eqmem_limit(5)

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.

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 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).

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 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 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.