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Semaphore Management in UNIX
Oracle Tips by Burleson Consulting


Semaphore Management in UNIX

A semaphore is a term used for a signal flag used by the Navy to communicate between ships. In some dialects of UNIX, semaphores are used by Oracle to serialize internal Oracle processes and guarantee that one thing happens before another thing. Oracle uses semaphores in HP/UX and Solaris to synchronize shadow processes and background processes. However, AIX UNIX does not use semaphores, and a post/wait driver is used instead to serialize tasks.

The number of semaphores for an Oracle database is normally equal to the value of the processes initialization parameter. For example, a database where processes=200 would need to have 200 UNIX semaphores allocated for the Oracle database.

When allocating semaphore in UNIX, it is critical that your UNIX kernel parameter semmns be set to at least double the high-water mark of processes for every database instance on your server. If you fail to allocate enough semaphores by setting semmns too low, your Oracle database will fail at startup time with this message:

ORA-7279: spcre: semget error, unable to get first semaphore set

Let’s talk about setting the semmns kernel parameter. To make changes in kernel shared memory or semaphore parameters, you need to perform the following steps:

1. Shut down any running Oracle instances
2. Locate the kernel configuration file for your OS
3. Make the necessary changes using the system utilities or the vi editor.

Today, most dialects of UNIX have specialized System Administration utilities to perform kernel management (Table 2-4)

UNIX Dialect
Utility Name
Table 4: UNIX kernel management tools

The above is an excerpt from the "Oracle9i UNIX Administration Handbook" by Oracle press, authored by Donald K. Burleson.


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