP, note that the semaphore must have been created as a set of 3 sema- phores (for example, by specifying 3 as the nsems parameter when calling the C semget() function), otherwise PHP will be unable to access the semaphore. SEE ALSO
sem_acquire(3), sem_release(3), ftok(3). PHP Documentation Group SEM_GET(3)
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