desfree_pct man page on HP-UX

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desfree_pct(5)							desfree_pct(5)

       desfree_pct - sets the lower bound before paging daemon starts stealing
       pages, specified as a percentage of available physical memory

   Allowed values
       Must not be greater than (see lotsfree_pct(5)).

       is a tunable parameter to set the lower bound before the paging	daemon
       start stealing pages.  It is specified as a percentage of physical mem‐
       ory available after boot.

       must be a positive integer between 0 and 30 and must  be	 equal	to  or
       less than (see lotsfree_pct(5)).	 Trying to set higher than results in

       The paging daemon in HP-UX acts on a "two hand" model.  The daemon runs
       at least once a second, with one part marking pages of virtual  address
       space  as "unused".  If the page is referenced before this "aging" hand
       returns, it will be marked as "used" again.   Another  section  of  the
       daemon,	the  "steal"  hand  follows the age hand (the distance between
       them varies in a well bounded range)  and  processes  pages  which  are
       still  marked "unused", since needed or frequently accessed pages would
       have a high likelihood of being	referenced  in	the  gap  between  the

       Exactly	how  the  steal hand treats the pages still marked as "unused"
       depends on the comparison of free system physical memory and three pag‐
       ing  parameters:	 and  Between and is a periodically re-calculated and,
       hence floating, threshold known as Thus and are	the  upper  and	 lower
       bounds between which moves.

       is  an  expression  of how much the system maintains, expressed in per‐
       centage terms (percentage of physical memory available after boot).

       ·  If is greater than the  system's  memory  availability  is  in  good
	  shape.   The	steal  hand  does  nothing as a lot of memory is still
	  available on the system, so there is no need to "steal' a page  that
	  might be needed soon.

       ·  In  the  more	 common	 case, when is lower than (but higher than the
	  steal hand will begin to steal pages that have remained unreferenced
	  from	when  the age hand last marked them.  "Stealing" a page refers
	  to the process of freeing the page from being allocated for  a  par‐
	  ticular  virtual page and making it available for general allocation
	  again.  If falls below but is still more than memory availability is
	  still	 in  reasonable	 shape,	 but the paging daemon begins stealing
	  pages more aggressively, including putting lower priority  processes
	  to sleep to free up their memory.

       ·  When	is  less than free memory in the system is getting more criti‐
	  cal; the paging daemon begin deactivating low priority processes, in
	  other words, swapping out pages to the swap device.

       Since  the exact memory topography varies widely across supported plat‐
       forms, the paging parameters and are tunable to allow for  cases	 where
       the  default values are insufficient, or where the system administrator
       requires more control over the actions of the paging daemon.   In  gen‐
       eral,  however,	the  automatic calculation performed by default should
       suffice for most systems.  is a system calculated value.

   Who Is Expected to Change This Tunable?
       Anyone with super-user privileges.

   Restrictions on Changing
       Changes to this tunable take effect immediately.

   When Should the Value of This Tunable Be Raised?
       This tunable should be raised on systems where system memory is	criti‐
       cal  for short-term, high priority processes, but is being held by low-
       priority, long running processes.  For example, when one web server  is
       forking	off  per-connection  threads  while  50 RCS clients run in the

       Increasing (and hence would raise the likelihood	 of  the  low-priority
       processes  being	 put  to sleep, allowing the high-priority processes a
       chance to acquire the pages that were previously allocated to the  for‐

   What are the Side Effects of Raising the Value?
       As  increases,  more  low  priority processes might be forced to sleep.
       Depending on what these processes are actually doing, this may  not  be
       the right approach.

   When Should the Value of This Tunable Be Lowered?
       Raising	too high can impair performance of lower priority processes as
       previously described.

       In addition, raising and increases system overhead (it  takes  time  to
       swap  a page to disk), and should be weighed carefully against the need
       or desire to keep a large pool of physical memory unused.

       Some free memory is essential, a moderate amount is  probably  good,  a
       lot  of	free  memory  just  sitting idle is a waste.  If the available
       physical memory on the machine seems generous  and  heavy  swapping  is
       occurring,  lowering  and/or  will  increase  memory usage but decrease
       swapping overhead.

   What are the Side Effects of Lowering the Value?
       More physical memory will be in use before processes are put  to	 sleep
       to try to reclaim their frames.

   What Other Tunable Values Should Be Changed at the Same Time?
       should  be  considered  when modifying for reasons previously described
       above.  acts as a firm upper bound to so attempting  to	set  above  is
       meaningless and will result in an error.	 See lotsfree_pct(5).

       Starting	 with HP-UX 11i Version 2, HP recommends the use of to specify
       the lower bound for the paging daemon.

       Memory resource management infrastructure is busy.
		      Please try later.

       Attempting to set

       An invalid value is entered that is not an integer between
		      0 and 30, such as an negative number.

       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.


			   Tunable Kernel Parameters		desfree_pct(5)

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