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1 - About
On Linux, A
semaphore is a
System V IPC object that is used to control utilization of a particular process.
Semaphores are a
shareable resource that take on a
non-negative integer value. They are manipulated by the P (wait) and V
(signal) functions, which decrement and increment the semaphore,
respectively. When a process needs a resource, a “wait” is issued and
the semaphore is decremented. When the semaphore contains a value of
zero, the resources are not available and the calling process spins or
blocks (as appropriate) until resources are available. When a process
releases a resource controlled by a semaphore, it increments the
semaphore and the waiting processes are notified.
The Semaphore
Kernel parameters
| Semaphore | Description | Minimum |
| SEMMSL | maximum number of semaphores per array | 128 |
| SEMMNS | maximum semaphores system-wide | |
| SEMOPM | maximum operations per semop call | |
| SEMMNI | maximum arrays | |
2 - Articles Related
3 - How to
3.1 - Display them ?
This command displays the value of the semaphore parameters:
# /sbin/sysctl -a | grep sem
3.2 - Calculate them ?
sum (process parameters of all database instances on the system) + system and other application requirements
Set semmns (total semaphores systemwide) to this total.
Set semmsl (semaphores per set) to 256.
Set semmni (total semaphores sets) to semmns / semmsl rounded up to the nearest multiple of 1024.
The following formula can be used as a guide, although in practice,
SEMMNS and SEMMNU can be much less than SEMMNI * SEMMSL because not
every program in the system needs semaphores.
3.3 - Set them ?
In the file, /etc/sysctl.conf
kernel.sem = 2200 6400 200 25
Where:
kernel.sem = SEMMSL SEMMNS SEMOPM SEMMNI
Then reboot or run this command:
# /sbin/sysctl -p
4 - Documentation / Reference
