shmmax.5 (2010 09)

shmmax(5) shmmax(5)

(Tunable Kernel Parameters)

NAME

shmmax - maximum size (in bytes) for a System V shared memory segment

VALUES

Default

1GB

Allowed values

Minimum: 2048

Maximum: 0x40000000000

DESCRIPTION

Shared memory is an efﬁcient InterProcess Communications (IPC) mechanism. One process creates a

shared memory segment and attaches it to its address space. Any processes looking to communicate with

this process through the shared memory segment, then attach the shared memory segment to their

corresponding address spaces as well. Once attached, a process can read from or write to the segment

depending on the permissions speciﬁed while attaching it.

This tunable sets the maximum size for such a segment within the system, and is dynamic as of HP-UX

11i.

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect immediately.

When Should the Value of This Tunable Be Raised?

shmmax should be raised if it is below the maximum and user programs are attempting to shmget

seg-

ments larger than the current value, and receiving an [EINVAL] error message.

What Are the Side Effects of Raising the Value?

The only effect is that user programs can use

shmget to get larger segments.

When Should the Value of This Tunable Be Lowered?

If you wish to enforce behavior on the user’s code, limiting the maximum size of System V segments.

What Are the Side Effects of Lowering the Value?

None.

What Other Tunable Values Should Be Changed at the Same Time?

shmseg and shmmni should be considered because enforcing smaller segments may cause the user’s

code to try to create more segments to accomplish the task.

WARNINGS

All HP-UX kernel tunable parameters are release speciﬁc. This parameter may be removed or have its

meaning changed in future releases of HP-UX.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parame-

ter values. After installation, some tunable parameters may no longer be at the default or recommended

values. For information about the effects of installation on tunable values, consult the documentation for

the kernel software being installed. For information about optional kernel software that was factory

installed on your system, see HP-UX Release Notes at http://www.hp.com/go/hpux-core-docs

AUTHOR

shmmax was developed by HP.