shmseg.5 (2010 09)

s
shmseg(5) shmseg(5)
(Tunable Kernel Parameters)
NAME
shmseg - maximum number of System V shared memory segments per process
VALUES
Default
300 segments
Allowed values
Minimum:
1
Maximum: Any value less than or equal to
shmmni.
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 immediately.
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 previ-
ously 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 pro-
cess 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 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.
Installation of optional kernel software, from HP or other vendors, may cause changes to tunable parame-
ter 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://www.hp.com/go/hpux-core-docs.
AUTHOR
shmseg was developed by HP.
SEE ALSO
kctune(1M), shmmni(5).
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1