ITFN 2601Introduction to Operating

Systems

Lecture 15

Page Replacement Algorithms

Agenda

Not Recently UsedFirst In-First Out

Second ChanceClocking

LRUHardware/SoftwareAging

Working Sets

Page Replacement Algorithms

OptimalSwap unnecessary pagesLook forward in time

Find page with longest wait until being usedSwap it outMinimal damage/effect

Unimplementable

Not Recently Used

Not Recently Used00 – Not Referenced, Not Modified01 – Not Referenced, Modified10 – Referenced, Not Modified11 – Referenced, Modified

First bit reset on clock-timerSwap any page from lowest possible

group

First in Algorithms

FIFOSwap the oldest page currently in memoryCould still be very active!

Second ChanceReset “access” bitReset load time

First in, Clocking

ClockMove the clock hand to point to “oldest”

page

Least Recently Used

ListingsMove a page to the front/end of the list each

time it is accessedVery consuming

CounterEach entry gets a “last used” timeEntry with oldest one goes *poof*

More LRU

TableN pagesNxN matrix of bitsEach time page k is used

Row k set to 1’sColumn k set to 0’s

Swap row with the most 0’s

Software LRU

Not Frequently UsedOS maintains countersClock tick => Counter + Read bit Never forgets, old data remains important

AgingShift counter rightAppend Read bit at beginning

Aging example

Working Set

Working setPages used by a process

Working set as a pageBefore the process loads, load the pagesLoad an entire working set at once

If it doesn’t all fit, back to normal paging

OS must track the pages in the working set

PRA’s (Summary)