The document discusses CPU scheduling within operating systems, focusing on the process of selecting which process to execute next among those ready to run. It explains different types of schedulers, including long-term, short-term, and medium-term schedulers, along with various scheduling algorithms like FCFS, SJF, priority scheduling, and round-robin. Key objectives and criteria for effective scheduling are highlighted, such as CPU utilization, throughput, fairness, and minimizing wait times.

1. OPERATING SYSTEM:CPU SCHEDULING
• Dr Vatan sehrawat
• Asst. Professor, Computer Science & engg dept.
• RPSCET, Balana, Mahendragarh
• vatansehrawat@gmail.com
• 8059211113

2. It is the change of ready state to running state of the
process. CPU scheduler selects process among the processes
that are ready to execute and allocates CPU to one of them.
Short term scheduler also known as dispatcher, execute most
frequently and makes the fine grained decision of which
process to execute next.
CPU scheduling decisions may take place when a process:
Switches from running to waiting state.
Switches from running to ready state.
Switches from waiting to ready.
Terminates

3. Scheduling Criteria
Criteria for best scheduling algorithm:
CPU Utilization : Computer usage of memory and processing
resources or amount of work handle by a CPU.
Throughput : It is the total number of process completed per unit
time.
Load average : average number of processes sitting in the
ready queue waiting for that turn to get onto CPU.
Fairness : All the jobs should get fair chance to get the use of
CPU.
Efficiency : scheduler should keep the system busy whole time, if
CPU and all i/o devices can be kept running all the time more
work can be done per second.

4. Response / service time : Amount of time taken from when a request was
submitted until first response is produced is called response / service
time.
Response time = Time at which the process gets the CPU for the first time - Arrival
time
Turn around time : It is the total amount of time the process executed itself
on the CPU.
Turnaround time = Burst time + Waiting time
Turnaround time = Exit time - Arrival time
Waiting time : It is the amount of time a process waits to load itself on the
CPU.
Waiting time = Turnaround time - Burst time
Waiting Time = Service Time - Arrival Time
Arrival Time: Time at which the process arrives in the ready queue.
Completion Time: Time at which process completes its execution.
Burst Time: Time required by a process for CPU execution.

5. Scheduling Objectives
When to schedule :
• When a new process is created , a decision needs to be made whether to run the
parent process or child process.
• A scheduling decision must be made when a process exits.
• When I/O interrupt occurs , a scheduling decision may be made.
Main objective of scheduling is :
• Be Fair
• Make the system fast
• Maximize throughput
• Increasing the profit.
• Increasing the output.
• Maximize number of users receiving acceptible response times.
• Reducing the manufacturing time.
• Minimizing the production costs
• Give better service to processes that have desirable behaviour patterns

6. Schedul
er
• When a computer is multiprogrammed , it frequently
has multiple process or threads competing for the CPU at
the same time.
• This situation occurs whenever two or more of them
are simultaneously in the ready state.
• If only one CPU is available , a choice has to be
made which process to run next.
• The part of the operating system that makes the choice
is called the scheduler.

7. Types of
schedulers
Schedulers are of 3 types :
1. Long term scheduler
2. Short term scheduler
3. Medium term scheduler

8. Long term
schedulers
• It is also called job scheduler
• It determine which program are admitted to a system
for processing .
• It selects process from queue and loads them in to memory
for execution .
• On some system it may not me available or may be minimal .
• Time sharing operating systems have no long term scheduler.
• When process changes the state from new to ready, then there
is use of long term scheduler.

9. Short term
schedulers
• It is also called CPU scheduler.
• Main objective is increasing system performance in accordance
with the chosen set of criteria .
• CPU scheduler selects process among the processes that are ready
to execute and allocates CPU to one of them.
• When process changes the state from ready to running, then there
is use of long term scheduler.
• They are also called as dispatchers.
• Context switching is done by dispatchers.
• Faster than long term schedulers .

10.  Dispatchers :
• It gives control of CPU to the process selected by short
term scheduler.
• This involves switching context , switching of user mode
, jumping to proper location in the user program to
restart that program .
 Dispatcher latency :
• Time taken by dispatcher to stop one process and
start another .

11. Medium term schedulers
• It is the part of swapping .
• It removes the processes from memory .
• A running process may become suspended if it does not
get I/O resource needed.
• In this condition the process is move to the
secondary storage until it gets the resources.
• It is the part of time sharing operating system.

12. Comparison among Scheduler

13. Maximum CPU utilization.
Max throughput.
Minimum turn around time.
Minimum waiting time.
Minimum response time

14. • First Come First Serve (FCFS) Scheduling
• Shortest-Job-First (SJF) Scheduling
• Priority Scheduling
• Round Robin(RR) Scheduling
• Multilevel Queue Scheduling

15. First-Come ,First-Served(FCFS)Scheduling
Since CPU gets a lot of processes to handle. So in
FCFS CPU will take the first process in the list and
will process it then it will take the second process
and so on. For example let there be three processes:
PROCESSES BURSTS TIME
P1 24
P2 3
P3 3

16. Suppose that the processes arrive in order p1,p2,p3
So the waiting time for p1=0,p2=24,p3=27
The average waiting time=(0+24+27)/3=17.
One of the major drawback of this scheme is that the
Average waiting time is often quite long.

17. It is best approach to minimize waiting time. it
Selects the waiting process with the smallest
execution time to execute next.
Processes Bursts time
P1 6
P2 8
P3 7
P4 3

18. Average waiting time=(3+16+9+0)/4=7.
The disadvantage of using SJF is that the total
execution time of a job must be known before
execution. while it is not possible to perfectly
predict execution time.

19. Priority scheduling is a method of scheduling
processes based on priority. In this method the
scheduler chooses the tasks to work as per the
priority.
Processes Burst time Priority
P1 10 3
P2 1 1
P3 2 4
P4 1 5
P5 5 2

20. Average waiting time=(6+0+16+18+1)/5=8.2
The drawback of priority scheduling is starvation
of low priority process.

21. In the round robin scheduling, processes are
dispatched in a FIFO manner but are given a
limited amount of CPU time called a time-slice
or a quantum.
If a process does not complete before its CPU-time
expires, the CPU is preempted and given to the
next process waiting in a queue. The preempted
process is then placed at the back of the ready
list.

22. A CPU Scheduling Mechanism Should
• Favour short jobs.
• Favour I/O bound jobs to get good I/O device
utilization.
• Determine the nature of a job and schedule
accordingly.