Tunable Kernel Parameters
NAME [Toc] [Back]
msgssz - number of bytes in a System V IPC message segment
VALUES [Toc] [Back]
Failsafe [Toc] [Back]
Allowed Values [Toc] [Back]
The product of msgseg*msgssz must not exceed a 32 bit integer value of
DESCRIPTION [Toc] [Back]
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
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?
Restrictions on Changing [Toc] [Back]
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
When Should the Value of This Tunable Be Lowered?
Lower this tunable when applications no longer require as much message
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Tunable Kernel Parameters
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 [Toc] [Back]
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
AUTHOR [Toc] [Back]
msgssz was developed by AT&T.
SEE ALSO [Toc] [Back]
msgsnd(2), mesg(5), msgmap(5), msgmax(5), msgmnb(5), msgmni(5),
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