NAME
swapmem_on — allow physical memory size to exceed the available swap space
VALUES
Allowed values
0 (off)
or
1 (on)
DESCRIPTION
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?
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 tunable
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 allocation 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 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
http://docs.hp.com.
AUTHOR
swapmem_on
was developed by HP.
