Operating System Concepts and

Techniques Lecture 7

Scheduling-3

M. Naghibzadeh

ReferenceM. Naghibzadeh, Operating System Concepts and Techniques, First ed., iUniverse Inc., 2011.

To order: www.iUniverse.com, www.barnesandnoble.com, or www.amazon.com

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Computation of Turnaround Time (TT)

Turnaround Time: is the time span starting from when a request is submitted and ending

whenever the requested execution is completedWe take Average TT to be the only performance

measure of schedulersThree schedulers FCFS, SPN, and RR will be

consideredA system with the following three processes is

examined

Process Arrival time CPU burst

P1 10 4

P2 11 3

p3 12 2

FCFS schedulerThe following table shows how

FCFS behaves

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Process Arrival time CPU burst Completion time

Turnaround time

P1 10 4 14 4

P2 11 3 17 6

P3 12 2 19 7

Single-programming and FIFO scheduling

Average turnaround time = 17/3 = 5.66

SPN schedulerThe following table shows how SPN behaves

With SPN, whenever a scheduling decision has to be made, the decision is based on available

information, not of future information, therefore at time 10 process 1 is picked but at time 14 process

3 is picked

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Single-programming and SPN scheduling

Average turnaround time = 16/3 = 5.33

Process Arrival time CPU burst Completion time

Turnaround time

P1 10 4 14 4

P2 11 3 19 8

P3 12 2 16 4

RR schedulerThe following chart shows how SPN behaves

When processes do not have any I/O operation and do not have to wait for any event, RR scheduler may perform worse than FCFS and

SPNIn cases of I/O, depending on average CPU wait ration, usually RR

performs better

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Multi-programming and RR scheduling

Average turnaround time = 23/3 = 7.66

10.0 11.0 12.0 18.0 19.0 Time

1.0 0.5 2.0 0.5

0.5 2.0 0.5

Processes

P3

P2

P1

2.0

RR for Processes with I/OHere, we consider the same set of tasks but the average I/O ratio

w=0.4, i.e., when there is only one process 40% of CPU time is wastedWhen there are two processes CPU wait fraction is w2=0.16 hence 84%

of CPU time is useful, every process’s share is 42%When there are three processes CPU wait fraction is w3=0.064 hence

approximately 94% of CPU time is useful, every process’s share is 31.3%

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Multi-programming and RR scheduling with w=0.4

Average turnaround time = 8.55

10.00 11.00 12.00 18.41 19.79 20.46 Time

0.6 0.42 2.0 0.58 0.4

0.42 2.0 0.58

Processes

P3

P2

P1

2.0

Disk I/O SchedulingDisks have their own special schedulersUnit of work is reading/writing a sectorDisks constantly receive requests from

different processesRequest arrivals is not uniform; therefore a pool for outstanding

requests is neededDisk scheduler decides which request to pick from the pool when the disk head is

finished its previous request

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Disk propertiesThe simplest kind of disk is a one surface with one

moving headIt is composed of many concentric tracks with every track having the same number of sectors, see below

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Track

Sector

Moving head

Reading/writing a sectorReading/Writing a sector comprises

Seek time: the time length for the read/write head to move from current position to the desired track

Rotational delay: is the length of time for the beginning of sector to reach the read/write head,

after the head has settled on the desired track Average rotational delay is considered to be half the time

of one complete disk rotation Transfer time: The time to transfer one sector of

data depends on the rotation speed, track capacity, and sector size, This is equal to:

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timeRotationcapacityTrack

sizeBlocktimetransferBlock

SchedulersRotational delay and transfer time is

fixedThe performance of a disk scheduler thus depends on the order in which outstanding requests are answered

A track trace is set of requests in the order of arrival

We study, FIFO, LIFO, SSTF, Scan, C-Scan schedulers

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FIFO

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From 50 34 57 28 65 190 120 163 163 180

To 34 57 28 65 190 120 163 163 180 46

Seek length

16 23 29 37 125 70 43 0 17 134 Total = 494

Track trace 34, 57, 28, 65, 190, 120, 163, 163, 180, 46

Let’s assume head’s current position is on track 50

The shortest seek length for is zero The longest seek length is n-1With FIFO there is no risk of starvationNew request may not be served before existing ones

LIFO

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From 50 34 57 28 65 190 120 163 163 180

To 34 57 28 65 190 120 163 163 180 46

Seek length

16 23 29 37 125 70 43 0 17 134 Total = 494

Track trace 34, 57, 28, 65, 190, 120, 163, 163, 180, 46

Let’s assume head’s current position is on track 50

The shortest seek length for is zero The longest seek length is n-1 An existing request may never get the chance to be executed because new requests keep coming, dtarvation With LIFO, arrival of new requests causes the serving of previous ones to delay

Shortest Service Time First

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Track trace 34, 57, 28, 65, 190, 120, 163, 163, 180, 46

Let’s assume head’s current position is on track 50

The shortest seek length for is zero The longest seek length is n-1 SSTF may cause starvation for some requestsWith SSTF, arrival of new requests may cause the serving of previous ones to delay

From 50 46 57 65 34 28 120 163 163 180

To 46 57 65 34 28 120 163 163 180 190

Seek length

4 11 8 31 6 92 43 0 17 10 Total = 222

Scan

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Track trace 34, 57, 28, 65, 190, 120, 163, 163, 180, 46

Let’s assume head’s current position is on track 50

The shortest seek length for is zero The longest seek length is n-1. Scan is considered starvation-freeWith SSTF, arrival of new requests may cause the serving of previous ones to delay

From 50 57 65 120 163 163 180 190 46 34

To 57 65 120 163 163 180 190 46 34 28

Seek length

7 8 55 43 0 17 10 144 12 6 Total = 302

C-Scan

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Track trace 34, 57, 28, 65, 190, 120, 163, 163, 180, 46

Let’s assume head’s current position is on track 50

The shortest seek length for is zero The longest seek length is n-1. Scan is considered starvation-freeWith SSTF, arrival of new requests may cause the serving of previous ones to delay

From 50 57 65 120 163 163 180 190 28 34

To 57 65 120 163 163 180 190 28 34 46

Seek length

7 8 55 43 0 17 10 162 6 12 Total = 320

SummaryA scheduling strategy is usually designed to

attain a defined objective, although multi-objective strategies are also possible

Average turnaround time (ATT) may be used to estimate the expected time length in which a request is completed after being submitted to

the system; this could be a good measure of performance

Based on ATT different scheduling algorithms were investigated

Besides, in this chapter, I/O scheduling was studied and different schedulers such as FIFO, LIFO, SSTF, Scan, and C-Scan were introduced

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Find outHow ATT is computed for nonpreemptive

schedulers such as FIFO and SJN where w0How FIFO, SJN, and RR behave with respect to

throughputHow I/O schedulers work for two-surface disks

How I/O schedulers work for disk packs, i.e., disks with multiple plates each having two

surfacesHow I/O schedulers work for multiples

Read/write heads per surfaceWhat the role of cylinders are with respect to

I/O schedulers

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Any questions?