Page Replacement Algorithms

1. Page replacement algorithms
Ubaidullah alias kashif
MSCCN-III
Sukkur IBA.

2. Paging:- In computer operating systems, paging
is one of the memory-management schemes by
which a computer can store and retrieve data
from secondary storage for use in main
memory.
Page fault The main functions of paging are
performed when a program tries to access
pages that are not currently mapped to physical
memory (RAM). This situation is known as a
page fault.

3. Page replacement algorithm
Page replacement algorithms decide which
memory pages to page out (swap out, write
to disk) when a page of memory needs to be
allocated.
Paging happens when a page fault occurs and
a free page cannot be used to satisfy the
allocation, either because there are none, or
because the number of free pages is lower
than some threshold.

4. Clock
The clock algorithm keeps a circular list of pages in
memory, with the "hand" (iterator) pointing to the last
examined page frame in the list.

5. When a page fault occurs and no empty frames exist, then the R
(referenced) bit is inspected at the hand's location. If R is 0, the
new page is put in place of the page the "hand" points
to, otherwise the R bit is cleared. Then, the clock hand is
incremented and this process is repeated until a page is found
with R = 0

6. LRU & NRU
LRU keeps track of page usage over a short period of
time, while NRU just looks at the usage in the last
clock interval.

7. Adaptive replacement cache
ARC improves the basic LRU strategy by splitting the
cache directory into two lists, T1 and T2, for recently
and frequently referenced entries.
In turn, each of these is extended with a ghost list (B1
or B2), which is attached to the bottom of the two
lists. These ghost lists act as scorecards by keeping
track of the history of recently evicted (expelled)
cache entries, and the algorithm uses ghost hits to
adapt to recent change in resource usage.

8. T1, for recent cache entries.
T2, for frequent entries, referenced at least twice.
B1, ghost entries recently evicted from the T1 cache, but are still
tracked.
B2, similar ghost entries, but evicted from T2.
T1 and B1 together are referred to as L1, a combined history of
recent single references. Similarly, L2 is the combination of T2
and B2

9. ...[ B1 <-[ T1 <-!-> T2 ]-> B2 ] . ….
[ ……………. . . . [ . . . . . . ! . .^. …………...... ] …… . . . ]
[ fixed cache size (c) ]