Managing Oracle Shared Memory on AIX

Shared memory uses common virtual memory resources across processes. Processes share virtual memory segments through a common set of virtual memory translation resources, for example, tables and cached entries, for improved performance.

Shared memory can be pinned to prevent paging and to reduce Input-Output overhead. To perform this, set the LOCK_SGA parameter to true. On AIX 5L, the same parameter activates the large page feature whenever the underlying hardware supports it.

Run the following command to make pinned memory available to Oracle Database:

$ /usr/sbin/vmo -r -o v_pinshm=1 

Run a command similar to the following to set the maximum percentage of real memory available for pinned memory, where percent_of_real_memory is the maximum percent of real memory that you want to set:

$ /usr/sbin/vmo -r -o maxpin%=percent_of_real_memory

When using the maxpin% option, it is important that the amount of pinned memory exceeds the Oracle SGA size by at least 3 percent of the real memory on the system, enabling free pinnable memory for use by the kernel.

Use the svmon command to monitor the use of pinned memory during the operation of the system. Oracle Database attempts to pin memory only if the LOCK_SGA parameter is set to true.

To turn on and reserve 10 large pages each of size 16 MB on a POWER4 or POWER 5 system, run the following command:

$ /usr/sbin/vmo -r -o lgpg_regions=10 -o lgpg_size=16777216

This command proposes bosboot and warns that a restart is required for the changes to take affect.
Oracle recommends specifying enough large pages to contain the entire SGA. The Oracle Database instance attempts to allocate large pages when the LOCK_SGA parameter is set to true. If the SGA size exceeds the size of memory available for pinning, or large pages, then the portion of the SGA exceeding these sizes is allocated to ordinary shared memory.