vps_chatr_ceiling(5)                                   vps_chatr_ceiling(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      vps_chatr_ceiling - maximum (in kilobytes) of user selectable page
      size

 VALUES    [Toc]    [Back]
    Default
      1048576 (KB)

    Allowed values    [Toc]    [Back]
      Minimum: 4 (KB)

      Maximum: 4194304 (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 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 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 1K of memory.  Using
      the usual 4K 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 every fifth element.

      This tunable sets the upper bound for virtual page size requested by a
      user application, as set with the chatr command on the binary.

    Who is Expected to Change This Tunable?
      Anyone.

    Restrictions on Changing    [Toc]    [Back]
      Changes to this tunable take effect at the next reboot.

    When Should the Value of This Tunable Be Raised?
      This tunable should be raised when a user application with known large
      memory set usage (such as a database) is expected to need larger pages
      than the current value allows.




 Hewlett-Packard Company            - 1 -      HP-UX 11i Version 2: Sep 2004






 vps_chatr_ceiling(5)                                   vps_chatr_ceiling(5)
                          Tunable Kernel Parameters



    What Are the Side Effects of Raising the Value?
      The side effects depend on the actual memory usage of the chatr'ed
      application, and if many users on the system chatr their applications
      for no good reason.  In the first case, mistakenly chatr'ing an
      application to use a large page size (512 MB or more) when the
      application uses memory in a sparse pattern, or has a much smaller
      working set in general. For example, an application uses a shell
      script which only needs 64KB of memory total, or a scientific spare
      array analysis program that works on large data sets, but only on very
      small portions of the data where the rest can be swapped out or not
      even allocated.  Setting this value for the application will result in
      several frames of physical memory being wasted if any of a virtual
      large page is in core memory then all of it must be there.  This is
      true because a virtual large page must be constructed of contiguous
      physical frames, which may not always be available in the quantity
      desired, leading to unneeded allocation delays.

      In the second case, where several users chatr their application for no
      good reason, keeping this tunable low minimizes the performance hit on
      the rest of the system.

    When Should the Value of This Tunable Be Lowered?
      This tunable should be lowered if no user application actually needs
      large pages sized at the current tunable value to minimize the chance
      of a mistaken or malicious user causing wasted physical frames when
      using chatr with their applications.

    What Are the Side Effects of Lowering the Value?
      The only side effect is that applications must run with smaller page
      sizes.

    What Other Tunables Should Be Changed at the Same Time?
      None.

 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_chatr_ceiling was developed by HP.




 Hewlett-Packard Company            - 2 -      HP-UX 11i Version 2: Sep 2004






 vps_chatr_ceiling(5)                                   vps_chatr_ceiling(5)
                          Tunable Kernel Parameters



 SEE ALSO    [Toc]    [Back]
      chatr(1).


 Hewlett-Packard Company            - 3 -      HP-UX 11i Version 2: Sep 2004