msgssz(5)

NAME

msgssz — number of bytes in a System V IPC message segment

VALUES

Default

96

Failsafe

96

Allowed Values

Minimum: 65

Maximum: (2^32-1)/"msgseg"

The product of msgseg*msgssz must not exceed a 32 bit integer value of 4,294,967,295 (2^32)-1.

DESCRIPTION

The msgssz tunable specifies the size, in bytes, of a "segment" of memmory space reserved for storing IPC messages. Space for messages is acquired in segment-sized increments as required to contain the message. Separate messages do not share segments. Messages of size less than or equal to 64 bytes are allocated in a different area and do not require a segment.

The total available space for messages greater than 64 bytes in size on the system is defined by the product of msgseg*msgssz, the number of segments multiplied by the segment size.

It is best to select a segment size equal to the size of most messages sent by applications. This ensures optimal memory utilization of the segments.

For more information about System V message queues, refer to the Overview section of the mesg(5) manpage.

Who Is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect at the next reboot.

When Should the Value of This Tunable Be Raised?

Raise this tunable if applications block too frequently in msgsnd(), requiring more or larger messages be held in the kernel message queues.

When Should the Value of This Tunable Be Lowered?

Lower this tunable when applications no longer require as much message space.

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

All the System V message queue tunables are interrelated and should not be treated as independent variables. The set must be evaluated as a system to ensure the tunables reflect the application requirements. The message tunables include msgmap, msgmax, msgmnb, msgmni, msgseg, msgssz, and msgtql. Specifically, the msgseg and msgtql tunables may need to be adjusted in conjunction with changes in the msgssz tunable.

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. The 64 byte threshold may change in future releases of HP-UX.

A table of message segments is allocated in kernel memory at boot time. This memory is reserved whether or not it is used.

The memory requirement is the product the msgssz and msgseg tunables. Use care when setting these two tunables, as they can have a profound effect on memory utilization, due to the multiplicative effect.

Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parameter 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://docs.hp.com.

AUTHOR

msgssz was developed by AT&T.