msgtql.5 (2010 09)

m
msgtql(5) msgtql(5)
(Tunable Kernel Parameters)
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 com-
bined size, in bytes, in the system.
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.
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 mes-
sages 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 vari-
ables. 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.
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