kmem_do_largepage_cache_dItanium(R)-based Skmem_do_largepage_cache_demotion(5)NAMEkmem_do_largepage_cache_demotion - tunable to enable memory reclamation
by demoting large page translations cached for kernel allocations
VALUES
Failsafe
Default
Allowed values
DESCRIPTION
The 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 por‐
tions 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-inten‐
sive 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 applications. If this tunable is enabled,
under low memory situations, the kernel demotes large page translations
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.
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 translation sizes for in-use kernel memory and
increases TLB miss faults as a result. Of course, the actual perfor‐
mance 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
When does this tunable change take effect?
Changes to this tunable take effect immediately without any reboot.
ERRORS
An invalid value is entered that is neither 0 nor 1.
WARNINGS
All HP-UX kernel tunable parameters are release specific. This parame‐
ter 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 val‐
ues. For information about the effects of installation on tunable val‐
ues, 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
AUTHOR
was developed by HP.
SEE ALSOkctune(1), kmem_aggressive_caching(5).
Tunable Kernel Pkmem_do_largepage_cache_demotion(5)
