Chapter 2 processor scheduling part iv
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CHAPTER 2 PROCESSOR SCHEDULING PART IV. by Uğur HALICI. 2.3.3 Shortest-Remaining-Time-First (SRTF). The scheduling algorithms we discussed so far are all non-preemptive algorithms. That is, once a process grabs the processor, it keeps the processor until it terminates or it requests I/O.

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CHAPTER 2 PROCESSOR SCHEDULING PART IV

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Chapter 2 processor scheduling part iv

CHAPTER 2PROCESSOR SCHEDULINGPART IV

by Uğur HALICI


2 3 3 shortest remaining time first srtf

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • The scheduling algorithms we discussed so far are allnon-preemptive algorithms.

  • That is, once a process grabs the processor, it keeps the processor until it terminates or it requests I/O.

  • To deal with this problem (if so), preemptive algorithms are developed.

  • In this type of algorithms, at some time instant, the process being executed is forced to preempt CPU in order to execute a new selected process.

  • The preemption conditions are up to the algorithm design.


2 3 3 shortest remaining time first srtf1

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • SPF algorithm can be modified to be preemptive.

  • Assume while one process is executing on the processor, another process arrives.

  • The new process may have a predicted next processor burst time shorter than what is left of the currently executing process.

  • If the SPF algorithm is preemptive, the currently executing process will preempt the processor and the new process will start executing.

  • The modified SPF algorithm is named as Preemptive SPF or Shortest-Remaining-Time-First (SRTF) algorithm.


2 3 3 shortest remaining time first srtf2

2.3.3 Shortest-Remaining-Time-First (SRTF)

Start : The process has just arrived.

Ready : The process is waiting to grab the processor.

Running : The process has been allocated by the processor.

Waiting : The process is doing I/O work or blocked.

Halted : The process has finished and is about to leave the system

preemption

START

READY

HALTED

RUNNING

I/O completed

I/O requested

WAITING


2 3 3 shortest remaining time first srtf3

2.3.3 Shortest-Remaining-Time-First (SRTF)

SRTF

next_cpu_burst : remaining_time

FCFS


2 3 3 shortest remaining time first srtf4

2.3.3 Shortest-Remaining-Time-First (SRTF)

next_cpu_burst : remaining_time

continue


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2.3.3 Shortest-Remaining-Time-First (SRTF)

continue


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2.3.3 Shortest-Remaining-Time-First (SRTF)


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2.3.3 Shortest-Remaining-Time-First (SRTF)


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2.3.3 Shortest-Remaining-Time-First (SRTF)

preemption

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2.3.3 Shortest-Remaining-Time-First (SRTF)

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2.3.3 Shortest-Remaining-Time-First (SRTF)

preemption

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2.3.3 Shortest-Remaining-Time-First (SRTF)

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2.3.3 Shortest-Remaining-Time-First (SRTF)

preemption

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2.3.3 Shortest-Remaining-Time-First (SRTF)

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2.3.3 Shortest-Remaining-Time-First (SRTF)

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2.3.3 Shortest-Remaining-Time-First (SRTF)

continues

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2 3 3 shortest remaining time first srtf22

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Processor utilization = (35 / 35) * 100 = 100 % 

  • Throughput = 4 / 35 = 0.11

p

p

p


2 3 3 shortest remaining time first srtf23

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Turn around time: 

    tatA = 27 – 0 = 27

    tatB = 35 – 2 = 33

    tatC = 11 – 3 = 8

    tatD = 14 – 7 = 7

    tatAVG = (27 + 33 + 8 + 7) / 4 = 18.75

p

p

p


2 3 3 shortest remaining time first srtf24

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Turn around time: 

    tatA = 27 – 0 = 27

    tatB = 35 – 2 = 33

    tatC = 11 – 3 = 8

    tatD = 14 – 7 = 7

    tatAVG = (27 + 33 + 8 + 7) / 4 = 18.75

p

p

p


2 3 3 shortest remaining time first srtf25

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Turn around time: 

    tatA = 27 – 0 = 27

    tatB = 35 – 2 = 33

    tatC = 11 – 3 = 8

    tatD = 14 – 7 = 7

    tatAVG = (27 + 33 + 8 + 7) / 4 = 18.75

p

p

p


2 3 3 shortest remaining time first srtf26

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Turn around time: 

    tatA = 27 – 0 = 27

    tatB = 35 – 2 = 33

    tatC = 11 – 3 = 8

    tatD = 14 – 7 = 7

    tatAVG = (27 + 33 + 8 + 7) / 4 = 18.75

p

p

p


2 3 3 shortest remaining time first srtf27

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Turn around time: 

    tatA = 27 – 0 = 27

    tatB = 35 – 2 = 33

    tatC = 11 – 3 = 8

    tatD = 14 – 7 = 7

    tatAVG = (27 + 33 + 8 + 7) / 4 = 18.75

p

p

p


2 3 3 shortest remaining time first srtf28

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Waiting time: 

    wtA = (0 – 0) + (8 – 8) + (12 - 9) + (14 – 13) + (23 - 20) = 7

    wtB = (6 – 2) + (16 – 7) + (27-24) = 16

    wtC = (4 – 3) + (9 – 9) = 1

    wtD = (7 – 7) + (11 – 10) + (13 – 13) = 1

    wtAVG = (7 + 16 + 1 + 1) / 4 = 6.25

p

p

p


2 3 3 shortest remaining time first srtf29

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Waiting time: 

    wtA = (0 – 0) + (8 – 8) + (12 - 9) + (14 – 13) + (23 - 20) = 7

    wtB = (6 – 2) + (16 – 7) + (27-24) = 16

    wtC = (4 – 3) + (9 – 9) = 1

    wtD = (7 – 7) + (11 – 10) + (13 – 13) = 1

    wtAVG = (7 + 16 + 1 + 1) / 4 = 6.25

p

p

p


2 3 3 shortest remaining time first srtf30

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Waiting time: 

    wtA = (0 – 0) + (8 – 8) + (12 - 9) + (14 – 13) + (23 - 20) = 7

    wtB = (6 – 2) + (16 – 7) + (27-24) = 16

    wtC = (4 – 3) + (9 – 9) = 1

    wtD = (7 – 7) + (11 – 10) + (13 – 13) = 1

    wtAVG = (7 + 16 + 1 + 1) / 4 = 6.25

p

p

p


2 3 3 shortest remaining time first srtf31

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Waiting time: 

    wtA = (0 – 0) + (8 – 8) + (12 - 9) + (14 – 13) + (23 - 20) = 7

    wtB = (6 – 2) + (16 – 7) + (27-24) = 16

    wtC = (4 – 3) + (9 – 9) = 1

    wtD = (7 – 7) + (11 – 10) + (13 – 13) = 1

    wtAVG = (7 + 16 + 1 + 1) / 4 = 6.25

p

p

p


2 3 3 shortest remaining time first srtf32

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Waiting time: 

    wtA = (0 – 0) + (8 – 8) + (12 - 9) + (14 – 13) + (23 - 20) = 7

    wtB = (6 – 2) + (16 – 7) + (27-24) = 16

    wtC = (4 – 3) + (9 – 9) = 1

    wtD = (7 – 7) + (11 – 10) + (13 – 13) = 1

    wtAVG = (7 + 16 + 1 + 1) / 4 = 6.25

p

p

p


2 3 3 shortest remaining time first srtf33

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Response time: 

    rtA = 0 – 0 = 0

    rtB = 6 – 2 = 4

    rtC = 4 – 3 = 1

    rtD = 7 – 7 = 0

    rtAVG = (0 + 4 + 1 + 0) / 4 = 1.25

p

p

p


2 3 3 shortest remaining time first srtf34

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Response time: 

    rtA = 0 – 0 = 0

    rtB = 6 – 2 = 4

    rtC = 4 – 3 = 1

    rtD = 7 – 7 = 0

    rtAVG = (0 + 4 + 1 + 0) / 4 = 1.25

p

p

p


2 3 3 shortest remaining time first srtf35

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Response time: 

    rtA = 0 – 0 = 0

    rtB = 6 – 2 = 4

    rtC = 4 – 3 = 1

    rtD = 7 – 7 = 0

    rtAVG = (0 + 4 + 1 + 0) / 4 = 1.25

p

p

p


2 3 3 shortest remaining time first srtf36

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Response time: 

    rtA = 0 – 0 = 0

    rtB = 6 – 2 = 4

    rtC = 4 – 3 = 1

    rtD = 7 – 7 = 0

    rtAVG = (0 + 4 + 1 + 0) / 4 = 1.25

p

p

p


2 3 3 shortest remaining time first srtf37

2.3.3 Shortest-Remaining-Time-First (SRTF)

  • Response time: 

    rtA = 0 – 0 = 0

    rtB = 6 – 2 = 4

    rtC = 4 – 3 = 1

    rtD = 7 – 7 = 0

    rtAVG = (0 + 4 + 1 + 0) / 4 = 1.25

p

p

p


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