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Configuring Shared Memory in Oracle

April 13, 2010

Configuring Shared Memory
The Oracle database uses shared memory in UNIX to allow processes to access common data structures and data. These data structures and data are placed in a shared memory segment to allow processes the fastest form of Interprocess Communications (IPC) available. The speed is primarily a result of processes not needing to copy data between each other to share common data and structures – relieving the kernel from having to get involved.
Oracle uses shared memory in UNIX to hold its Shared Global Area (SGA). This is an area of memory within the Oracle instance that is shared by all Oracle backup and foreground processes. It is important to size the SGA to efficiently hold the database buffer cache, shared pool, redo log buffer as well as other shared Oracle memory structures. Inadequate sizing of the SGA can have a dramatic decrease in performance of the database.
To determine all shared memory limits you can use the ipcs command. The following example shows the values of my shared memory limits on a fresh RedHat Linux install using the defaults: # ipcs -lm
—— Shared Memory Limits ——–
max number of segments = 4096
max seg size (kbytes) = 32768
max total shared memory (kbytes) = 8388608
min seg size (bytes) = 1Let’s continue this section with an overview of the parameters that are responsible for configuring the shared memory settings in Linux.
SHMMAX
The SHMMAX parameter is used to define the maximum size (in bytes) for a shared memory segment and should be set large enough for the largest SGA size. If the SHMMAX is set incorrectly (too low), it is possible that the Oracle SGA (which is held in shared segments) may be limited in size. An inadequate SHMMAX setting would result in the following: ORA-27123: unable to attach to shared memory segment
You can determine the value of SHMMAX by performing the following: # cat /proc/sys/kernel/shmmax
33554432As you can see from the output above, the default value for SHMMAX is 32MB. This is often too small to configure the Oracle SGA. I generally set the SHMMAX parameter to 2GB.

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