vps_pagesize(5)vps_pagesize(5)NAMEvps_pagesize - minimum (in kilobytes) of system-selected 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 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 exam‐
ple, 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 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 tunable, and if the selected size is larger, the value of is used
instead. Then, the value is compared against the minimum page size as
set by and the larger of the two values is used.
Who Is Expected to Change This Tunable?
Anyone.
Restrictions on Changing
Changes to this tunable take effect for subsequent physical memory
allocations. Physical memory already in use is not affected.
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 previ‐
ously 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 behavior.
This is especially true when many processes with small or fragmented
data/code sets are active. Every virtual page referenced by the appli‐
cation, 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.
Modern architectures support very large page sizes (up to 4 GB for and
up to 1 GB for PA-RISC). Setting the value very high (greater than 64
KB) can cause excessive memory consumption and quickly deplete the free
memory on the system.
When Should the Value of This Tunable Be Lowered?
The tunable should be lowered if physical memory fragmentation is pre‐
venting small memory processes from running due to waiting on contigu‐
ous 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 is lowered as well, applications with large data sets (such as data‐
bases) may suffer a performance degradation due to increased page
faults. This can be corrected with a of the appropriate application.
If 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 application not changed with the command.
What Other Tunables Should Be Changed at the Same Time?
should be considered, being the minimum bound on the kernel heuristic
range.
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 ALSOvps_ceiling(5).
Tunable Kernel Parameters vps_pagesize(5)