eqmem_limit.5 (2010 09)

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eqmem_limit(5)
PA-RISC System Only
eqmem_limit(5)
(Tunable Kernel Parameters)
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
0 - 419304
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 mappings. 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
eqmem_limit is set to its default the kernel will compute an appropri-
ate 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 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
equivalently mappable memory may be less than either the
eqmem_limit 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 inter-
leaved 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 eqmem_limit 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).
HP-UX 11i Version 3: September 2010 − 1 − Hewlett-Packard Company 1