3. CPU Scheduling

3. CPU Scheduling Operating System Concepts with Java – 8th Edition Silberschatz, Galvin and Gagn

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operating system – CPU Scheduling 3

4 operating system – CPU Scheduling

Long-short-medium Scheduler Long term Short Term Medium Term

job scheduler CPU scheduler process swapping Scheduler

Slower than short term

Fastest Between two

Control degree of multiprogramming

provides lesser control over degree of Multiprogramming

reduces the degree of multiprogramming

Minimal in time sharing system

Minimal in time sharing system

part of Time sharing systems

selects processes from pool and loads them into memory for execution

selects those processes which are ready to Execute

can re-introduce the process into memory and execution can be continued

5 operating system – CPU Scheduling

Dispatcher selects from among the processes in memory that

are ready to execute, and allocates the CPU to one of them

�decide when a process � 1. running to waiting state 2. running to ready state 3. waiting to ready state 4. Terminates

�Scheduling under 1 and 4 is nonpreemptive

�All other scheduling is preemptive


Scheduling Factor 1. CPU utilization - keep the CPU as busy as possible must be maximized 2. Throughput - number of processes that complete

their execution per time unit must be maximized 3. �Turnaround time - amount of time to execute a

particular process must be minimized 4. Waiting time - amount of time a process has been

waiting in the ready queue must be minimized 5. �Response time - amount of time it takes from when

a request was submitted until the first response is produced, not output �for timesharing environment�

must be minimized operating system – CPU Scheduling 7

SCHEDULING METHOD

▪ FIRST COME FIRST SERVE �FCFS / FIFO�

▪ SORTHEST JOB FIRST �SJF� ▪ PRIORITY BASED ▪ ROUND ROBIN


First-Come, First-Served �FCFS� Proces Execute

time Service Time

p1 24 0

p2 3 24

p3 3 27


processes arrive in the order: P1, P2, P3

Waiting time for P1= 0; P2= 24; P3 = 27 �Average waiting time: (0 + 24 + 27)/3 = 17

processes arrive in the order: P2, P3, P1

Waiting time for P1= 6; P2= 0; P3 = 3 �Average waiting time: (6 + 0 + 3)/3 = 3 Conclusion ?

CONVOY EFFECT short process behind long

process

Proces Execute time

Service Time

p1 24 ??

p2 3 ??

p3 3 ??

FCFS �another example�


Shortest Job First �SJF�

Best approach to minimize waiting time.

Impossible to implement

Processer should know in advance how much time process will take.


Proces Execute time

Service Time

p1 6 3

p2 8 16

p3 7 9

p4 3 0 Average waiting time = (3 + 16 + 9 + 0) / 4 = 7

SJF �another example�


Priority Scheduling Each process is assigned a priority. Process with highest priority is to be executed first and so on.

Processes with same priority are executed on FCFS

Priority can be decided based on memory requirements, time requirements or any other resource requirement.


Round Robin Scheduling Each process is provided a fix time to execute called

quantum Q.

Once a process is executed for given time period. Process is preempted and other

process executes for given time period. �If there are nprocesses in the ready queue and the time quantum is Q, then each process gets 1/nof the CPU time in chunks of at most Q time units at once. No process waits more than �n-1�Q time units.

Performance

Q large -� FIFO

Q small -� overhead is too high


RR with Quantum - 4


Proces Execute time

p1 24

p2 3

p3 3

Round Robin �another example�


Multi Queue Scheduling Multiple queues are maintained for processes.

Each queue can have its own scheduling algorithms.

Priorities are assigned to each queue.


Example of Multilevel Three queues� Q0 - RR with Q 8 milliseconds

Q1 - RR with Q 16 milliseconds

Q2 - FCFS

A new job enters queue Q0 which is served FCFS. When it gains CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1 .

At Q1 job is again served FCFS and receives 16 additional milliseconds. If it still does not complete, it is preempted and moved to queue Q2


The END . . .

References 1. Abraham Silberschatz, Peter Baer Galvin, Greg

Gagne, Operating System Concepts With Java, Wiley

2. Tutorialpoint.com


Next Week It is time for

▪ QUIZ - Learn how to calculate scheduling

▪ Practical Hardware TroubleShoot – Install OS on real PC �minimum complete PC

provided by lecturer� – Optional may bring own PC �2 teams 1 complete

PC and extension power cable� in order to gain self experience


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3. CPU Scheduling