Semaphore

A semaphore can be thought of as a counter that is used to control access to a shared resource. Semaphores provide low level synchronization between processes (or threads within a process) so that only one process (or thread) has access to the shared segment, thereby ensuring the integrity of the shared resource.

Oracle recommeds setting some of the semaphore parameters to the largest PROCESS instance parameter setting in the database parameter files for all database hosted in the Linux system plus 10.

In Linux environments semaphore is controlled by parameter kernel.sem. This parameter is set along with other parameters in the configuration file /etc/sysctl.conf.

Following commands helps to check the current setting of semaphore parameter

# cat /etc/sysctl.conf | grep sem

# sysctl -a | grep kernel.sem

# ipcs -l

Example

# cat /etc/sysctl.conf | grep sem

kernel.sem = 250 32000 100 128

# sysctl -a | grep kernel.sem

kernel.sem = 250 32000 100 128

# ipcs -l

------ Shared Memory Limits --------

max number of segments = 4096

max seg size (kbytes) = 67108864

max total shared memory (kbytes) = 100663296

min seg size (bytes) = 1

------ Semaphore Limits --------

max number of arrays = 1000

max semaphores per array = 1000

max semaphores system wide = 32000

max ops per semop call = 1000

semaphore max value = 32767

------ Messages: Limits --------

max queues system wide = 16

max size of message (bytes) = 65536

default max size of queue (bytes) = 65536

To change the semaphore settings, the "kernel.sem" parameter value need to be changed in the /etc/sysctl.conf. This change wont take effect until server is rebooted. In order to make the change to take effect immediately the following command need to be executed.

# sysctl -p