vps_ceiling.5 (2010 09)

v
vps_ceiling(5) vps_ceiling(5)
(Tunable Kernel Parameters)
NAME
vps_ceiling - maximum (in kilobytes) of system-selectable page size
VALUES
Default
16 KB
Allowed values
Minimum:
4 KB
Maximum:
4194304 KB
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. Historically, 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 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 is 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 ele-
ment.
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 com-
pared against the minimum page size as set by vps_pagesize, 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 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 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 KB pages are now addressed by a single 256 KB 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 upper 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, if either
vps_pagesize is also
raised or the kernel heuristic chooses a larger value. This can lead to undesired behavior. For example,
when a program is reading in the last 4 KB of code from disk with the default value, this means 16 KB of
contiguous physical memory must be found and set up with the appropriate virtual translation, even
though, only 4 KB of data will actually be on it. Consider the maximum, where 64 megabytes of contigu-
ous physical memory is allocated for every new virtual page the program uses, even if, only 4 KB of that
is actually used. Besides the wasted physical memory here, there is also an issue of delays due to frag-
mentation that many contiguous frames of physical memory may not be available and a process may be
stalled waiting on the allocation when the amount of memory it actually needs is available.
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1