eqmem_limit man page on HP-UX

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eqmem_limit(5)							eqmem_limit(5)

       eqmem_limit  -  determines  the	maximum amount (in MB) of equivalently
       mapped memory which can be allocated after boot


   Allowed values
       When an explicit value is used,	it  is	interpreted  as	 a  number  of

   Recommended value
       The system-determined default.

       Equivalently  mapped  memory  is memory which has the same physical and
       virtual address.	 On PA-RISC systems, this type of memory  is  required
       for some kernel structures.  Most of these are allocated in early boot,
       but some are allocated whenever memory is added to the system;  equiva‐
       lently  mapped  memory  may  also be used by some device drivers, and a
       very small number of kernel intrusive applications.

       Upon boot, the HP-UX kernel selects some memory as potentially  equiva‐
       lently  mappable.   Only	 those pages can ever be given equivalent map‐
       pings.  When not required for equivalent memory, these  pages  can  and
       will  be used for other purposes, so it's generally desirable to have a
       large number of pages designated as equivalently mappable.  If  the  is
       set  to its default the kernel will compute an appropriate limit.  This
       limit will be conservative, intended  to	 ensure	 that  there  will  be
       enough to support Online Addition (OLA) of a maximum amount of memory.

       If  the	tunable	 is set to a non-default value then that limit will be

       Note that this tunable only provides an upper bound.  Given the	reali‐
       ties  of dynamic addition of memory, it is legal - and normal - for the
       limit to be set to more memory than is  currently  present.   Moreover,
       memory  can only be used for equivalently mapped kernel memory if it is
       non-ejectable and its physical address corresponds to a	legal  virtual
       address	for dynamic kernel memory.  Thus, the actual amount of equiva‐
       lently mappable memory may be less than either the or the total	memory
       on the system.

   Who is Expected to Change This Tunable?
       Cache-Coherent  Non-Uniform  Memory Access (ccNUMA) systems making lim‐
       ited use of online addition of memory.

       Systems making extreme use of online addition of memory, and also using
       significant equivalently mapped memory for other purposes.

   Restrictions on Changing
       This tunable only exists on PA-RISC systems.

       Changes to this tunable take effect at the next reboot.

   When Should the Value of This Tunable Be Changed?
       For  best performance on ccNUMA systems, certain shared data structures
       should be allocated from interleaved memory.  If all  available	inter‐
       leaved memory has been designated equivalently mappable, and cell local
       memory is available, these structures will be  allocated	 out  of  cell
       local memory.

       If a system has a mix of interleaved and cell local memory, and there's
       no intention of performing online addition of memory, or the total that
       will  ever  be  added is much less than the maximum supported, then the
       tunable can be used to limit the total that will be designated  equiva‐
       lently  mappable,  thereby causing more of these structures to be allo‐
       cated from interleaved memory.

       It may also be desired to reconfigure such  systems  with  more	inter‐
       leaved memory and less cell local memory.

       A conservative lower limit for the value of this tunable would be 2% of
       the total memory expected to ever be  added  online,  or	 a  couple  of
       megabytes,  whichever is larger.	 A less conservative limit would be 1%
       of total memory (initial and OLA'd, or 0 if no OLA will ever occur, and
       it's  known that there are no drivers or applications requiring equiva‐
       lent memory).

       It is not especially useful to change this  tunable  on	systems	 which
       support	neither	 online	 memory	 addition nor cell local memory.  It's
       also generally not especially useful to change it on small ccNUMA  sys‐
       tems, having only a handful of localities (cells).

       The  default value is extremely conservative, and should support online
       addition of more memory than is actually possible, with lots of equiva‐
       lently  mappable memory left over for the rare applications and drivers
       which might need it -- provided that there's sufficient physical memory
       that's  actually	 equivalently mappable (non-ejectable, not cell local,
       and having physical addresses in the legal  range  for  kernel  dynamic
       virtual	addresses).   However,	memory designated as equivalently map‐
       pable may be used for other purposes, and it's conceivable  that	 usage
       patterns	 might	result	in its unavailability when needed for OLA.  If
       such a problem is encountered, it may be useful to  raise  the  tunable
       value  above the computed default value.	 (This should probably be done
       only on the advice of HP customer support engineers.)

   What Are the Side Effects of Changing the Value?
       If the total amount of memory designated as  equivalently  mappable  is
       too  low,  allocations  will  fail.   This can result in the failure of
       online addition of memory, or of devices whose drivers require  equiva‐
       lent memory.

       If  the	total  amount of memory designated as equivalently mappable is
       too high somewhat reduced performance may be experienced	 for  accesses
       to shared structures on ccNUMA systems.

       Values  above 104856 (1024 * 1024 MB, i.e. 1 Terabyte) are unsupported,
       and should be avoided except on the advice of a customer support	 engi‐

   What Other Tunable Values 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.

PA-RISC System Only	   Tunable Kernel Parameters		eqmem_limit(5)

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