vps_ceiling(5)							File Formats Manual						    vps_ceiling(5)

NAME

       vps_ceiling - maximum (in kilobytes) of system-selectable page size

VALUES

   Default
   Allowed values
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  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', that  can	be  repre-
       sented  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 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 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  tun-
       able 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 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 contiguous 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 fragmentation 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.

       Therefore, it is best to only raise this tunable if you know precisely the memory usage of the system.  In general, increasing the variable
       page size on a per application basis for known applications, such as, databases which scan large amounts of data with only one page  fault,
       is a much better practice.

       Modern  architectures  support  very large pages (up to 4 GB for Itanium and up to 1 GB for PA-RISC).  Setting to very large sizes (greater
       than 64 KB) should be done with extreme caution since it can cause excessive memory consumption and quickly deplete the amount of free mem-
       ory available on the machine.

   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 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?
       Applications such as databases will suffer more page faults to get their working set into memory, but this can be handled by using with the
       appropriate application.

   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 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  tun-
       able  values,  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_pagesize(5).

							     Tunable Kernel Parameters						    vps_ceiling(5)