7. Scheduling.ppt

8/14/2019 7. Scheduling.ppt

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OSes: 7. CPU Scheduling 1

Operating Systems

Objectivesexamine and compare some of the

common CPU scheduling algorithms

Certificate Program in Software DevelopmentCSE-TC and CSIM, AITSeptember -- November, 2003

7. CPU Scheduling(Ch. 5, S&G)

ch 6 in the 6th ed.


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Contents

1. CPU Scheduling

what it is, burst cycles, criteria

2. Scheduling Algorithms

FCFS, SJF, Priority, RR, multilevel

3. Algorithm Evaluationdeterministic, queueing, simulation


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1. CPU Scheduling

One of the main aims of an OS is to

maximise the utilization of the CPU by

switching it between processeswhen should the switching occur?

which processes should be executed next?


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1.1. What is a Process? Fig. 4.1, p.90

terminated

ready

waiting

running

new

exit

I/O eventor wait

dispatch

interrupt

event

completed


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1.2. Scheduling Opportunities

When the running process yields the CPU:

the process enters a wait state

the process terminates

When an interrupt occurs:

the current process is still ready

process switches from wait state to ready

Preemptive vs. non-preemptive scheduling


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1.3. CPU and I/O Burst Cycle

:

load

store

add

storeread from file

wait for I/O

store

increment index

write to file

wait for I/O

load

store

:

CPU burst

I/O burst

CPU burst

I/O burst

CPU burst

Fig. 5.1, p.124


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CPU Burst Duration (ms)

Fig. 5.2, p.125;VUW CS 305

02

48

1624

3240

frequency

020

40

60

80

100

120

140

160



1.4. Scheduling Criteria

CPU utilization

keep the CPU as busy as possible

Throughput

no. of processes completed per time unit

Turnaround time

how long it takes to complete a process

continued



Waiting time

the total time a process is in the ready queue

the measure used in chapter 5

Response time

time a process takes to start responding



2. Scheduling Algorithms

2.1. First-Come, First-Served (FCFS)

2.2. Shortest Job First (SJF)

2.3. Priority Scheduling

2.4. Round Robin (RR)

2.5. Multilevel Queue Scheduling

2.6. Multilevel Feedback Queue Scheduling



2.1. FCFS Scheduling

When the CPU is available, assign it to the

process at the start of the ready queue.

Simple to implement

use a FIFO queue

Non-preemptive


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Example (v.2)

Process Burst Time

P2 3

P3 3P1 24

Gantt Chart:

Average waiting time:

(6 + 0 + 3)/3 = 3 ms

P1P2 P3

0 3 6 30


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FCFS Features

May not give the best average waiting time.

Average times can vary a lot depending onthe order of the processes.

Convoy effectsmall processes can get stuck behind a big

process


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2.2. Shortest Job First Scheduling (SJF)

When the CPU is available, assign it to the

process with the smallest next CPU burst

duration

better name is shortest next CPU burst

Can be preemptive or non-preemptive


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Non-preemptive Example

Process Burst Time

P1 6

P2 8

P3 7P4 3

Gantt Chart:


(3 + 16 + 9 + 0)/4 = 7 ms

FCFS gives 10.25 ms

p.131

P2P4 P1

0 3 9 24

P3

16


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SJF Features

Provably optimal

gives the minimum average waiting time

Problem: it is usually impossible to know

the next CPU burst duration for a process

solution: guess (predict)


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Predicting the next CPU burst time

Use the formula:

Tn+1= w tn+ (1- w) Tn

Meaning:Tn+1= prediction for the next (n+1

th) CPU burst

duration

tn= known duration of current (nth) CPU burst

Tn= previous prediction (based on the sequence

of old titimes)

w = a weight (0


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Preemptive SJF

When a new process arrives, ifit has a

shorter next CPU burst duration than what

is left of the currently executing processthenpreempt the current process.


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Example

Process Arrival Time Burst Time

P1 0 8

P2 1 4

P3 2 9P4 3 5

Gantt Chart:

p.133

P1P1 P2

0 1 5 26

P4

10

P3

17

continued


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( (10-1) + (1-1) + (17-2) + (5-3) )/4

= 6.5 ms

Non-preemptive SJF gives 7.75 ms

start time arrival time


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2.3. Priority Scheduling

Associate a priority with each process and

the CPU is allocated to the process with the

highest priority.

FCFS, SJF are special cases.

Low numbers = high priority.


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Example

Process Burst Time Priority

P1 10 3

P2 1 1

P3 2 3P4 1 4

P5 5 2

Gantt Chart:

Average waiting time: 8.2 ms

p.134

P3P2 P5

0 1 6 19

P1

16

P4

18


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Features

Internal/external priorities.

Preemptive or non-preemptive.

How to avoid starvation?

aging


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2.4. Round Robin Scheduling (RR)

A small unit of time (a time quantum, a time

slice) is defined

typically 10 - 100 ms

The ready queue is treated as a circular queue.

The CPU scheduler goes around the queue

giving each process one time quantum

preemptive


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Example

Time quantum = 4 ms. At time 0.

Process Burst Time

P1 24

P2 3

P3 3

Gantt Chart:

Average waiting time: 17/3 = 5.67 ms

p.135

P1P1 P20 4 7 30

P1

22

P1

2610 14 18

P3 P1 P1


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RR Features

Average waiting time can be quite long.

Context switching is an important overhead

when the time quantum is small.

continued


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Average turnaround time of a set ofprocesses does not necessarily improve as

the time quantum size increase:

Fig. 5.5, p.137

9.5

10

10.5

11

11.5

12

12.5

1 2 3 4 5 6 7

Process Time

P1 6

P2 3

P3 1P4 7

Time Quantum

Average

Turnar

oundTime


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2.5. Multilevel Queue SchedulingFig. 5.6, p.138

system processes

interactive processes

interactive editing processes

batch processes

student processes

highest priority

lowest prioritycontinued


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Scheduling between queues:

fixed priority preemptive scheduling

varying time slices between the queues


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2.6. Multilevel Feedback Queue Scheduling

Allow a process to

move between queues

priority sinks whenquantum exceeded

greater discrimination

against longer jobs

better response for

shorter jobs

Q0

Q1> Q0

Q2> Q1

FCFS

Fig. 5.7, p.140;VUW CS 305


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3. Algorithm Evaluation

3.1. Deterministic Modelling

3.2. Queueing Models

3.3. Simulation


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Common Issues

Model/simulation is based on some pattern

of use of a real machine

e.g. students use of bazooka: lots ofinteractive, small jobs, and a few large ones

How to represent the changesin usage:changes in problems, programs, users


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3.1. Deterministic Modelling

Take a given workload and calculate the

performance of each scheduling algorithm:FCFS, SJF, and RR (quantum = 10 ms)


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Example

At time 0.

Process Burst Time

P1 10

P2 29

P3 3

P4 7

P5 12

p.145

continued


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Gantt Charts and Times

1. FCFS:


(0 + 10 + 39 + 42 + 49)/5= 28 ms

P1 P2

0 10 39 61

P5

42 49

P3 P4

continued


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2. Non-preemptive SJF:


(10 + 32 + 0 + 3 + 20)/5

= 13 ms

continued

P3 P4

0 3 10 61

P2

20 32

P1 P5


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3. RR:


(0 + 32 + 20 + 23 + 40)/5

= 23 ms

P1 P2

0 10 20 23 30

P3 P4

50 52

P2 P5

61

P2P5

40


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Results

For this mix of processes and CPU burst

durations:

SJF 13 ms

RR 23 ms

FCFS 28 ms


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3.2. Queueing Models

Usually the process mix varies greatly in an

OS, but it may still be possible to determine

statistical distributions for the CPU and I/Obursts.

The mathematics is difficult, and onlyapplies to limited cases.


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3.3. Simulations

Often driven by random number generators

to model CPU burst durations, process

arrival, departure, etc.

Trace tapes

records of actual events in a real system, whichcan be used to drive simulations

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7. Scheduling.ppt