copy_on_write man page on HP-UX

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copy_on_write(5)					      copy_on_write(5)

       copy_on_write  -	 dynamically enable or disable copy-on-write semantics
       for privately mapped files, private segments  of	 child	process	 after
       fork(), and private segments locked by mlock()

	  Allowed values

       This tunable dynamically enables or disables the copy-on-write behavior
       in private and system calls.  When the tunable  is  disabled,  copy-on-
       access  behavior is followed; this is the legacy behavior (HP-UX 11i v3
       and previous releases) where private segments are copied	 upon  access.
       Disabling this tunable will preserve that old behavior.

       Following is a summary of what happens when the tunable is enabled:

	      ·	 For  a process' private data (data segment, stack, heap, RSE)
		 is copied only when the child process writes  for  the	 first

	      ·	 For if mapping is created for a file for which exists, a sep‐
		 arate copy of the page is created  for	 only  when  it	 first
		 writes	 to  the  page.	  As long as reads, it shares the page
		 with mapping.	That is, updates made by shared	 mapping  will
		 be visible to private mapping until private mapping writes.

	      ·	 For  locking  privately  mapped  segments  does not result in
		 copying those segments.  However, segments are locked in mem‐
		 ory  and only when the content of the page changes, a copy is
		 made and the lock is transferred to the new segment.

   Who Is Expected to Change This Tunable?
       System administrators who wish  to  change  the	default	 copy-on-write

   Restrictions on Changing
       Changes to this tunable take effect immediately.

   When Should the Tunable Be Turned Off?
       If  an  application depends on copy-on-access behavior, where a process
       accessing a privately mapped file for the first time gets a copy of the
       page  and  the contents of the page does not change unless private map‐
       ping writes to it, the tunable should be turned	off  to	 get  copy-on-
       access behavior.

   What Are the Side Effects of Turning the Tunable Off?
       Some  workloads	might  see  increased memory consumption since private
       segments are copied when accessed for the first time.

   When Should the Tunable Be Turned On?
       If an application wants industry standard semantics, which allows users
       to  share  data	via private mapping of a file for which shared mapping
       exists, the tunable should be enabled.  Turning the tunable on can also
       reduce  memory consumption on some workloads since private segments are
       only copied upon write.

   What Are the Side Effects of Turning the Tunable On?
       Applications that privately map a file for which shared mapping	exists
       can now see the updates performed by shared mapping.

   What Other Tunables Should Be Changed at the Same Time?

       All HP-UX kernel tunable parameters are release specific.  This parame‐
       ter may be removed or have its meaning changed in  future  releases  of

       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

       was developed by HP.

       fork(2), mlock(2), mmap(2).

			   Tunable Kernel Parameters	      copy_on_write(5)

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