HP-UX Reference (11i v1 05/09) - 5 Miscellaneous Topics (vol 9)

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shmseg(5) shmseg(5)
(Tunable Kernel Parameters)
NAME
shmseg - maximum number of System V shared memory segments per process
VALUES
Default
120 segments
Allowed values
Minimum: 1
Maximum: Any value less than or equal to
shmmni, if ShmemExtensions is not installed or if the system is
32 bit. If the system is 64 bit, ShmemExtensions is installed and
shmmni_extended is set to a non-zero
value, then any value less than or equal to
shmmni_extended
is allowed instead.
DESCRIPTION
Shared memory is an efficient InterProcess Communications (IPC) mechanism. One process creates a
shared memory segment and attaches it to its address space. Any processes looking to communicate with
this process through the shared memory segment, then attach the shared memory segment to their
corresponding address spaces. Once attached, a process can read from or write to the segment depending
on the permissions specified while attaching it.
This tunable sets an upper limit to the number of segments which can be attached per process.
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?
If user processes reach their limit, more segments are desired, and the current value is less than the total
number of segments in the system (
shmmni).
What Are the Side Effects of Raising the Value?
A single process will be able to acquire more segments, possibly starving another process which previously
was able to acquire all the segments it needed. In this case, shmmni should be raised if it is below it’s
maximum.
When Should the Value of This Tunable Be Lowered?
This tunable should only be lowered to enforce segment policy on user processes, or if one runaway process
is hogging the global-segment pool. Otherwise, keeping the maximum higher than the common usage is
harmless.
What Are the Side Effects of Lowering the Value?
None.
What Other Tunable Values Should Be Changed at the Same Time?
shmmni or shmmni_extended should be considered, as previously described.
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.
AUTHOR
shmseg was developed by HP.
SEE ALSO
shmmni(5).
Section 5−−342 Hewlett-Packard Company − 1 − HP-UX 11i Version 1: September 2005