msgtql(5)

NAME

msgtql — maximum number of System V IPC messages in the system at any time

VALUES

Default

1024

Failsafe

1024

Allowed values

Minimum: 1

Maximum: 2147483647

Recommended values

The largest number of messages expected.

DESCRIPTION

The msgtql tunable specifies the maximum total system-wide individual messages across all message queues. Every message has a header to specify message type and location and the total number of headers is limited by msgtql.

Note that if a msgsnd() system call attempts to exceed the limit imposed by msgtql, it will block until message space becomes available or return EAGAIN if IPC_NOWAIT is set.

Other kernel tunables may limit the maximum number of messages supported. Limiting factors can be msgmnb, the maximum number of bytes in a queue, and msgmbs, the maximum allowable total combined size, in bytes, in the system.

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

This tunable is dynamic. Changes to to this tunable take effect immediately.

When Should the Value of This Tunable Be Raised?

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

What are the Side Effects of Raising the Value?

Raising this value increases the possible number of messages which may be queued at any point in time. This can result in msgsnd(2) having to block less frequently. Since message queue headers are stored in kernel memory, that memory is unavailable for use by other system services.

When Should the Value of This Tunable Be Lowered?

Lower this tunable when applications no longer require as many total messages.

What are the Side Effects of Lowering the Value?

Lowering the value increases the risk of application failure due to the inability to send more messages. Lowering the tunable has no effect on any queued messages, even if the new tunable value is less than the number of queued messages. However, no new messages may be queued until the number of messages falls below the setting of msgtql.

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 msgmbs, msgmnb, msgmni, and msgtql. Specifically, the msgmbs and msgmnb tunables may need to be adjusted along with the msgtql 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.

System resource limits (such as, memory) may limit the number and/or total size of queued messages. These system limits may be encountered before the limit values of the msgtql and msgmbs tunables.

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

msgtql was developed by AT&T.