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

9. Software LRU • Not Frequently Used • OS maintains counters • Clock tick => 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 doesn’t all fit, back to normal paging • OS must track the pages in the working set

12. PRA’s (Summary)