desfree_pct.5 (2010 09)

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desfree_pct(5) desfree_pct(5)
(Tunable Kernel Parameters)
NAME
desfree_pct - sets the lower bound before paging daemon starts stealing pages, specified as a percentage
of available physical memory
VALUES
Failsafe
0
Default
0
Allowed values
0 to 30. Must not be greater than
lotsfree_pct (see lotsfree_pct (5)).
DESCRIPTION
desfree_pct is a tunable parameter to set the lower bound before the paging daemon start stealing
pages. It is specified as a percentage of physical memory available after boot.
desfree_pct must be a positive integer between 0 and 30 and must be equal to or less than
lotsfree_pct (see lotsfree_pct (5)). Trying to set
desfree_pct higher than lotsfree_pct results
in [EINVAL].
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.
desfree_pct is an expression of how much desfree the system maintains, expressed in percentage
terms (percentage 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
freemen 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 is still 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, free memory in the system is getting more critical; the paging
daemon begin deactivating low priority processes, in other words, swapping out pages to the swap dev-
ice.
Since the exact memory topography varies widely across supported platforms, the paging parameters
desfree and lotsfree 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 calculated value.
Who Is Expected to Change This Tunable?
Anyone with super-user privileges.
Restrictions on Changing
Changes to this tunable take effect immediately.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1