Week15n16DiskScheduling [Compatibility Mode]

8/3/2019 Week15n16DiskScheduling [Compatibility Mode]

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Mass-Storage Systems

Disk Structure

Disk Scheduling


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Disk Structure

Disk drives are addressed as large 1-dimensional arraysof logical blocks, where the logical block is the smallestunit of transfer.

The 1-dimensional array of logical blocks is mapped intothe sectors of the disk sequentially.

Sector 0 is the first sector of the first track on the outermostcylinder.

Mapping proceeds in order through that track, then the restof the tracks in that cylinder, and then through the rest of thecylinders from outermost to innermost.


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Disk Scheduling

Several algorithms exist to schedule the servicing of diskI/O requests.

Disk scheduling algorithms can be illustrated with arequest queue

98, 183, 37, 122, 14, 124, 65, 67

Suppose the Initial Head Position (IHP) is 53


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FCFS

Illustration shows total head movement of 640 cylinders.


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SSTF

Selects the request with the minimum seek time from thecurrent head position.

SSTF scheduling is a form of SJF scheduling; may causestarvation of some requests.



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SSTF


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SCAN

The disk arm starts at one end (say END1) of the diskand moves toward the other end (say END2) whileservicing requests until it gets to END2 of the disk. Thenform there, the head movement is reversed and servicingcontinues on its way to the END1. So, it keeps moving

from one end to the other end.

Sometimes called the elevator algorithm.



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SCAN


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C-SCAN

Provides a more uniform wait time than SCAN.

The head moves from one end (say END1) of the disk tothe other (say END2) while servicing requests as it goes.When it reaches the END2, however, it immediatelyreturns to the beginning of the disk (END1) withoutservicing any requests on the return trip.

Treats the cylinders as a circular list that wraps aroundfrom the last cylinder to the first one.


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C-SCAN


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LOOK

The disk arm starts from the presentposition and goes as far as the finalrequest in each direction.

It does not go to the end of the disk. It looks for a request before continuing to

move in a given direction.


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LOOK


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C-LOOK

Version of C-SCAN

Arm only goes as far as the last request in each direction,then reverses direction immediately, without servicing anyrequest while going all the way to the other side of thedisk.

Movement of the head is from request to request fromone side to the other side of the disk.


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C-LOOK


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TEST SAMPLE

50 REQUESTS

INITIAL HEAD POSITION AT 53


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Head Move Sequence Queue FCFS SSTF LOOK-UPLOOK-DOWN

C-LOOK SCAN C SCAN

IHP 53 53 53 53 53 53 53 53

1 272 272 50 58 50 58 50 58

2 69 69 58 69 40 69 40 69

3 23 23 69 73 39 73 39 73

4 58 58 73 80 32 80 32 80

5 190 190 80 83 27 83 27 83

6 205 205 83 83 25 83 25 83

7 39 39 83 88 23 88 23 88

8 117 117 88 94 19 94 19 94

9 25 25 94 117 15 117 15 117

10 213 213 117 121 11 121 11 121

11 121 121 121 122 9 122 9 122

12 290 290 122 134 8 134 8 134

13 310 310 134 138 7 138 7 138

14 19 19 138 150 5 150 5 150

15 80 80 150 167 4 167 4 167

16 8 8 167 176 2 176 2 176

17 94 94 176 190 58 190 0 190

22 303 303 230 240 83 240 83 240

23 260 260 240 242 88 242 83 242

24 150 150 242 253 94 253 88 253

25 83 83 253 260 117 260 94 260

26 167 167 260 263 121 263 117 263

27 32 32 263 272 122 272 121 272

28 27 27 272 290 134 290 122 290

29 253 253 290 291 138 291 134 291

30 122 122 291 301 150 301 138 301

31 73 73 301 303 167 303 150 303

32 134 134 303 310 176 310 167 310

33 11 11 310 312 190 312 176 312

34 5 5 312 319 205 319 190 319

35 83 83 319 50 213 2 205 999

36 50 50 40 40 222 4 213 0

37 2 2 39 39 2 30 5 222 2

38 240 240 32 32 240 7 230 4

39 242 242 27 27 242 8 240 5

40 301 301 25 25 253 9 242 7

41 4 4 23 23 260 11 253 8

42 263 263 19 19 263 15 260 9

43 138 138 15 15 272 19 263 11

44 7 7 11 11 290 23 272 15

45 291 291 9 9 291 25 290 19

46 319 319 8 8 301 27 291 23

47 9 9 7 7 303 32 301 25

48 15 15 5 5 310 39 303 27

49 230 230 4 4 312 40 310 32

50 88 88 2 2 319 50 312 39

40


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Queue FCFS

53 53

272 272

69 69

23 23

58 58

190 190

205 205

39 39

117 117

25 25

213 213

121 121

290 290

310 310

19 19

80 80

8 8

94 94

303 303

260 260

150 150

83 83

167 167

32 32

27 27

253 253

122 122

73 73

134 134

11 11

5 5

83 83

50 50

2 2

240 240

242 242

301 301

4 4

263 263

138 138

7 7

291 291

5 32 7 2

6 92 3

5 8

1 9 02 0 5

3 91 1 7

2 52 13

1 2 12 9 0

3 1 01 9

808

9 42 2 2

4 01 7 6

3 1 23 0 3

2 6 01 5 0

8 31 67

3 22 7

2 5 31 2 2

7 313 4

1 15

8 35 0

22 4 02 4 2

3 0 14

26 31 3 8

72 9 1

3 1 9

91 5

2 3 08 8

025

50

75

100

125

150

175

200

225

250

275

300

325

350

FCFS5851 HEAD MOVES


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Queue SSTF

53 53

272 50

69 58

23 69

58 73

190 80

205 83

39 83

117 88

25 94

213 117

121 121

290 122

310 134

19 138

80 150

8 167

94 176

303 230

260 240

150 242

83 253

167 260

32 263

27 272

253 290

122 291

73 301

134 303

11 310

5 312

83 319

50 40

2 39

240 32

242 27

301 25

4 23

263 19

138 15

7 11

291 9

5 35 05 8

6 97 3

8 08 38 38 89 4

1 1 71 2112 2

1 3 41 38

1 5 01 6 71 7 6

1 9 02 0 52 1 32 2 22 3 02 4 024 2

2 5 32 6 02 6 32 72

2 9 02 9 13 0 13 0 33 1 0

31 23 1 9

4 03 9

3 22 72 52 31 91 511987

542

025

50

75

100

125

150

175

200

225

250

275

300

325

350

SSTF589 HEAD MOVES


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Queue LOOK-UP

53 53

272 58

69 69

23 73

58 80

190 83

205 83

39 88

117 94

25 117

213 121

121 122

290 134

310 138

19 150

80 167

8 176

94 190

303 240

260 242

150 253

83 260

167 263

32 272

27 290

253 291

122 301

73 303

134 310

11 312

5 319

83 50

50 40

2 39

240 32

242 27

301 25

4 23

263 19

138 15

7 11

291 9

535 86 97 38 0

8 38 3889 4

1 1712 112 2

13 413 8

1 501 6 71 76

1 902 052 1 32222 3 024 02 4 2

2 5326 02 6 32 72

2 9 02913 0 13 0 33 1 03 1 2

3 195 0

403 9

3 22 72 5231 91 51 1987

542

025

50

75

100

125

150

175

200

225

250

275

300

325

350

LOOK-UP583 HEAD MOVES


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QueueLOOK-DOWN

53 53

272 50

69 40

23 39

58 32

190 27

205 25

39 23

117 19

25 15

213 11

121 9

290 8

310 7

19 5

80 4

8 2

94 58

303 83

260 88

150 94

83 117

167 121

32 122

27 134

253 138

122 150

73 167

134 176

11 190

5 205

83 213

50 222

2 230

240 240

242 242

301 253

4 260

263 263

138 272

7 290

291 291

5 35 0

4 039

3 2

2 72 52 31 91 51 1987542

5 86 97 38 08 38 38 894

1 1 71 2 11 2 2

1 3 41 3 8

1 5 01 6 71 7 6

1 9 0

2 0 52 1 32 2 22 302 4 02 42

2 5 32 6 02 632 7 2

2 9 02 9 13 0 13 0 3

3 1 03 1 23 1 9

025

50

75

100

125

150

175

200

225

250

275

300

325

350

LOOK-DOWN368 HEAD MOVES


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Queue C-LOOK

53 53

272 58

69 69

23 73

58 80

190 83

205 83

39 88

117 94

25 117

213 121

121 122

290 134

310 138

19 150

80 167

8 176

94 190

303 240

260 242

150 253

83 260

167 263

32 272

27 290

253 291

122 301

73 303

134 310

11 312

5 319

83 2

50 4

2 5

240 7

242 8

301 9

4 11

263 15

138 19

7 23

291 25

5 35 8

6 9738 0

83838 89 4

1 171 2 112 2

13 41 38

1501 6 71 76

19 020 52 1 322 223 02 402 42

25 32 6 02 6 327 2

29 02 9 13 013 0331 031 2

3 19245789111 5192 325273 2

394 05 0

025

50

75

100

125

150

175

200

225

250

275

300

325

350

C - LOOK631 HEAD MOVES


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Queue SCAN

53 53

272 50

69 40

23 39

58 32

190 27

205 25

39 23

117 19

25 15

213 11

121 9

290 8

310 7

19 5

80 4

8 2

94 0

303 83

260 83

150 88

83 94

167 117

32 121

27 122

253 134

122 138

73 150

134 167

11 176

5 190

83 205

50 213

2 222

240 230

242 240

301 242

4 253

263 260

138 263

7 272

291 290

5350

403 9

32

2 7252 31 9151 19875420

5 86973808383889 4

11 712 112 2

13413 8

1501 671 7 6

19 020 521322 223 024 024 2

25 32 6 02632 72

2 902 9 13 01

3 0331 031 2

025

50

75

100

125

150

175

200

225

250

275

300

325

350

SCAN372 HEAD MOVES


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Queue C SCAN

53 53

272 58

69 69

23 73

58 80

190 83

205 83

39 88

117 94

25 117

213 121

121 122

290 134

310 138

19 150

80 167

8 176

94 190

303 240

260 242

150 253

83 260

167 263

32 272

27 290

253 291

122 301

73 303

134 310

11 312

5 319

999

0

83 2

50 4

2 5

240 7

242 8

301 9

4 11

263 15

138 19

7 23

291 25

5 35 8

6 97 38 08 38 38 89 4

1 1 71 2 11 2 2

1 3 41 3 81 5 0

1 6 71 7 6

1 9 02 0 52 1 3

2 2 22 3 02 4 02 4 2

2 5 32 6 02 6 32 7 2

2 9 02 9 13 0 13 0 33 1 03 1 23 1 9

9 9 90245789

1 11 51 92 32 52 73 2

3 94 05 0

0

100

200

300

400

500

600

700

800

900

C SCAN1995 HEAD MOVES


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HEAD MOVEMENTS

5851

589 583 368 631 372

1995

0

1000

2000

3000

4000

5000

6000

7000

FCFS SSTF LOOK-UP LOOK-

DOWN

C-LOOK SCAN C SCAN


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DEADLINE BASED ALGORITHMS


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Earliest Dead line First (EDF)

In this scheme, the blocks with the nearest deadlines are read first. It has poorthroughput and excessive seek time. Only the dead lines are taken into account, but not

the track numbers. It is very similar to FCFS

Disadvantage: Inefficient. Doesnt reflect the geographical position of tracks


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SCAN EDF

This algorithm combines the advantages of SCAN and EDF. Requests with the

earliest deadline is read first. Amongst the requests with same deadline, requestsare served in accordance with track locations.


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Accurate SCAN EDFThe algorithm is a variation of Scan EDF, in which along with the deadlines the time constraintis also considered. This algorithm rearranges the request queue according to the time stamp(i.e. the one with the lowest time stamp is entertained first) .The request with same time

stamp will be entertained by the deadline seniority criteria. The scan is carried out primarily onthe basis of time stamp and then on the basis of deadline.


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TEST SAMPLE No 1

10 REQUESTS



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Request Queue Algorithms- Head Movement Sequence

RequestNumber

TrackNo

Deadline( 0-9 )

Timestamp

EDF SCAN EDF Accurate SCAN EDF

IHP 0 0 0 0 0 0 0 0 0 0

1 20 0 0 20 0 20 0 20 00

2 22 1 3 22 1 12 1 12 10

3 40 2 2 12 1 22 1 50 30

4 12 1 0 45 1 45 1 45 11

5 45 1 1 40 2 42 2 42 21

6 42 2 1 42 2 40 2 16 22

7 50 3 0 16 2 16 2 40 22

8 16 2 2 50 3 24 3 22 13

9 30 3 3 30 3 30 3 24 33

10 24 3 3 24 3 50 3 30 33


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0

20

22

12

45

40

42

16

50

30

24

0 20 40 60

RequestNumber

TrackNo

Deadline( 0-9 )

IHP 0 0

1 20 0

2 22 1

3 40 2

4 12 1

5 45 1

6 42 2

7 50 3

8 16 2

9 30 3

10 24 3

EDF

0 0

20 0

22 1

12 1

45 1

40 2

42 2

16 2

50 3

30 3

24 3

EDF

158 HAED MOVES


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RequestNumber

TrackNo

Deadline( 0-9 )

IHP 0 0

1 20 0

2 22 1

3 40 2

4 12 1

5 45 1

6 42 2

7 50 3

8 16 2

9 30 3

10 24 3

SCANEDF

0 0

20 0

12 1

22 1

45 1

42 2

40 2

16 2

24 3

30 3

50 3

0

2 0

1 2

2 2

4 5

4 2

4 0

1 6

2 4

3 0

5 0

0 2 0 4 0 6 0

SCAN EDF

124 HAED MOVES


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RequestNumber

TrackNo

Deadline( 0-9 )

Timestamp

IHP 0 0 0

1 20 0 0

2 22 1 3

3 40 2 2

4 12 1 0

5 45 1 1

6 42 2 1

7 50 3 0

8 16 2 2

9 30 3 3

10 24 3 3

AccurateSCAN EDF

0 0 0

20 00

12 10

50 30

45 11

42 21

16 22

40 22

22 13

24 33

30 3 3

0

20

12

50

45

42

16

40

22

24

30

0 20 40 60

ACCURATESCAN EDF

150 HAEDMOVES


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HEAD MOVEMENTS

158124

150

0

20

40

60

80

100

120

140

160

180

EDF SCAN EDF ASCAN EDF


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TEST SAMPLE No 2

20 REQUESTS



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Request Queue Algorithms- Head Movement Sequence

Request

NumberTrack No

Deadline

( 0-9 )

TimeStamp( 0-9 )


IHP 0 0 0 0 0 0 0 0 0 0

1 270 1 3 235 0 0 0 135 1 0

2 235 0 6 40 0 40 0 80 2 0

3 40 0 1 125 0 60 0 10 4 0

4 10 4 0 60 0 125 0 0 0 1

5 80 2 0 0 0 235 0 40 0 1

6 135 1 0 270 1 135 1 17 1 1

7 284 4 2 135 1 17 1 195 2 1

8 350 2 4 17 1 270 1 54 5 1

9 125 0 3 80 2 195 2 60 0 2

10 7 4 2 350 2 170 2 7 4 2

11 60 0 2 170 2 80 2 284 4 2

12 170 2 4 195 2 350 2 125 0 313 95 3 6 95 3 301 3 270 1 3

14 0 0 1 301 3 95 3 12 3 3

15 54 5 1 12 3 12 3 301 3 3

16 301 3 3 10 4 7 4 170 2 4

17 195 2 1 284 4 10 4 350 2 4

18 24 4 4 7 4 24 4 24 4 4

19 12 3 3 24 4 284 4 235 0 6


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RequestNumber

Track NoDeadline ( 0-

9 )

IHP 0 0

1 270 1

2 235 0

3 40 0

4 10 4

5 80 2

6 135 1

7 284 4

8 350 2

9 125 0

10 7 4

11 60 0

12 170 2

13 95 3

14 0 0

15 54 5

16 301 3

17 195 2

18 24 4

19 12 3

20 17 1

EDF

0 0

235 0

40 0

125 0

60 0

0 0

270 1

135 1

17 1

80 2

350 2

170 2

195 2

95 3

301 3

12 3

10 4

284 4

7 4

24 4

54 5

0

235

40

125

60

0

270

135

17

80

350

170

195

95

301

12

10

284

7

24 54

0 100 200 300 400

EDF

2896 HAEDMOVES


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RequestNumber

Track No

Deadline( 0-9 )

IHP 0 0

1 270 1

2 235 0

3 40 0

4 10 4

5 80 2

6 135 1

7 284 4

8 350 2

9 125 0

10 7 4

11 60 0

12 170 2

13 95 3

14 0 0

15 54 5

16 301 3

17 195 2

18 24 4

19 12 3

20 17 1

SCAN EDF

0 0

0 0

40 0

60 0

125 0

235 0

135 1

17 1

270 1

195 2

170 2

80 2

350 2

301 3

95 312 3

7 4

10 4

24 4

284 4

54 5

0

0

40

60

125

235

135

17

270

195

170

80

350

301

95

12

7

10

24

28454

0 100 200 300 400

SCAN EDF

2016 HAEDMOVES


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RequestNumber

TrackNo

Deadline( 0-9 )

TimeStamp( 0-9 )

IHP 0 0 0

1 270 1 3

2 235 0 6

3 40 0 1

4 10 4 0

5 80 2 0

6 135 1 0

7 284 4 2

8 350 2 4

9 125 0 310 7 4 2

11 60 0 2

12 170 2 4

13 95 3 6

14 0 0 1

15 54 5 1

16 301 3 3

17 195 2 1

18 24 4 4

19 12 3 3

20 17 1 1

AccurateSCAN EDF

0 0 0

135 1 0

80 2 0

10 4 0

0 0 1

40 0 1

17 1 1

195 2 1

54 5 1

60 0 2

7 4 2

284 4 2

125 0 3

270 1 3

12 3 3

301 3 3

170 2 4350 2 4

24 4 4

235 0 6

95 3 6

0

135

80

10

0

40

17

195

54

60

7

284

125

270

12

301

170

350

24

23595

0 100 200 300 400

ACCURATESCAN EDF

2827 HAEDMOVES


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HEAD MOVEMENTS

2896

2016

2827

0

500

1000

1500

2000

2500

3000

3500



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TEST SAMPLE No 3

40 REQUESTS



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TrNo

DL( 0-9 )

TS( 0-9 )

EDFSCANEDF

ACCURATESCAN EDF

124 2 1 270 1 270 1 90 0 3

45 1 3 10 2 166 2 45 1 3

5 1 2 16 2 140 2 96 2 3

50 31

166 2 124 2166

23

85 3 7 23 2 90 2 95 0 4

50 1 7 40 2 55 2 24 3 4

55 2 1 140 2 42 2 99 3 4

60 1 1 42 2 40 2 11 4 4

6 1 8 124 2 23 2 40 0 5

70 4 9 55 2 16 2 23 2 5

8 10

96 2 10 2100

35

80 01

1 3 1 3270

16

86 1 2 24 3 24 3 10 2 6

96 2 3 30 3 30 3 40 2 6

90 03

50 3 50 3140

26

91 1 0 85 3 85 3 1 3 6

95 0 4 99 3 99 3 50 1 7

99 3 4 100 3 100 3 85 3 7

100 3 5 11 4 70 4 6 1 8

270 1 6 70 4 11 4 70 4 9

TrNo

DL( 0-9 )

TS( 0-9 )

EDFSCANEDF

ACCURATESCAN EDF

124 2 1 270 1 270 1 90 0 3

45 1 3 10 2 166 2 45 1 3

5 1 2 16 2 140 2 96 2 3

50 3 1 166 2 124 2 166 2 3

85 3 7 23 2 90 2 95 0 4

50 1 7 40 2 55 2 24 3 4

55 2 1 140 2 42 2 99 3 4

60 1 1 42 2 40 2 11 4 4

6 1 8 124 2 23 2 40 0 5

70 4 9 55 2 16 2 23 2 5

8 1 0 96 2 10 2 100 3 5

80 0 1 1 3 1 3 270 1 6

86 1 2 24 3 24 3 10 2 6

96 2 3 30 3 30 3 40 2 6

90 0 3 50 3 50 3 140 2 6

91 1 0 85 3 85 3 1 3 6

95 0 4 99 3 99 3 50 1 7

99 3 4 100 3 100 3 85 3 7

100 3 5 11 4 70 4 6 1 8

270 1 6 70 4 11 4 70 4 9


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TrNo

DL( 0-9)

124 2

45 1

5 150 3

85 3

50 1

55 2

60 1

6 1

70 4

8 1

80 0

86 1

96 2

90 0

91 1

95 0

99 3

100 3

270 1

EDF

270 1

10 2

16 2166 2

23 2

40 2

140 2

42 2

124 2

55 2

96 2

1 3

24 3

30 3

50 3

85 399 3

100 3

11 4

70 4

TrNo

DL( 0-9 )

124 2

45 1

5 1

50 3

85 3

50 1

55 2

60 1

6 1

70 4

8 1

80 0

86 1

96 2

90 0

91 195 0

99 3

100 3

270 1

EDF

270 1

10 2

16 2166 2

23 2

40 2

140 2

42 2

124 2

55 2

96 2

1 3

24 3

30 3

50 3

85 399 3

100 3

11 4

70 4

0

2 02 5

2 73 3

4 08 0

9 09 5

1 2

4 22 2

6 64 5

55 0

6 06

88 6

9 1

2 7 01 0

1 61 6 6

2 34 0

1 4 04 2

1 2 4

5 59 6

12 4

3 05 0

8 59 9

1 0 0

1 1

0 1 0 0 2 0 0 3 0 0

EDF

2014 HEADMOVES


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TrNo

DL( 0-9 )

124 2

45 1

5 1

50 3

85 3

50 1

55 2

60 1

6 1

70 4

8 1

80 0

86 1

96 2

90 0

91 1

95 0

99 3

100 3

270 1

SCANEDF

270 1

166 2

140 2

124 2

90 2

55 2

42 2

40 2

23 2

16 2

10 2

1 3

24 3

30 3

50 3

85 3

99 3

100 3

70 4

11 4

TrNo

DL( 0-9 )

124 2

45 1

5 1

50 3

85 3

50 1

55 2

60 1

6 1

70 4

8 1

80 0

86 1

96 2

90 0

91 1

95 0

99 3

100 3

270 1

SCANEDF

270 1

166 2

140 2

124 2

90 2

55 2

42 2

40 2

23 2

16 2

10 2

1 3

24 3

30 3

50 3

85 3

99 3

100 3

70 4

11 4

0

2 02 5

2 73 3

4 0

8 09 0

9 59 1

8 66 6

6 05 0

4 5

4 22 2

1 28

6

52 7 0

1 6 61 4 0

1 2 49 0

5 5

4 24 0

2 31 6

1 01

2 4

3 05 0

8 59 9

1 0 07 0

1 1

0 1 0 0 2 0 0 3 0 0

SCAN EDF

907 HEADMOVES


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TrNo

DL( 0-9 )

TS(0-9)

124 2 1

45 1 3

5 1 2

50 3 1

85 3 7

50 1 7

55 2 1

60 1 1

6 1 8

70 4 9

8 1 0

80 0 1

86 1 2

96 2 3

90 0 3

91 1 0

95 0 4

99 3 4

100 3 5

270 1 6

ACCURATESCAN EDF

90 0 3

45 1 3

96 2 3

166 2 3

95 0 4

24 3 4

99 3 4

11 4 4

40 0 5

23 2 5

100 3 5

270 1 6

10 2 6

40 2 6

140 2 6

1 3 6

50 1 7

85 3 7

6 1 8

70 4 9

TrNo

DL(0-9 )

TS(0-9)

124 2 1

45 1 3

5 1 2

50 3 1

85 3 7

50 1 7

55 2 1

60 1 1

6 1 8

70 4 9

8 1 0

80 0 1

86 1 2

96 2 3

90 0 3

91 1 0

95 0 4

99 3 4

100 3 5

270 1 6

ACCURATESCAN EDF

90 0 3

45 1 3

96 2 3

166 2 3

95 0 4

24 3 4

99 3 4

11 4 4

40 0 5

23 2 5

100 3 5

270 1 6

10 2 6

40 2 6

140 2 6

1 3 6

50 1 7

85 3 7

6 1 8

70 4 9

0

33

8

12

42

66

9120

27

80

22

60

55

124

30

50

25

586

16

42

90

45

96

166

95

24

99

11

4023

100

270

10

40

140

1

50

85

670

0 100 200 300

ACCURATESCAN EDF

2346 HEADMOVES


47/47

HEAD MOVEMENTS

2014

907

2346

0

500

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Documents

Week15n16DiskScheduling [Compatibility Mode]