msgseg(5)

NAME

msgseg — number of System V IPC message segments in the system

VALUES

Default

8192

Failsafe

8192

Allowed Values

Minimum: 1

Maximum: 32767 or ((2^31-1)/msgssz), whichever is smaller.

The product of msgseg*msgssz must not exceed a 32 bit integer value of 2,147,483,647 (2^31)-1.

DESCRIPTION

The msgseg specifies the total number of "segments" of system-wide shared memory message storage space which is shared among all IPC message queues.

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

Segments are only used to store messages larger than 64 bytes long. Messages smaller than or equal to 64 bytes long are stored in a different area and do not consume a segment.

If applications have consumed all message segments, the msgsnd() call will block until segments become available or return EAGAIN if IPC_NOWAIT was specified.

Messages larger than a single segment are allocated in a number of contiguous segments sufficient to hold the message. As all segments are allocated as a table (contiguous memory), fragmentation can occur as messages are added and removed. Therefore, if message sizes often exceed a single segment, consider allocating more segments than are actually needed for message data, to allow for some fragmentation.

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 msgssz and msgtql tunables may need to be adjusted in conjunction with changes in the msgseg 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

msgmni was developed by AT&T.