Oracle Semaphore Management
in UNIX Administration
Oracle UNIX/Linux Tips by Burleson Consulting
Semaphores and Oracle 11g
Semaphores are data structures within the operating system that
function like signals for memory process communications. For
Oracle, semaphores tell the Oracle processes when to stop, wait or
start operations. In addition, semaphores tell Oracle processes
when they are to resume processing. Each Oracle process has its
own semaphore assigned to it.
For example, DBWR (Database Writer process) needs to flush the
database buffer cache to write to the data files on disk. When it
is time for DBWR to perform this task, DBWR semaphore signals to
DBWR to flush the data from the buffer out to disk. With System V
implementations and variations of the UNIX and LINUX operating
system, some of the key parameters that manage the use and
allocation of semaphores with Oracle 11g exist in the following
operating system level parameters:
- SEMMNS: number of semaphores in the operating system.
- SEMMSL: size limit for a single semaphore set.
- SEMMNI: number of semaphore set identifiers in the
- SEMMNU: number of semaphore undo structures in the
The output of semaphores can be viewed by using the ipcs
command for UNIX and Linux platforms as will be observed in the
following section on shared memory and Oracle 11g.
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 the 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
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 kernel management tools
System Default values for semaphores
The number of UNIX semaphores is determined
by the value of the semmns UNIX kernel parameter.
Viewing semaphores in HP/UX version 11
In HP/UX v 11, the command to display kernel
parameters is kmtune and we can grep to see the semaphore settings.
Counting Used Semaphores
The ipcs UNIX command has a ?sa option that
can be used to display semaphores. The total number of semaphores is
determined by summing the NSEMS column in the ipcs display. In
the example below we see that there are 4 semaphores held by the
ROOT user and 475 semaphores held by the ORACLE user in 3 database
IPC status from /dev/kmem as of Mon Sep 10 17:25:21 2001
T ID KEY
MODE OWNER GROUP CREATOR
s 15 0x00000000 --ra-r----- oracle dba
oracle dba 400
To display semaphores with the ipcs
command and remove them with ipcs -pmb, see my notes on
Managing UNIX memory with IPCS
Next, let?s take a look at how we can
examine UNIX system log messages.
Displaying System log Messages
In UNIX, a system failure will often
precipitate an Oracle crash. Any hardware associated with
disk, CPU or RAM may cause an Oracle database crash, and the UNIX
system logs can be used to identify the initial cause of the Oracle
The following commands are used to display
the UNIX error logs. We need to note that you should regularly
check the UNIX logs, even if there is not Oracle failure. These
commands can also be useful for detecting transient disk I/O
problems, memory failures, etc.
Show server log on HP/UX
In HP/UX we have a file in the /var/adm
directory called syslog to hold all UNIX system messages. In
the command below we search the syslog for any lines that contain
the word ?error?.
?i error /var/adm/syslog/syslog.log|more
1 20:30:08 sprihp01 syslog: NetWorker media: (warning)
dev/rmt/c5t6d0BESTn reading: I/O error
Show server log on AIX
In the IBM AIX operating system we use the
errpt command to display the contents of the system log.
Tue May 9 10:34:47
Sequence Number: 24908
Machine Id: 000138644C00
Node Id: sp2k6n03
Resource Name: SYSPROC
Again, checking the UNIX system logs should
be a regular activity for the Oracle DBA. Next, let?s take a look at
how UNIX can be monitored for performance problems.
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