Tunable Kernel Parameters
NAME [Toc] [Back]
swapmem_on - allow physical memory size to exceed the available swap
VALUES [Toc] [Back]
Default [Toc] [Back]
Allowed values [Toc] [Back]
0 (off) or 1 (on)
DESCRIPTION [Toc] [Back]
In previous versions of HP-UX, system configuration required
sufficient physical swap space for the maximum possible number of
processes on the system. This is because HP-UX reserves swap space
for a process when it is created, to ensure that a running process
never needs to be killed due to insufficient swap.
This was difficult, however, for systems needing gigabytes of swap
space with gigabytes of physical memory, and those with workloads
where the entire load would always be in core. This tunable was
created to allow system swap space to be less than core memory. To
accomplish this, a portion of physical memory is set aside as
'pseudo-swap' space. While actual swap space is still available,
processes still reserve all the swap they will need at fork or execute
time from the physical device or file system swap. Once this swap is
completely used, new processes do not reserve swap, and each page
which would have been swapped to the physical device or file system is
instead locked in memory and counted as part of the pseudo-swap space.
Who Is Expected to Change This Tunable?
Restrictions on Changing [Toc] [Back]
Changes to this tunable take effect on the next reboot.
When Should the Tunable Be Turned On?
If this tunable is set to 0 (off), and a fork() or exec() process is
failing with an [ENOMEM] error message and has sufficient system
memory but insufficient unreserved swap space, then setting this
tunable to 1 (on) will allow these processes to use pseudo-swap space
What Are the Side Effects of Turning the Tunable On?
Some physical memory is set aside for pseudo-swap, but since the
kernel can steal pages from this allocation if needed (for locked
memory or kernel memory), and the rest of the pages will only be used
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: Sep 2004
Tunable Kernel Parameters
when physical swap is completely filled anyway, this is quite
When Should the Tunable Be Turned Off?
This tunable can be turned off if there are sufficient physical swap
devices or file systems such that the system workload never fails to
reserve swap. However, turning this feature off really does not gain
the system anything.
What Are the Side Effects of Turning the Tunable Off?
Processes will be limited to physical swap devices or file systems for
reserving their swap on a fork() or an exec().
What Other Tunables Should Be Changed at the Same Time?
WARNINGS [Toc] [Back]
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 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
AUTHOR [Toc] [Back]
swapmem_on was developed by HP.
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: Sep 2004 [ Back ]