This document discusses various CPU scheduling algorithms such as FCFS, SJF, priority scheduling, and round robin. It covers the basic concepts of scheduling, criteria for evaluating algorithms, examples of single processor and multiprocessor scheduling, and examples of scheduling in operating systems like Windows and Mac OS. The goal of scheduling is to allocate CPU time effectively among competing processes or threads.

1. CPU SCHEDULING
GUIDED BY:
MR. VENKATESWARAN.R (ASST. PROFESSOR M.TECH)
CREATED BY:
VINOTH.J
SARAVANA KUMAR.R
KARTHICK SEKAR.S
ASIR ALLOSANAI GANA SEKAR
MUTHURAMALINGAM.E
MUNEESWARAN.M
INFANT JESUS COLLEGE OF ENGINEERING

2. OVERVIEW
 Basic Concepts
 Scheduling Criteria
 Scheduling Algorithms
 Multiple-Processor Scheduling
 Operating Systems Examples

3. OVERVIEW
 In computer science, scheduling is the method by
which threads, processes or data flows are given
access to system resources (e.g. processor time,
communications bandwidth). This is usually done to
load balance a system effectively or achieve a target
quality of service.

4. OVERVIEW
 Throughput:
 No of processes that complete the execution per unit
time
 Response time: Amount of time it takes from
when a request was submitted until the first
response is produced
 Waiting time:Amount of time a process has been
waiting in the ready queue Response time = waiting
time + execution time

5. Types of scheduling methods
 Preemptive scheduling:A higher priority process
can preempt a currently running low priority process
to avoid priority inversion
 Nonpreemptive scheduling:A higher priority
process waits a currently running process to finish
regardless of the priority e.g., FCFS

6. Types of scheduling methods
 Long and medium term scheduling
 Long term scheduling: which determines which
programs are admitted to the system for execution
and when, and which ones should be exited.
 Medium term scheduling: which determines when
processes are to be suspended and resumed;
 Short term scheduling (or dispatching): which
determines which of the ready processes can have
CPU resources, and for how long.

7. Single Processor Scheduling Algorithms
 First Come, First Served (FCFS)
 Shortest Job First (SJF)
 Priority
 Round Robin (RR)

8. First Come, First Served (FCFS)
 First Come, First Served (FCFS), is the simplest
scheduling algorithm, FIFO simply queues processes
in the order that they arrive in the ready queue.
 Throughput can be low, since long processes can
hold the CPU
 It is based on Queuing
 No prioritization occurs, thus this system has trouble
meeting process deadlines.

9. Example

10. MERITS AND DEMERITS
 MERITS:Easy to understand and easy to program
 It is fair
 DEMERITS:
 Does not perform well in real systems
 Ignores the service time request and all other criteria

11. Shortest-Job-First (SJR) Scheduling
 1. non preemptive – once CPU given to the
process it cannot be preempted until completes its
CPU burst.
 2. preemptive – if a new process arrives with CPU
burst length less than remaining time of current
executing process, preempt. This scheme is know as
the Shortest-Remaining-Time-First (SRTF).
 SJF is optimal – gives minimum average waiting
time for a given set of processes.

12. Example

13. MERITS AND DEMERITS
 MERITS:
 SJF is optimal – gives minimum average waiting
time for a given set of processes

 DEMERITS:
 Need to have a good heuristic to guess the next
CPUexecution time
 Short duration processes will starve longer ones

14. Priority Scheduling
 The SJF algorithm is a special case of the general
priority scheduling algorithm
 A priority number (integer) is associated with each
process
 The CPU is allocated to the process with the highest
priority (smallest integer = highest priority)

15. Example

16. MERITS AND DEMERITS
 MERITS:
 Simplicity
 Reasonable support for priority.
Suitable for applications with varying time and resource
requirements.
 DEMERITS:
 Indefinite blocking or starvation.
 A priority scheduling can leave some low priority waiting
processes indefinitely for CPU.

17. Round Robin (RR)
 Each process gets a small unit of CPU time (time
quantum), usually 10-100 milliseconds. After this
time has elapsed, the process is preempted and
added to the end of the ready queue.
 Performance
 1. q large _ FIFO
 2. q small _ q must be large with respect to
context switch, otherwise overhead is too high.

18. Example

19. MERITS AND DEMERITS
 MERITS:
 Fair allocation of CPU across jobs
 Low average waiting time when job lengths vary
 DEMERITS:
 It is suitable for task with varied burst time. But for task
with same time makes trouble.

20. Multilevel Queue Scheduling

21. Multilevel Feedback Queue
 A process can move between the various queues;
 Multilevel-feedback-queue scheduler defined by
the following parameters:
 1. number of queues
 2. scheduling g algorithms for each queue

22. Example of Multilevel Feedback Queue
Scheduling algorithm CPU Overhead Throughput Turnaround time Response time
First In First Out Low Low High Low
Shortest Job First Medium High Medium Medium
Priority based
Medium Low High High
scheduling
Round-robin scheduling High Medium Medium High
Multilevel Queue
High High Medium Medium
scheduling

23. Multiprocessor Scheduling
 '' On a multiprocessor, scheduling is two
dimensional. The scheduler has to decide which
process to run and which CPU to run it on. This extra
dimension greatly complicates scheduling on
multiprocessors.

24. OPERATING SYSTEM EXAMPLES
 Windows
 Very early MS-DOS and Microsoft Windows systems
were non-multitasking, and as such did not feature a
scheduler. Windows 3.1x used a non-preemptive
scheduler, meaning that it did not interrupt
programs.

25. OPERATING SYSTEM EXAMPLES
 Mac OS
 Mac OS 9 uses cooperative scheduling for threads,
where one process controls multiple cooperative
threads, and also provides preemptive scheduling for
MP tasks.

26. CONCLUSION
 The above mentioned are the various CPU
scheduling used in the present day. These algorithms
are inherited based on the recuirement of the
processor.