desfree_pct.5 (2010 09)

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desfree_pct(5) desfree_pct(5)

(Tunable Kernel Parameters)

When Should the Value of This Tunable Be Raised?

This tunable should be raised on systems where system memory is critical for short-term, high priority

processes, but is being held by low-priority, long running processes. For example, when one web server is

forking off per-connection threads while 50 RCS clients run in the background.

Increasing

desfree_pct (and hence desfree) would raise the likelihood of the low-priority processes

being put to sleep, allowing the high-priority processes a chance to acquire the pages that were previously

allocated to the former.

What are the Side Effects of Raising the Value?

desfree_pct increases, more low priority processes might be forced to sleep. Depending on what

these processes are actually doing, this may not be the right approach.

When Should the Value of This Tunable Be Lowered?

Raising

desfree_pct too high can impair performance of lower priority processes as previously

described.

In addition, raising

desfree_pct and lotsfree_pct increases system overhead (it takes time to

swap a page to disk), and should be weighed carefully against the need or desire to keep a large pool of

physical memory unused.

Some free memory is essential, a moderate amount is probably good, a lot of free memory just sitting idle

is a waste. If the available physical memory on the machine seems generous and heavy swapping is

occurring, lowering

desfree_pct and/or lotsfree_pct will increase memory usage but decrease

swapping overhead.

What are the Side Effects of Lowering the Value?

More physical memory will be in use before processes are put to sleep to try to reclaim their frames.

What Other Tunable Values Should Be Changed at the Same Time?

lotsfree_pct should be considered when modifying desfree_pct for reasons previously described

above. lotsfree_pct acts as a ﬁrm upper bound to desfree_pct, so attempting to set

desfree_pct above lotsfree_pct is meaningless and will result in an [EINVAL] error. See

lotsfree_pct (5).

Notes

Starting with HP-UX 11i Version 2, HP recommends the use of

desfree_pct to specify the lower bound

for the paging daemon.

ERRORS

[EBUSY] Memory resource management infrastructure is busy. Please try later.

[EINVAL] Attempting to set

desfree_pct above lotsfree_pct.

[EINVAL] An invalid value is entered that is not an integer between 0 and 30, such as an negative

number.

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

installed on your system, see HP-UX Release Notes at

http://www.hp.com/go/hpux-core-docs

AUTHOR

desfree_pct was developed by HP.