vps_chatr_ceiling(5)vps_chatr_ceiling(5)NAMEvps_chatr_ceiling - maximum (in kilobytes) of user selectable page size
VALUES
Default
Allowed values
Minimum:
Maximum:
DESCRIPTION
The Translation Look-aside Buffer (TLB) is a microprocessor feature for
virtual memory, where the most recent physical to virtual address
translations are cached, in the expectation that these translations are
likely to be needed again soon. This is based on the principles of
spatial and temporal locality of address references in programs. His‐
torically, the TLB was entirely managed within hardware to achieve
speed optimizations while sacrificing the flexibility of software
implementations, for example, easily changed algorithms or table imple‐
mentations.
In recent years, the flexibility of a software implementation of the
TLB has regained importance over pure hardware speed. Specifically,
the idea of logical grouping of physical frames (whose size is fixed in
hardware) into "superpages" or "large pages", which can be represented
in software TLB algorithms using a single base address translation for
many physical frames, significantly reduces the lost cycles due to page
faults assuming reasonable spatial and temporal locality. For example,
consider a scientific application working on an array where each ele‐
ment requires 1K of memory. Using the usual 4K physical frame size,
and referencing the array sequentially causes a page fault that
requires the page be read into memory from disk or swap, and loads the
TLB with the frame base address translation every fifth element.
The tunable parameter sets the upper bound for virtual page size on
files that a user specifies, through or any programmatic interfaces or
other mechanisms. (Programmatic interface refers to an interface that
can be called from a program.)
Originally, this tunable was created to set an upper bound for virtual
page size set by the user through the command on the binary. Hence the
parameter name (where "vps" refers to variable page size).
Who is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect for any subsequent physical memory
allocations. It does not affect any physical memory that has already
been allocated.
When Should the Value of This Tunable Be Raised?
This tunable should be raised when a user application, or in general,
the system with known large memory set usage (such as a database) is
expected to need larger pages than the current value allows.
What Are the Side Effects of Raising the Value?
The side effects depend on the actual memory usage in the system and
whether large page sizes are used indiscriminately, that is, with no
performance justification. In the first case, mistakenly indicating
that an application use a large page size (512 MB or more, for example)
when the application accesses memory in a sparse pattern, or has a much
smaller working set, in general. For example, an application uses a
shell script which only needs 64 KB of memory total, or a scientific
sparse array analysis program that works on large data sets, but only
on very small portions of the data, where the rest can be swapped out
or not even allocated. Setting this value for the application results
in several frames of physical memory being wasted, because if any of a
virtual large page is in core memory, then all of it must be loaded.
This could potentially lead to unnecessary pageouts and pageins, that
is, excessive disk activity, leading to performance degradation.
In the second case, where several users request large page sizes for
their application for no good reason, keeping this tunable low mini‐
mizes the performance hit on the rest of the system.
Modern architectures support very large page sizes (up to 4 GB for IA
and up to 1 GB for PA-RISC). Setting vps_chatr_ceiling to very high
values (anything above 64 KB) should be done with extreme caution. It
can lead to excessive memory consumption and quickly deplete the amount
of free memory available on the system.
When Should the Value of This Tunable Be Lowered?
This tunable should be lowered if no user application actually needs
large pages sized at the current tunable value, to minimize the chance
of a mistaken or malicious user causing wasted physical frames by
requesting unnecessarily large page sizes.
What Are the Side Effects of Lowering the Value?
One side effect is that applications run with smaller page sizes.
Also, the system uses smaller page sizes for certain system pages.
What Other Tunables Should Be Changed at the Same Time?
None.
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 at
AUTHOR
was developed by HP.
SEE ALSOchatr(1).
Tunable Kernel Parameters vps_chatr_ceiling(5)
