vps_pagesize man page on HP-UX

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vps_pagesize(5)						       vps_pagesize(5)

NAME
       vps_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 ALSO
       vps_ceiling(5).

			   Tunable Kernel Parameters	       vps_pagesize(5)
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