msgmbs(5)

NAME

msgmbs — maximum number of megabytes for all System V IPC message queues

VALUES

Default

8

Failsafe

1

Allowed Values

Minimum: 1

Maximum: 0x100000

DESCRIPTION

The msgmbs tunable specifies the maximum allowable total combined size, in megabytes, of all System V IPC messages in the system at any one time.

If a msgsnd() system call attempts to exceed the limit imposed by msgmbs, it will block (or return EAGAIN if IPC_NOWAIT was specified) until a caller of msgrcv() decreases the number of queued message bytes sufficiently such that the new message, along with other messages still on the queue, will fit within the msgmnb limit.

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. Changes affect subsequent msgsnd() operations.

When Should the Value of This Tunable Be Raised?

Raise this tunable if applications have a need to store more data on a system.

What are the Side Effects of Raising the Value?

Raising this value increases the total message data 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 have no need to store large amounts of data on a single queue.

What are the Side Effects of Lowering the Value?

Lowering the tunable value has no effect on any active messages, even if the new tunable value is less than the total number of message bytes currently enqueued. However, no new messages may be queued until the total number of bytes falls below the setting of msgmbs.

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 msgmnb, msgmni, and msgtql tunables.

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

msgmnb was developed by AT&T.