== Linux Page Replacement Requirements ==

As any other OS linux has the usual 'cheap' requirements for its page
replacement. However several other requirements make implementing page
replacement algorithms as found in papers/textbooks non-trivial.


== Multiple Zones ==

Unlike most, linux has multiple memory zones; that is, memory is not viewed as
one big continuous section. There are specific sections of memory where it is
desirable to have the ability to free pages in. Think of NUMA topologies or
DMA engines that cannot operate on the full address space. Hence memory is
viewed in multiple zones.

For traditional page replacement algorithms this is not a big issue since we
just implement per zone page replacement; eg. a CLOCK per zone. However with
the introduction of non-resident page state tracking in the recent algorithms
this does become a problem. Since a page can fault into a different zone than
were it came from the non-resident page state tracking needs to be over all
memory, not just a single zone.

This makes for per zone resident page tracking and global non-resident page
tracking; this separation is not present in several proposed algorithms and
hence makes implementing them a challenge.


== Asynchronous Page-Out ==

The page-out operation is not synchonous. Dirty pages that are selected for
reclaim are not directly freed, writeback is started against them (PG_writeback
is set) and they are fed back to the resident list. When on completion of the
write to their backing-store the reference bit is still unset a callback is
invoked to place them so that they are immediate candidates for reclaim again
(rotate_reclaimable_page).

When scanning for reclaimable pages make sure you are not stuck on a
writeback saturated list.


== Insert Referenced ==

Since we fault in pages it is per definition that the page is going to be used
(readahead?) right after we switch back to userspace. Hence we effectifly
insert page with their reference bit set. Since most algorithms assume we
insert pages with their reference bit unset the need arises to modify the
algorithms so that pages are not promoted on their first reference (use-once).


== Expensive Referenced Check ==

Because multiple page table entries can refer to the same physical page
checking the referenced bit is not as cheap as most algorithms assume it is
(rmap). Hence we need to do the check without holding most locks. This
suggests a batched approach to minimize the lock/unlock frequency. Modifying
algorithms to do this is not usualy very hard.
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CategoryAdvancedPageReplacement