ITFN 2601 Introduction to Operating Systems

1
ITFN 2601Introduction to Operating Systems

• Lecture 15
• Page Replacement Algorithms

2
Agenda

• Not Recently Used
• First In-First Out
• Second Chance
• Clocking
• LRU
• Hardware/Software
• Aging
• Working Sets

3
Page Replacement Algorithms

• Optimal
• Swap unnecessary pages
• Look forward in time
• Find page with longest wait until being used
• Swap it out
• Minimal damage/effect
• Unimplementable

4
Not Recently Used

• Not Recently Used
• 00 Not Referenced, Not Modified
• 01 Not Referenced, Modified
• 10 Referenced, Not Modified
• 11 Referenced, Modified
• First bit reset on clock-timer
• Swap any page from lowest possible group

5
First in Algorithms

• FIFO
• Swap the oldest page currently in memory
• Could still be very active!
• Second Chance
• Reset access bit
• Reset load time

6
First in, Clocking

• Clock
• Move the clock hand to point to oldest page

7
Least Recently Used

• Listings
• Move a page to the front/end of the list each
  time it is accessed
• Very consuming
• Counter
• Each entry gets a last used time
• Entry with oldest one goes poof

8
More LRU

• Table
• N pages
• NxN matrix of bits
• Each time page k is used
• Row k set to 1s
• Column k set to 0s
• Swap row with the most 0s

9
Software LRU

• Not Frequently Used
• OS maintains counters
• Clock tick gt Counter Read bit
• Never forgets, old data remains important
• Aging
• Shift counter right
• Append Read bit at beginning

10
Aging example
11
Working Set

• Working set
• Pages used by a process
• Working set as a page
• Before the process loads, load the pages
• Load an entire working set at once
• If it doesnt all fit, back to normal paging
• OS must track the pages in the working set

12
PRAs (Summary)