System V Shared Memory
System V shared memory allows the creation of a segment by a
process. Cooperating processes can attach to the memory segment (subject to access permissions on
the segment) and gain access to the data contained in the segment. This
capability is implemented as a loadable module. Entries in the /etc/system file
must contain the shmsys: prefix. Starting with the Solaris 7 release, the keyserv
daemon uses System V shared memory.
A special kind of shared memory known as intimate shared memory (ISM) is used by
DBMS vendors to maximize performance. When a shared memory segment is made into
an ISM segment, the memory for the segment is locked. This feature enables
a faster I/O path to be followed and improves memory usage. A
number of kernel resources describing the segment are then shared between all processes that
attach to the segment in ISM mode.
For information about the changes to shared memory resources in the Solaris 10
release, see System V IPC Configuration.
For detailed information about using the new resource controls in the Solaris 10
release, see Chapter 6, Resource Controls (Overview), in System Administration Guide: Virtualization Using the Solaris Operating System.
For legacy information about the obsolete System V shared memory parameters, see Parameters That Are Obsolete or Have Been Removed.
Identifies pages of system memory that cannot be allocated for ISM shared memory.
- Data Type
5 percent of available system memory when the first ISM segment is created
0 to 50 percent of physical memory
None. Values that are too small can cause the system to hang or performance to severely degrade when memory is consumed with ISM segments.
- When to Change
On database servers with large amounts of physical memory using ISM, the value of this parameter can be decreased. If ISM segments are not used, this parameter has no effect. A maximum value of 128 Mbytes (0x4000) is almost certainly sufficient on large memory machines.
- Commitment Level