eqmemsize(5)

DESCRIPTION

This tunable has been obsoleted and removed.

If it is desired to control the total amount of equivalently mapped memory available to the kernel after boot, then use the new tunable eqmem_limit (see eqmem_limit(5)).

Note that generally speaking, systems where it was useful to set eqmemsize will not need to set eqmem_limit.

Equivalently mapped memory is memory which is given the same physical and virtual address. On PA-RISC systems, this is required to support on-line addition of memory, and may be useful for some applications and some I/O devices.

HP-UX 11i Version 2 maintained a (small) reserve of equivalently mapped pages, which could be used for no other purpose. It could also potentially equivalently map any page having a physical address below the maximum kernel virtual address, but only if it happened to find both the virtual and physical addresses available; this rarely happened, except immediately after boot. The eqmemsize tunable was used to size this reserve. It was kept quite small, except on systems known to use such memory, where the reserve pool size would be increased using the eqmemsize tunable.

The equivalent memory allocator was completely rewritten after HP-UX 11i Version 2. The current version of the equivalent memory allocator decides, at boot, which pages it will consider to be equivalently mappable. It makes the corresponding virtual addresses unavailable for other purposes, thereby ensuring that if the physical page is available, it will be possible to map it equivalently. This allows such pages to be used for other purposes, and still be reliably reused for equivalent mappings. Thus no reserve is required. The eqmem_limit tunable places a cap on the total amount of memory which will be considered equivalently mappable.

Such pages are treated almost identically to other pages, but not quite. The differences only matter on Cache-Coherent Non-Uniform Memory Access (ccNUMA) systems, where in some circumstances these differences can result in reduced performance. On such systems the eqmem_limit tunable may be used to reduce the total amount of memory that will be designated equivalently mappable down to the maximum expected to actually be needed. (Normally the kernel makes a very conservative estimate of the total amount that might be needed.) See eqmem_limit(5) for details.

AUTHOR

eqmemsize was developed by HP.

SEE ALSO

eqmem_limit(5).