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 pagezero_daemon_enabled(5)                       pagezero_daemon_enabled(5)
                          Tunable Kernel Parameters



 NAME    [Toc]    [Back]
      pagezero_daemon_enabled - zeroing of free memory in the background is
      enabled

 VALUES    [Toc]    [Back]
    Default
      1

    Allowed values    [Toc]    [Back]
      Minimum: 0

      Maximum: 1

 DESCRIPTION    [Toc]    [Back]
      HP-UX provides improved security by zeroing out any memory that is
      being assigned to user space.  This ensures that no user can read what
      may have been written by some other user.  Normally, the zeroing is
      done when the physical page is allocated to the user -- often when the
      application touches the page for the first time.  Some system calls
      like mlock() also result in the zeroing of pages.  The time taken for
      these kind of system calls and accesses depends upon the size of the
      memory being zeroed out.  A 4G page may easily take many seconds to be
      allocated.  A large database shared memory segment may take many
      minutes to be allocated.  Allocation of a small page is generally
      unobservable.

      The pagezero daemon is a performance enhancement that reduces the
      elapsed time for completing kernel operations like page faults,
      mlock(), and so on.  The idea is to zero out large free pages (4MB and
      above in size) during times when the CPU is idle.  The daemon has been
      specially designed to ensure that it only executes for brief times
      when the CPU is idle.

      However, under certain conditions, where some resource (CPU, TLB or
      Memory bandwidth) is very highly utilized, it is possible that the
      operation of the daemon adversely impacts performance.  For example,
      if the application is limited by the memory bandwidth, then it may be
      better to disable the daemon.  This situation is expected to be rare.
      Most workloads will not need to disable the daemon.

      This tunable allows system administrators to disable and enable the
      pagezero daemon.  Once the daemon has been disabled, it will not zero
      out any more pages.  Any pages already in the process of being zeroed
      will be zeroed out.  When the daemon is enabled, it will zero out any
      unzeroed free pages of size 4MB and more.

    Who is Expected to Change This Tunable?
      Anyone.

    Restrictions on Changing    [Toc]    [Back]
      Changes to this tunable take effect immediately.



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






 pagezero_daemon_enabled(5)                       pagezero_daemon_enabled(5)
                          Tunable Kernel Parameters



    When Should the Value of This Tunable Be Changed to 0?
      This tunable should be changed only if the system is using large pages
      (i.e.  vps_ceiling is set to 4MB or above).  Changing it if the system
      is not using pages of size 4M and above will have no effect.  It may
      be changed to 0 if the system is bottle-necked on some hardware
      resource.  In particular, disabling the daemon may help in decreasing
      TLB misses and increasing the available CPU time and reducing memory
      latencies.

    What Are the Side Effects of changing the value to 0?
      Disabling the daemon will increase the time it takes to process page
      faults and to complete system calls that result in the allocation of
      memory (e.g.  mlock()).

 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]
      pagezero_daemon_enabled was developed by HP.

 SEE ALSO    [Toc]    [Back]
      vps_ceiling(5).


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