What is semaphore ?
Semaphore can be used when number of processes try to access the shared resource or same file,Semaphore stored in kernel, so that it can be accessed by all the processes,
- Semaphore can be identified unique id in linux kernel and it can be deleted using semdelete function,
- semaphore values can be incremented or decremented by using functions wait and signal,
- If we are using ONFS (Oracle over network file system) in linux, we need to increase the kernel.sem value to improve system performance
How to Increase semaphore value in linux?
[root@server ~]# sysctl -A | grep kernel.sem
kernel.sem = 250 32000 32 128
[root@server ~]# ipcs -ls
------ Semaphore Limits --------
max number of arrays = 128
max semaphores per array = 250
max semaphores system wide = 32000
max ops per semop call = 32
semaphore max value = 32767
Increase semop value from 32 to 100, it can be increased upto 250 which is equal to semaphores per array,
Add the following line into /etc/sysctl.conf file,
#vi /etc/sysctl.conf
kernel.sem = 250 32000 100 128
or
#sysctl -w "kernel.sem = 250 32000 100 128"
or
#sysctl -w "kernel.sem=4096 512000 1600 2048"
kernel.sem: max_sem_per_id max_sem_total max_ops_sem_call max_sem_ids
Now we have modified the kernel.sem value,
Please run the following command to update the changes
[root@server ~]# sysctl -p
Now semaphore value got changed on server.you can verify using ipcs command.
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