05 lcd slides 1 - cpu scheduling (powerpoint)

CPU Scheduling

Classification of CPU Scheduling Performance Criteria of Good Scheduler CPU Scheduling Algorithms

*Property of STI J0024

Classification of CPU SchedulingCPU scheduling may be classified as: Non-preemptive scheduling

CPU scheduling is non-preemptive if once the CPU has been assigned to a process and the process starts executing, the CPU cannot be taken away from that process.

Preemptive schedulingIn preemptive CPU scheduling, even though the CPU has been assigned to a process and the process is already executing, the CPU scheduler may decide to assign the CPU to another process in the ready queue.


Performance Criteria of a Good SchedulerA good scheduler should optimize the following performance criteria:

CPU utilization Throughput Turnaround time Response time Waiting time


CPU Scheduling AlgorithmsThe different CPU scheduling algorithms are:

First-Come, First-Served Algorithm Shortest Process First Algorithm Shortest Remaining Time First Algorithm Round Robin Algorithm Priority Scheduling Multilevel Feedback Queues


CPU Scheduling AlgorithmsFirst-Come, First-Served Algorithm (FCFS)- the one that enters the Ready queue first gets to execute at the CPU first

a.

b.

c.

A

0 8

A

0

B

8 12

A

0

B C

8 12 17


CPU Scheduling AlgorithmsFirst-Come, First-Served Algorithm (FCFS)

d.

e.

WTA = 0 – 0 = 0 ms

WTB = 8 – 3 = 5 ms

WTC = 12 – 4 = 8 ms

WTD = 17 – 6 = 11 ms

WTE = 20 – 10 = 10 ms

The average waiting time is(0 + 5 + 8 + 11 + 10)/5 = 34/5 = 6.8 ms

A

0

B C D

8 12 17 20

A

0

B C D

8 12 17 20

E

22


CPU Scheduling AlgorithmsFirst-Come, First-Served Algorithm (FCFS)

TAA = 8 - 0= 8 ms

TAB = 12 - 3 = 9 ms

TAC = 17 - 4 = 13 ms

TAD = 20 - 6 = 14 ms

TAE = 22 - 10 = 12 ms

The average turnaround time is(8 + 9 + 13 + 14 + 12)/5 = 56/5 = 11.2 ms


CPU Scheduling AlgorithmsShortest Process First Algorithm (SPF)- the process with the shortest CPU burst

a.

b.

c.

A

0 8

A

0

D

8 11

A

0

D

8 11 13

E


CPU Scheduling AlgorithmsShortest Process First Algorithm (SPF)

d.

e.

WTA = 0 – 0 = 0 ms

WTB = 13 – 3 = 10 ms

WTC = 17 – 4 = 13 ms

WTD = 8 – 6 = 2 ms

WTE = 11 – 10 = 1 ms


A

0

BD

8 11 13 17

E

A

0

B CD

8 11 13 17

E

22


CPU Scheduling AlgorithmsShortest Process First Algorithm (SPF)

TAA = 8- 0 = 8 ms

TAB = 17 - 3 = 14 ms

TAC = 22 - 4 = 18 ms

TAD = 11 - 6 = 5 ms

TAE = 13 - 10 = 3 ms



CPU Scheduling AlgorithmsShortest Remaining Time First Algorithm (SRTF)- the preemptive version of SPF

a.

b.

c.

d.

Process ID Arrival Time CPU Burst

A 0 8

B 3 4

C 4 5

D 6 3

E 10 2

0

A

0

A

3

B

0

B

7

A

3

0

B

7

DA

3


CPU Scheduling AlgorithmsShortest Remaining Time First Algorithm (SRTF)

e.

f.

g.

h.

WTA = (0 – 0) + (12 – 3) = 9 ms

WTB = 3 – 3 = 0 ms

WTC = 17 – 4 = 13 ms

WTD = 7 – 6 = 1 ms

WTE = 10 – 10 = 0 ms


0

B D

7 10

A

3

0

B D

7 10 12

EA

3

A

0

B D

7 10 12 17

EA

3

A

0

B CD

7 10 12 17

E

22

A

3


CPU Scheduling AlgorithmsShortest Remaining Time First Algorithm (SRTF)

TAA = 17 - 0= 17 ms

TAB = 7 - 3 = 4 ms

TAC = 22 - 4 = 18 ms

TAD = 10 - 6 = 4 ms

TAE = 12 - 10 = 2 ms



CPU Scheduling AlgorithmsRound Robin Algorithm (RR)- the process are selected on a first-come, first-served basis. However, each

process is given a time limit to execute at the CPU.

Assume a time quantum of 3 ms.a.

b.

Process ID Arrival Time CPU Burst

A 0 8

B 3 4

C 4 5

D 6 3

E 10 2

0

A

3

0

B

6

A

3


CPU Scheduling AlgorithmsRound Robin Algorithm (RR)

c.

d.

e

f.

g.

h

i.

0

B A

6 9

A

3

0

B A

6 9 12

CA

3

D

0

B A

6 9 12 15

CA

3

D

0

B A

6 9 12 15

CA

3

B

16

D

0

B A

6 9 12 15

CA

3

B

16

A

18

D

0

B A

6 9 12 15

CA

3

B

16

A

18

E

20

D

0

B CA

6 9 12 15

C

22

A

3

B

16

A

18

E

20



WTA = (0 – 0) + (6 – 3) + (16 – 9) = 10 ms

WTB = (3 – 3) + (15 – 6) = 9 ms

WTC = (9 – 4) + (20 – 12) = 13 ms

WTD = 12 – 6 = 6 ms

WTE = 18 – 10 = 8 ms




TAA = 18 - 6= 18 ms

TAB = 16 - 3 = 13 ms

TAC = 22 - 4 = 18 ms

TAD = 15 - 6 = 9 ms

TAE = 20 - 10 = 10 ms



CPU Scheduling AlgorithmsPriority Scheduling- Each process is assigned a priority and the CPU scheduler selects the process in

the ready queue with the highest priority to execute next.

Using the non-preemptive priority algorithm:a.

b.

Process ID Arrival Time

CPU Burst Priority

A 0 8 4

B 3 4 3

C 4 5 1

D 6 3 2

E 10 2 2

A

0 8

A

0

C

8 13


CPU Scheduling AlgorithmsPriority Scheduling c.

d.

e.

WTA = 0 – 0 = 0 ms

WTB = 18 – 3 = 15 ms

WTC = 8 – 4 = 4 ms

WTD = 13 – 6 = 7 ms

WTE = 16 – 10 = 6 ms


A

0

C D

8 13 16

A

0

C D

8 13 16 18

E

A

0

BC D

8 13 16 18

E

22


CPU Scheduling AlgorithmsPriority Scheduling

TAA = 8 - 0= 8 ms

TAB = 22 – 3 = 19 ms

TAC = 13 - 4 = 9 ms

TAD = 16 - 6 = 10 ms

TAE = 18 - 10 = 8 ms




Using the preemptive priority algorithm:a.

b.

c.

d.

Process ID Arrival Time

CPU Burst Priority

A 0 8 4

B 3 4 3

C 4 5 1

D 6 3 2

E 10 2 2

0

A

0

BA

3

0

B C

4 9

A

3

0

B C

4 9

A

3

D



e.

f.

WTA = (0 – 0) + (17 – 3) = 14 ms

WTB = (3 – 3) + (14 - 4) = 10 ms

WTC = (4 – 4) = 0 ms

WTD = (9 – 6) = 3 ms

WTE = (12 – 10) = 2 ms


0

B C D

4 9 12

A

3

A

0

B C D

4 9 12 14

E

22

A

3

B

17



TAA = 22 – 0 = 22 ms

TAB = 17 - 3 = 14 ms

TAC = 9 - 4 = 5 ms

TAD = 12 - 6 = 6 ms

TAE = 14 - 10 = 4 ms



CPU Scheduling AlgorithmsMultilevel Feedback Queues

Q0All processes enter the queuing network

at the rear of the highest priority

queue

Processes that finish executing or request for an I/O operation within the given time slice leave the queuing network

FCFS (time slice = 10 ms)

Q1


Processes that do not finish executing within the given time slice move to the next

lower-priority queue


.

.

.

Qn

RR

Processes that do not finish executing within the given time slice move to the next

lower-priority queue

Q2




05 lcd slides 1 - cpu scheduling (powerpoint)

Software