How to Change Kernel Semaphore Limits in CentOS/RHEL
How to Change Kernel Semaphore Limits in CentOS/RHEL
The post explains how kernel semaphore limits can be changed in a CentOS/RHEL server.
Kernel Parameters to set semaphore limits
The kernel parameters to set the semaphore limits are:
kernel.sem = SEMMSL SEMMNS SEMOPM SEMMNI SEMMSL - max semaphores per array SEMMNS - max semaphores system wide SEMOPM - max ops per semop call SEMMNI - max number of arrays
Changing Semaphore Limits
If a limit needs to be changed there should be taken in to account that (max number of arrays)*(max semaphores per array) >= (max semaphores system-wide). No need to have more semaphores system-wide if limited by the amount of a possible number of arrays with max semaphores per it.
For example:
# sysctl -a| grep kernel.sem kernel.sem = 250 32000 32 128
For this example, we need to increase max semaphores per array limit from 128 to 192. The max semaphores system-wide can be also increased to 48000 ( 250*192 = 48000 >= 32000) or kept as 32000. In this scenario, it is kept at 32000.
To set kernel parameter dynamically i.e. without the need for server reboot.
# sysctl -w kernel.sem="250 32000 32 192" kernel.sem = 250 32000 32 192
If all is fine with the system as expected, modify kernel parameter by adding it to the /etc/sysctl.conf file to ensure the value persist after server reboot.
# cat /etc/sysctl.conf | grep kernel.sem kernel.sem = 250 32000 32 192
If you choose to set it directly in the file without using the sysctl -w, you’ll need to reload the file:
# sysctl -p
You can verify limits currently set by:
# ipcs -ls ------ Semaphore Limits -------- max number of arrays = 192 max semaphores per array = 250 max semaphores system wide = 32000 max ops per semop call = 32 semaphore max value = 32767
A semaphore is like a counter used to control access to shared resources by multiple processes. It is often used as a locking mechanism to prevent processes from accessing a particular resource while another process is performing operations on it. Semaphore value can be incremented or decremented until maximum set by variable SEMVMX, “semaphore max value”.
