kmem_do_largepage_cache_demotion.5 (2012 03)

k
kmem_do_largepage_cache_demotion(5) kmem_do_largepage_cache_demotion(5)
Itanium(R)-based Systems Only Itanium(R)-based Systems Only
(Tunable Kernel Parameters)
NAME
kmem_do_largepage_cache_demotion - tunable to enable memory reclamation by demoting large page
translations cached for kernel allocations
VALUES
Failsafe
0
Default
1
Allowed values
0 (off) and 1 (on)
DESCRIPTION
The
kmem_do_largepage_cache_demotion
tunable provides a way to reclaim unused portions of a
kernel large page that are kept cached for future allocations.
The Large Page Cache (LPC) layer provides a way to share a large page translation across many kernel
allocations. It also keeps unused portions of the large page cached and reduces TLB miss rate.
A kernel large page cannot be freed back to the system until all the objects allocated from that large page
are freed. Under kernel-intensive workloads, the objects allocated by the kernel may fragment the
underlying large pages, making the kernel memory consumption too high.
This may result in less memory availability and sometimes may result in sluggish behavior of user appli-
cations. If this tunable is enabled, under low memory situations, the kernel demotes large page transla-
tions to the extent where the free objects can be freed back to the system without affecting the in-use
objects that previously shared their translation.
kmem_do_largepage_cache_demotion
must be a positive integer, either 0 (off) or 1 (on).
Who Is Expected to Change This Tunable?
Administrators investigating the performance characteristics of kernel intensive workloads.
Restrictions on Changing
This tunable is present only on Itanium-based platforms. It is not available on PA-RISC platforms.
What are the Side Effects of enabling the tunable?
The tradeoff with demoting kernel dynamic memory is that this causes a trend towards smaller transla-
tion sizes for in-use kernel memory and increases TLB miss faults as a result. Of course, the actual per-
formance impact will depend on the data access pattern and the workload.
When Should the Value of This Tunable Be Lowered?
It should be lowered when the TLB miss rate for kernel accesses is affecting performance. The TLB miss
rates can be monitored using Caliper tool. For more details on Caliper, go to
http://hp.com/go/caliper.
When does this tunable change take effect?
Changes to this tunable take effect immediately without any reboot.
ERRORS
[EINVAL] An invalid value is entered that is neither 0 nor 1.
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
kmem_do_largepage_cache_demotion was developed by HP.
HP-UX 11i Version 3: March 2012 − 1 − Hewlett-Packard Company 1