Page Replacement

Sashikanta TaoremMTech – CSE1st Semester

Presented by:

Overview

1. Why we need page replacement?

2. Basic page replacement technique.

3. Different type of page replacement algorithm and their examples.

Why????

Limited physical memory --> limited number of frame --> limited number of frame allocated to a process.

Basic Page Replacement

1. Find the location of the desired page on the disk.

2. Find a free frame If there is a free frame use it. No free frame – use page replacement algorithm to

select a victim frame. Write the victim frame to disk, change the frame and

page tables accordingly.

Basic Page Replacement (Contd)

3. Read the desired page into the newly freed frame, change the page and frame tables.

4. Restart the user process.

Page Replacement

Victim

Swap desirepage in

Reset pagetable for new page

Change toinvaid

Page table

FrameValid invalid bit

2

4

3

0F

I V

Swap outvictim page

1

Physical memory

Overhead

If no free frames - 2 page transfers.

Solution : Modify bit or Dirty bit.

Replacement Policy

Which page to be replaced?

Page removed should be the page least likely to be referenced in the near future.

Most policies predict the future behavior on the basis of past behavior.

Page faults versus number of frames

Number of frames

Number of page faults

Replacement Algorithm

1. FIFO page replacement

2. Optimal page replacement

3. LRU page replacement

4. LRU-Approximation page replacement

5. Counting-Based page replacement

FIFO Page Replacement

Easy to understand and program.

Performance is not always good.

Belady’s Anomaly

Drawbacks - FIFO

A page which is being accessed quite often may also get replaced because it arrived earlier than those present

Ignores locality of reference. A page which was referenced last may also get replaced, although there is high probability that the same page may be needed again.

FIFO – An Example

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

707 7

01

201

231

230

430

420

423

023

013

012

712

702

701

Optimal Page Replacement

Lowest page fault rate of all algorithms

Free from Belady’s anomaly

Optimal Page Replacement (contd)

“Replace the page that will not be used for the longest period of time.”

Requires future knowledge of the reference string.

Used for comparison studies.

OPR – An Example

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

707 7

01

201

203

243

203

701

201

LRU Page Replacement

Free from Belady’s Anomaly.

Problem: How to order the frame defined by the time of last use.

Solution: Counters Queue

LRU – An Example

7 0 1 2 0 3 0 4 2 3 0 3 2 1 2 0 1 7 0 1

707 7

01

201

203

4

3

4

23

2

03

701

04

20

231

201

LRU – Approximation Page Replacement

Additional-Reference-Bits Algorithm

Second-Chance Algorithm

Enhanced Second-Chance Algorithm

Additional-Reference-Bits Algorithm

Uses reference bit, initially at 0 ,after reference h/w updates it to 1.

We can use 8-bit byte as reference bits history.

At regular interval – Shift the reference bit into the high-order bit of 8-bit byte,

shifting the other bits right by 1 bit and discarding the low-order bit.

Additional-Reference-Bits Algorithm

Example:00000000 – page has not been use for eight time

period.

11111111 – use at least once in each time period.

11000100 is use more recently than 01110111

Second-Chance Algorithm

Uses FIFO and a reference bit.

Implement using a circular queue.

Refer as CLOCK algorithm.

Enhanced Second-Chance Algorithm

Considers the Reference bit and the modify bit as an ordered pair, (Reference bit, Modify bit).

Four possible cases:1. (0,0) – neither recently used nor modified.

2. (0,1)

3. (1,0)

4. (1,1)

Counting-Based Page Replacement

Uses COUNTERS to count the number of references that have been made to each page.

1. Least Frequently used

2. Most frequently Used

Page-Buffering algorithm

Uses POOL of frames

When a page fault occurs, a victim is chosen but the page is read into a free frame from the pool before the victim is written out.

When a page fault occurs, we first check whether the desire page is in the free-frame pool. If it is not, we must select a free frame and read into it.

Objective Questions!!!

Ans: C

1. Dirty bit is used to show the

a. page with corrupted data

b. the wrong page in the memory

c. page that is modified after being loaded into cache memory

d. page that is less frequently accessed.

Ans: C

2. What replacement policy is used by Windows NT

a. LRU

b. NFU

c. FIFO

d. Clock Replacement

e. None of the above

CLOCK algorithm

3. Second-Chance page replacement algorithm is also known as ………………

Reference String

4. The string of memory references is called

___________.

Belady’s Anomaly

5. FIFO page replacement suffers from

________________anomaly.

Page fault increases with increase in number of frames.

6. What is Belady’s anomaly??

TRUE

7. Optimal Page Replacement algorithm has the lowest page fault rate.

TRUE/FALSE

The reference bit and the modify bit

8. Enhanced Second-Chance algorithm uses

_____________and ___________ as an ordered pair.