NCHU System & Network Lab Lab 12 Page Replacement Algorithm.

NCHU System & Network LabNCHU System & Network Lab

Lab 12Lab 12Page Replacement AlgorithmPage Replacement Algorithm

http://osnet.cs.nchu.edu.tw/


IntroductionIntroduction

• Why do we need page replacement algorithm ?– When page fault occur and there are no free

memory frame, we need an algorithm to choose a victim frame.

• What is page fault ?– Process wants to access the page but the page isn’t

placed in main memory.


Page FaultPage Fault

a

b

c

d

e

f

3 vi

8 vii

6 v

f

c

a

a b c d

e f

012345

0

1

2

3

4

5

0

3

6

8logicalmemory

page table

physical memory

secondary memory

frame

valid-invalid bit


Page Replacement AlgorithmPage Replacement Algorithm

• First In, First Out (FIFO)– Simplest– Associates with each page the time when it was

brought into memory.– victim page = oldest page– Just use a FIFO queue


FIFOFIFO

Reference string

a b c d a c a a b c

a

b

a

c

b

a

c

b

d

first in first out

c

a

d

c

a

d

c

a

d

c

a

d

b

a

d

b

a

c

page frames


ExampleExample

• A program that show FIFO replacement algorithm process

• Input – A file that has a reference string.

• Output– Page replacement process.– Number of page fault times.


Page Replacement AlgorithmPage Replacement Algorithm

• Least Recently Used (LRU)– Associates with each page the time of that page’s

last use– Victim page = the page that has not been used for

the longest period of time– Use counters or stack


LRU (stack)LRU (stack)

Reference string

a b c d a c a a b c

a

b

a

c

b

a

d

c

b

least-recently used

a

d

c

c

d

a

a

c

d

a

c

d

b

a

c

c

b

a

page frames


LRU (counter)LRU (counter)

Reference string

0 1 2 3 4 5 6 7 8 9

a b c d a c a a b ca 0 a 0

b 1

a 0

b 1

c 2

d 3

b 1

c 2

d 3

a 4

c 2

d 3

a 4

c 5

d 3

a 6

c 5

d 3

a 7

c 5

b 8

a 7

c 5

b 8

a 7

c 9

page frames

least-recently used


LabLab

• A program that show LRU replacement algorithm process

• Input – A file that has a reference string.

• Output– Page replacement process.– Number of page fault times.


ReferenceReference•Advanced Programming in the UNIX Environment 2nd

Author : Richard Stevens, Stephen A.Rago, Publisher : Addison-Wesley

•Beginning Linux ProgrammingAuthor : Richard Stones, Neil Matthew Publisher : Wrox

•Operating System Concepts 6th edition Author : Abraham Silberschatz, Peter Baer Galvin, Greg Gagne Publisher : John Wiley & Sons, inc.

NCHU System & Network Lab Lab 12 Page Replacement Algorithm.

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