HP-UX Reference (11i v2 04/09) - 5 Miscellaneous Topics (vol 9)

s
swapmem_on(5) swapmem_on(5)
(Tunable Kernel Parameters)
NAME
swapmem_on - allow physical memory size to exceed the available swap space
VALUES
Failsafe
1 (on)
Default
Failsafe
1 (on)
Allowed values
0 (off) or 1 (on)
DESCRIPTION
In previous versions of HP-UX, system configuration required sufficient physical swap space for the max-
imum possible number of processes on the system. This is because HP-UX reserves swap space for a pro-
cess 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?
Anyone.
Restrictions on Changing
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 tun-
able to 1 (on) will allow these processes to use pseudo-swap space and execute.
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 alloca-
tion if needed (for locked memory or kernel memory), and the rest of the pages will only be used when
physical swap is completely filled anyway, this is quite harmless.
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?
None.
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
HP-UX 11i Version 2: September 2004 − 1 − Hewlett-Packard Company Section 5−−357