vps_pagesize(5) vps_pagesize(5)
Tunable Kernel Parameters
NAME [Toc] [Back]
vps_pagesize - minimum (in kilobytes) of system-selected page size
VALUES [Toc] [Back]
Default
4 (KB)
Allowed values [Toc] [Back]
Minimum: 4 (KB)
Maximum: 65536 (KB)
DESCRIPTION [Toc] [Back]
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.
Historically, the TLB were entirely managed within hardware to achieve
speed optimizations while sacrificing the flexibility of software
implementations. For example, easily changed algorithms or table
implementations.
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 if fixed
in hardware) into 'superpages' or 'large pages', that 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 element requires 1 KB of memory. Using the usual
4 KB 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 at
every fifth element.
If a user application does not use the chatr command to specify a page
size for the program text and data segments, the kernel automatically
selects a page size based on system configuration and object size.
This selected size is then compared to the maximum page size defined
by the vps_ceiling tunable, and if the selected size is larger, the
value of vps_ceiling is used instead. Then, the value is compared
against the minimum page size as set by vsp_pagesize, and the larger
of the two values is used.
Who is Expected to Change This Tunable?
Anyone.
Restrictions on Changing [Toc] [Back]
Changes to this tunable take effect at the next reboot.
Hewlett-Packard Company - 1 - HP-UX 11i Version 2: Sep 2004
vps_pagesize(5) vps_pagesize(5)
Tunable Kernel Parameters
When Should the Value of This Tunable Be Raised?
This tunable can be raised when processes on the system access their
text and data in a regular fashion, and over a range of data larger
than the current value. For example, if this tunable is set to 16 KB,
but almost every process on the system repeatedly works with a four or
five distinct 256 KB data sets, then raising the tunable to 256 would
reduce the page faulting for these processes because 16 of the
previously 16 kilobyte pages are now addressed by a single 256
kilobyte translation.
Average system behavior is not likely to display uniformity of memory
access and the optimal value is not easy to determine, so this tunable
only represents the lower value for the kernel heuristic and may not
change the actual system behavior.
What Are the Side Effects of Raising the Value?
Memory allocations will require larger groups of contiguous pages
because the kernel heuristic was not already choosing the larger
value.
Requiring larger virtual pages may lead to undesirable system
behaviour. This is especially true when many processes with small or
fragmented data/code sets are active. Every virtual page referenced
by the application, regardless of actual usage within that page,
requires that the entire page work of contiguous physical frames of
memory be present. For example, you cannot swap out half of a large
virtual page. Many contiguous frames may not always be possible and
may cause memory stalls on allocation that are not strictly needed.
In addition, the waste of physical frames in this case would probably
lead to increase swap usage, further degrading system performance.
When Should the Value of This Tunable Be Lowered?
The tunable should be lowered if physical memory fragmentation is
preventing small memory processes from running due to waiting on
contiguous chunks of memory, or if the overall system usage of memory
displays poor spatial locality (virtual accesses are not close to each
other) producing wasted physical frames.
What Are the Side Effects of Lowering the Value?
If vps_ceiling is lowered as well, applications with large data sets
(such as databases) may suffer a performance degradation due to
increased page faults. This can be corrected with a chatr of the
appropriate application. If vps_ceiling is not modified, the side
effects should be minimal as the kernel will now have a larger range
to choose an appropriate page size for each non-chatr'ed application.
What Other Tunables Should Be Changed at the Same Time?
vsp_ceiling should be considered, being the minimum bound on the
kernel heuristic range.
Hewlett-Packard Company - 2 - HP-UX 11i Version 2: Sep 2004
vps_pagesize(5) vps_pagesize(5)
Tunable Kernel Parameters
WARNINGS [Toc] [Back]
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 parameter 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://docs.hp.com.
AUTHOR [Toc] [Back]
vps_pagesize was developed by HP.
SEE ALSO [Toc] [Back]
vps_ceiling(5).
Hewlett-Packard Company - 3 - HP-UX 11i Version 2: Sep 2004 [ Back ] |
