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.

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.

segspt_minfree

Description

Identifies pages of system memory that cannot be allocated for ISM shared memory.

Data Type

Unsigned long

Default

5 percent of available system memory when the first ISM segment is created

Range

0 to 50 percent of physical memory

Units

Pages

Dynamic?

Yes

Validation

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 MB (0x4000) is almost certainly sufficient on large memory machines.

Commitment Level

Unstable