OS SYSTEM PARAMETERS

SYSTEM SETTINGS

semaphores	Oracle uses shared memory and semaphores to communicate between processes and the SGA (System Global Area). Oracle uses semaphores to control concurrency between all the background processes (<="" a="" style="color: rgb(0, 0, 255); text-decoration: none; ">PMON, SMON, DBWn, LGWR, and oracle shadows). ipcs -sb will show semaphores currently allocated to the system at the moment. This will display all the semaphore sets allocated, their identifying number, the owner, the number of semaphores in each set, and more.  Occasionally, unexpected termination of Oracle processes will leave semaphore resources locked. If database is not running, but ipcs -sb shows that semaphore sets owned by oracle are still in use, then these will need to be deallocated (freed). Freeing semaphore sets is done with the ipcrm command. For each set that oracle has allocated, type ipcrm -s ID where ID is the set number you see from the ipcs output. Semaphores can also be freed by rebooting the system.
semmns	The number of semaphores in the system. kernel parameter controlling maximum number of semphores in the system. Related parameter: processes. To set the kernel parameter for SEMMNS, use this formula: For example, consider a server has three instances with processes set to: instance A has processes=100 instance B has processes=100 instance C has processes=200 The value of SEMMNS is calculated as follows: SEMMNS = ((A=100) + (B=100)) + ((C=200) * 2) + ((# of instances=3) * 10) = 630 or SEMMS = (total number of processes * 2) + (number of instances * 10)
semmni	Maximum number of semaphores identifiers in the system. Determines the number of semaphore sets that can be created at any one time. kernel parameter controlling maximum number of semaphore sets. Semphores in Unix are allocated in sets of 1 to SEMMSL.
semmsl	The maximum number of sempahores that can be in one semaphore set. Kernel parameter controlling maximum number of semaphores in a semphore set. It should be same size as maximum number of processes for all instances on the server. Related parameter: processes.
shmmhi	Kernel parameter controlling maximum number of shared memory segments in the system.
shmmax	The maximum size(in bytes) of a single shared memory segment. Kernel parameter controlling maximum size of one shared memory segment.  Related parameters: shared_pool_size, db_cache_size, log_buffer, large_pool_size, db_block_buffers
shmmin	The minimum size(in bytes) of a single shared memory segment. Kernel parameter controlling minimum size of one shared memory segment
shmmni	The number of shared memory identifiers.
shmseg	The maximum number of shared memory segments that can be attached by a process. Kernel parameter controlling maximum number of shared memory segments a process can attach
shmlba	kernel parameter controlling alignment of shared memory segments. All segments must be attached at multiples of this value
maxfiles	Soft file limit per process. Related parameter: db_files. Also see maxdatafiles.
nfile	Maximum number of simultaneously open files systemwide at any given time. Also see maxdatafiles.
nproc	Maximum number of processes that can exist simultaneously in the system. Related parameter: processes.
plimit	To find the file size limit for server: plimit $$
ulimit	To find the file size limit per current session: ulimit -Sa