HP-UX Reference (11i v3 07/02) - 5 Miscellaneous Topics (vol 9)

m
msgmbs(5) msgmbs(5)
(Tunable Kernel Parameters)
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) man-
page.
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 vari-
ables. 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.
278 Hewlett-Packard Company − 1 − HP-UX 11i Version 3: February 2007