eqmem_limit(5)							File Formats Manual						    eqmem_limit(5)

NAME

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

VALUES

   Default
       system-determined

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

   Recommended value
       The system-determined default.

DESCRIPTION

       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;
       equivalently 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 equivalently 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  mem-
       ory.

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

       Note that this tunable only provides an upper bound.  Given the realities 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 equiv-
       alently 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 limited 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
       interleaved 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
       equivalently mappable, thereby causing more of these structures to be allocated from interleaved memory.

       It may also be desired to reconfigure such systems with more interleaved 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 equivalent 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 systems, 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
       equivalently mappable memory left over for the rare applications and drivers which might need it -- provided that there's sufficient physi-
       cal  memory that's actually equivalently mappable (non-ejectable, not cell local, and having physical addresses in the legal range for ker-
       nel dynamic virtual addresses).	However, memory designated as equivalently mappable 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 equivalent 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
       engineer.

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

WARNINGS

       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 tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

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