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Understand Oracle Semaphores in UNIX


Oracle Semaphores are important serialization mechanisms in UNIX and the Oracle DBA must understand Oracle semaphore management.  See our notes on Oracle semaphore sizing:

 

 Related semaphore notes:

 

http://www.fors.com/velpuri2/LINUX/Semaf

 

Shared memory and semaphores are two important resources for an Oracle instance on UNIX. 

 

This paper primarily discusses the process Oracle goes through to allocate shared memory and semaphores at instance startup.

Also note for Oracle semaphore settings, Dr. Hall sez:

In 9i, the recommended setting in you /etc/sysctl.conf file is:

# semaphores: semmsl, semmns, semopm, semmni
kernel.sem = 100 256 100 100

As you can see, the semmns is set to 256, limiting your maximum number of semaphores, and therefore processes. You need to increase this number to allow a larger number of semaphores and as a result, a larger number of processes.

In 10g, the recommended setting is:

# semaphores: semmsl, semmns, semopm, semmni
kernel.sem = 250 32000 100 128


 

 
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Note: This Oracle documentation was created as a support and Oracle training reference for use by our DBA performance tuning consulting professionals.  Feel free to ask questions on our Oracle forum.

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