lotsfree_pct.5 (2010 09)

d
lotsfree_pct(5) lotsfree_pct(5)
(Tunable Kernel Parameters)
NAME
lotsfree_pct - sets the upper bound before paging daemon starts stealing pages, specified as a percentage
of available physical memory
VALUES
Default
0
Allowed values
0 to 30. Must not be less than
desfree_pct (see desfree_pct (5)).
DESCRIPTION
lotsfree_pct is a tunable parameter to set the upper bound before the paging daemon starts stealing
pages. It is specified as a percentage of physical memory available after boot.
lotsfree_pct must be a positive integer between 0 and 30 and must be equal to or greater than
desfree_pct. desfree_pct is the lower bound before paging daemon starts stealing pages (see
desfree_pct(5)).
The paging daemon in HP-UX acts on a "two hand" model. The daemon runs at least once a second, with
one part marking pages of virtual address space as "unused". If the page is referenced before this "aging"
hand returns, it will be marked as "used" again. Another section of the daemon, the "steal" hand follows
the age hand (the distance between them varies in a well bounded range) and processes pages which are
still marked unused, since needed or frequently accessed pages would have a high likelihood of being
referenced in the gap between the hands.
Exactly how the steal hand treats the pages still marked as unused depends on the comparison of free
system physical memory (
freemem) and three paging parameters: lotsfree, desfree, and min-
free. Between desfree and lotsfree is a periodically re-calculated and, hence, floating threshold
known as gpgslim. Thus desfree and lotsfree are the upper and lower bounds between which
gpgslim moves.
lotsfree_pct is an expression how much lotsfree the system maintains, in percentage terms (per-
centage of physical memory available after boot).
• If
freemem is greater than lotsfree, the system’s memory availability is in good shape. The steal
hand does nothing as a lot of memory is still available on the system, so there is no need to "steal" a
page that might be needed soon.
• In the more common case, when
freemem is lower than gpgslim (but higher than desfree), the
steal hand will begin to steal pages that have remained unreferenced from when the age hand last
marked them. "Stealing" a page refers to the process of freeing the page from being allocated for a
particular virtual page and making it available for general allocation again. If
freemem falls below
desfree, but still is more than minfree, memory availability is still in reasonable shape, but the
paging daemon begins stealing pages more aggressively, including putting lower-priority processes to
sleep to free up their memory.
• When
freemem is less than minfree, memory availability is getting more critical; and the paging
daemon begins deactivating low-priority processes, in other words, swapping out pages to the swap
device.
Since the exact memory topography varies widely across supported platforms, these three boundary
values are tunable to allow for cases where the default values are insufficient, or where the system
administrator requires more control over the actions of the paging daemon. In general, however, the
automatic calculation performed by default should suffice for most systems.
minfree is a system calcu-
lated value.
Who Is Expected to Change This Tunable?
Anyone with super-user privileges.
Restrictions on Changing
Changes to this tunable take effect immediately.
When Should the Value of This Tunable Be Raised?
The
lotsfree_pct tunable should only be raised if an administrator feels that the current lotsfree
value provides insufficient free physical memory required by a short lived but critical process. Increasing
lotsfree_pct (and hence lotsfree) to an amout equal to or greater than the amount of physical
memory required by this process allows the swapper to move out less frequently used pages from longer
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1