eqmemsize(5)

NAME

eqmemsize — determines the minimum size (in pages) of the equivalently mapped reserve pool

VALUES

Default

15 pages

Allowed values

0 or any positive integer.

However, because this represents physical pages no longer available for general system use, caution is urged before using large values.

DESCRIPTION

Equivalently mapped memory is a page which has the same physical and virtual address. This is useful for some applications, and for I/O. Since most memory is to be used in the traditional swapper/virtual address model, it is useful for the system to reserve some pages for this type of access at boot while the most physical memory is available.

eqmemsize provides the bottom value for the size of this reserved pool. The actual pool size is the sum of eqmemsize and a value determined dynamically at boot using the available size of physical memory, to account for large memory systems automatically. The scaled value increases the pool size by 1 page per 256 MB.

Who is Expected to Change This Tunable?

Anyone.

Restrictions on Changing

Changes to this tunable take effect at the next reboot.

When Should the Value of This Tunable Be Raised?

This tunable should raised if the customer sees the error message:

Equivalently mapped reserve pool exhausted;

Overall application performance may be improved by

increasing the "eqmemsize" tunable parameter

(currently set to "{X}").

What Are the Side Effects of Raising the Value?

Physical memory is reserved for this use and unavailable for the rest of the system. Increasing this tunable by a substantial amount would be roughly equivalent to removing the same amount of physical memory. The amount of memory represented by the increase in the tunable is not likely to be used for equivalent memory, and no one else can get to this memory either.

When Should the Value of This Tunable Be Lowered?

In general, if the tunable is set to more than 10 pages over 15+(physical_memory/256 MB), it's probably too high. Specifically, if the system swaps heavily, and the above is true, the value should be lowered to free the physical memory back to the system.

What Are the Side Effects of Lowering the Value?

Requests for equivalent memory may be denied. This is not a fatal error, but it should be avoided.

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 tunable values, consult the documentation for the kernel software being installed. For information about optional kernel software that was factory installed on your system, see HP-UX Release Notes at http://docs.hp.com.

AUTHOR

eqmemsize was developed by HP.