Uniprocessor Scheduling II

Chapter 9

Shortest Process Next

Nonpreemptive policyProcess with shortest expected processing

time is selected nextShort process jumps ahead of longer

processes

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5P3: 4 time unitsP4: 5 time unitsP5 2 time units

Shortest Process Next

Predictability of longer processes is reduced

If estimated time for process not correct, the operating system may abort it

Possibility of starvation for longer processes

Shortest Remaining Time

SRT is a preemptive version of shortest process next (SPN) policy

Must estimate processing time

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P2 Remaining 5 time units

P3 Remaining 4 time units

P2 Remaining 5 time unitsP4 Remaining 5 time unitsP5 Remaining 2 time units

Highest Response Ratio Next (HRRN)

Choose next process with the greatest value of RR

No PreemptiveRR =(w+s)/s =time spent waiting+expected service timeexpected service timeP5 RR=( (13-8) + 2)/2 = 3.5 // P5 has been chosen at 13P4 RR=( (13-6) + 5)/5 = 2.4

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P5 RR =3.5

P4 RR = 2.4P3 RR=2.25P4 RR = 1.6P5 RR = 1.5

Feedback

Scheduling is done on a preemptive basis, and a dynamic priority mechanisms is used.

When s process first enters the system, it is placed in RQ0. After its first execution, when it returns to the Ready state, it is placed in RQ1.

Feedback (q = 1)

Penalize jobs that have been running longer. Focus on the time spent in execution so far.

Don’t know remaining time process needs to execute

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RQ4

RQ3RQ2RQ1RQ0

RQ5

When a process first enters the system, it is placed in RQ0. After its first execution, when it returns to the Ready state, it is placed in RQ1 ……

In general, a process scheduled from RQi is allowed to execute 2 i time units (1, 2, 4, 8, …...) before preemption.

Feedback q=2i

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RQ2, 4 time unitsRQ1,2 time units

Question - Case Study (1)

Consider the following set of processes: Process Name Arrival Time Processing Time

1 0 32 1 53 3 24 9 55 12 5

Perform the analysis of a comparison of scheduling policies(FCFS, RR(q=1), RR(q=4), SPN, SRT, HRRN, Feedback(q=1), and Feedback(q=2 i) )

Solution of Question (2)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

FCFS 1 1 1 2 2 2 2 2 3 3 4 4 4 4 4 5 5 5 5 5

RR 1 2 1 2 3 1 2 3 2 4 2 4 5 4 5 4 5 4 5 5q=1RR 1 1 1 2 2 2 2 3 3 2 4 4 4 4 5 5 5 5 4 5q=4SPN 1 1 1 3 3 2 2 2 2 2 4 4 4 4 4 5 5 5 5 5

SRT 1 1 1 3 3 2 2 2 2 2 4 4 4 4 4 5 5 5 5 5

HRRN 1 1 1 2 2 2 2 2 3 3 4 4 4 4 4 5 5 5 5 5

FB 1 2 1 3 2 3 1 2 2 4 2 4 5 4 5 4 5 4 5 5q=1FB 1 2 1 1 3 2 2 3 2 2 4 4 5 4 4 5 5 4 5 5q=2i

* Each square represents one time unit. The number refers to the running process

Solution of Question - FCFS (3)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

FCFS 1 1 1 2 2 2 2 2 3 3 4 4 4 4 4 5 5 5 5 5

* Each square represents one time unit. The number refers to the running process

First Come First Served (FCFS)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 3 8 10 15 20Turnaround Time Tq 3.0 7.0 7.0 6.0 8.0 6.20 Tq = Tf - Ta Normalized Tq/Ts 1.0 1.4 3.5 1.2 1.6 1.74Turnaround Time

Solution of Question Round Robin q=1

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 P1P2P3P4P5 1 2 1 2 3 1 2 3 2 4 2 4 5 4 5 4 5 4 5 5

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Ready Time

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Solution of Question - RR (q=1) (4)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

RR 1 2 1 2 3 1 2 3 2 4 2 4 5 4 5 4 5 4 5 5q=1

* Each square represents one time unit. The number refers to the running process

Round Robin (RR q=1)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 6 11 8 18 20Turnaround Time Tq 6 10 5 9 8 7.6 Tq = Tf - Ta Normalized Tq/Ts 2.0 2.0 2.5 1.8 1.6 1.98Turnaround Time

Solution of Question Round Robin q=4

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 P1P2P3P4P5 1 1 1 2 2 2 2 3 3 2 4 4 4 4 5 5 5 5 4 5

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1214

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Ready Time

Solution of Question - RR (q=4) (5)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

RR 1 1 1 2 2 2 2 3 3 2 4 4 4 4 5 5 5 5 4 5q=4

* Each square represents one time unit. The number refers to the running process

Round Robin (RR q=4)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 3 10 9 19 20Turnaround Time Tq 3 9 6 10 8 7.20 Tq = Tf - Ta Normalized Tq/Ts 1.0 1.8 3.0 2.0 1.6 1.88Turnaround Time

Solution of Question - SPN (6)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

SPN 1 1 1 3 3 2 2 2 2 2 4 4 4 4 4 5 5 5 5 5

* Each square represents one time unit. The number refers to the running process

Shortest Process Next (SPN)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 3 10 5 15 20Turnaround Time Tq 3 9 2 6 8 5.60 Tq = Tf - Ta Normalized Tq/Ts 1.0 1.8 1.0 1.2 1.6 1.32Turnaround Time

Solution of Question - SRT (7)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

SRT 1 1 1 3 3 2 2 2 2 2 4 4 4 4 4 5 5 5 5 5

* Each square represents one time unit. The number refers to the running process

Shortest Remaining Time (SRT)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 3 10 5 15 20Turnaround Time Tq 3 9 2 6 8 5.60 Tq = Tf - Ta Normalized Tq/Ts 1.0 1.8 1.0 1.2 1.6 1.32Turnaround Time

Solution of Question - HRRN (8)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

HRRN 1 1 1 2 2 2 2 2 3 3 4 4 4 4 4 5 5 5 5 5

* Each square represents one time unit. The number refers to the running process

Highest Response Ratio Next (HRRN)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 3 8 10 15 20Turnaround Time Tq 3 7 7 6 8 6.20 Tq = Tf - Ta Normalized Tq/Ts 1.0 1.4 3.5 1.2 1.6 1.74Turnaround Time

* At time 3: P2 RR (2+5)/5 = 1.4; P3 RR= (0+2)/2 = 1. So Choose P2

Solution of Question Feedback q=1

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 P1P2P3P4P5 1 2 1 3 2 3 1 2 2 4 4 4 5 5 5 2 4 5 4 5

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3RQ0

RQ1

RQ3

RQ4

RQ2

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Solution of Question - FB q=1 (9)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

FB 1 2 1 3 2 3 1 2 2 4 4 4 5 5 5 2 4 5 4 5q=1

* Each square represents one time unit. The number refers to the running process

Feedback (FB q=1)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 7 16 6 19 20Turnaround Time Tq 7 15 3 10 8 8.6 Tq = Tf - Ta Normalized Tq/Ts 2.3 3.0 1.5 2.0 1.6 2.08Turnaround Time

Solution of Question Feedback q= 2i

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 P1P2P3P4P5 1 2 1 1 3 2 2 3 2 2 4 4 5 4 4 5 5 4 5 5

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2RQ0

RQ1RQ2

Solution of Question - FB q= 2i (10)

0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0

FB 1 2 1 1 3 2 2 3 2 2 4 4 5 4 4 5 5 4 5 5 q=2i

* Each square represents one time unit. The number refers to the running process

Feedback (FB q = 2i)Process Pi 1 2 3 4 5 AverageArrival Time Ta 0 1 3 9 12Service Time Ts 3 5 2 5 5Finish Time Tf 4 10 8 18 20Turnaround Time Tq 4 9 5 9 8 7.00 Tq = Tf - Ta Normalized Tq/Ts 1.3 1.8 2.5 1.8 1.6 1.81Turnaround Time

Question 1 - Exercise/Home Work (1)

Consider the following set of processes: Process Name Arrival Time Processing Time

1 0 12 1 93 2 14 3 9

Perform the analysis of a comparison of scheduling policies(FCFS, RR(q=1), RR(q=4), SPN, SRT, HRRN, Feedback(q=1), and Feedback(q=2 i) )