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Lecture notes study book Linking Thinking of James Trimble, Aisling Quirke, Audrey Byrne, Louise Boylan - ISBN: 9780717190140 (aptitude)
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OPERATING SYSTEMS NOTES II YEAR/I SEM MRCETUNIT-II
Process Scheduling: Foundation and Scheduling objectives, Types of Schedulers, Scheduling criteria:
CPU utilization, Throughput, Turnaround Time, Waiting Time, Response Time; Scheduling algorithms:
Pre-emptive and Non pre-emptive, FCFS, SJF, RR; Multiprocessor scheduling: Real Time scheduling: RM
and EDF.
Inter-process Communication: Critical Section, Race Conditions, Mutual Exclusion, Hardware Solution,
Strict Alternation, Peterson’s Solution, The Producer/Consumer Problem, Semaphores, Event Counters,
Monitors, Message Passing, Classical IPC Problems: Reader’s & Writer Problem, Dinning Philosopher
Problem etc.
PROCESS SCHEDULING:
CPU is always busy in Multiprogramming. Because CPU switches from one job to another job. But in
simple computers CPU sit idle until the I/O request granted.
scheduling is a important OS function. All resources are scheduled before use.(cpu,
memory, devices…..)
Process scheduling is an essential part of a Multiprogramming operating systems. Such
operating systems allow more than one process to be loaded into the executable memory at
a time and the loaded process shares the CPU using time multiplexing
. Scheduling Objectives
Maximize throughput.
Maximize number of users receiving acceptable response times.
Be predictable.
Balance resource use.
Avoid indefinite postponement.
Enforce Priorities.
Give preference to processes holding key resources
SCHEDULING QUEUES: people live in rooms. Process are present in rooms knows
as queues. There are 3types
1. job queue: when processes enter the system, they are put into a job queue, which
consists all processes in the system. Processes in the job queue reside on mass storage and await
the allocation of main memory.
2. ready queue: if a process is present in main memory and is ready to be allocated to
cpu for execution, is kept in readyqueue.
3. device queue: if a process is present in waiting state (or) waiting for an i/o event to
complete is said to bein device queue.(or)
The processes waiting for a particular I/O device is called device queue.
39
,OPERATING SYSTEMS NOTES II YEAR/I SEM MRCET
Schedulers : There are 3 schedulers
1. Long term scheduler.
2. Medium term scheduler
3. Short term scheduler.
Scheduler duties:
Maintains the queue.
Select the process from queues assign to CPU.
Types of schedulers
1. Long term scheduler:
select the jobs from the job pool and loaded these jobs into main memory (ready queue).
Long term scheduler is also called job scheduler.
2. Short term scheduler:
select the process from ready queue, and allocates it to the cpu.
If a process requires an I/O device, which is not present available then process enters device
queue.
short term scheduler maintains ready queue, device queue. Also called as cpu scheduler.
3. Medium term scheduler: if process request an I/O device in the middle of the
execution, then the process removed from the main memory and loaded into the waiting queue.
When the I/O operation completed, then the job moved from waiting queue to ready queue.
These two operations performed by medium term scheduler.
40
, OPERATING SYSTEMS NOTES II YEAR/I SEM MRCET
Context Switch: Assume, main memory contains more than one process. If cpu is executing a process, if
time expires or if a high priority process enters into main memory, then the scheduler saves information
about current process in the PCB and switches to execute the another process. The concept of moving CPU
by scheduler from one process to other process is known as context switch.
Non-Preemptive Scheduling: CPU is assigned to one process, CPU do not release until the competition of
that process. The CPU will assigned to some other process only after the previous process has finished.
Preemptive scheduling: here CPU can release the processes even in the middle of the
execution. CPU received a signal from process p2. OS compares the priorities of p1 ,p2. If
p1>p2, CPU continues the execution of p1. If p1<p2 CPU preempt p1 and assigned to p2.
Dispatcher: The main job of dispatcher is switching the cpu from one process to another
process. Dispatcher connects the cpu to the process selected by the short term scheduler.
Dispatcher latency: The time it takes by the dispatcher to stop one process and start another
process is known as dispatcher latency. If the dispatcher latency is increasing, then the degree of
multiprogramming decreases.
SCHEDULING CRITERIA:
1. Throughput: how many jobs are completed by the cpu with in a timeperiod.
2. Turn around time : The time interval between the submission of the process
and time of the completion is turn around time.
TAT = Waiting time in ready queue + executing time + waiting time in waiting queue for
I/O.
3. Waiting time: The time spent by the process to wait for cpu to beallocated.
4. Response time: Time duration between the submission and firstresponse.
5. Cpu Utilization: CPU is costly device, it must be kept as busy aspossible.
Eg: CPU efficiency is 90% means it is busy for 90 units, 10 units idle.
CPU SCHEDULINGALGORITHMS:
1. First come First served scheduling: (FCFS): The process that request the CPU
first is holds the cpu first. If a process request the cpu then it is loaded into the ready queue,
connect CPU to that process.
Consider the following set of processes that arrive at time 0, the length of the cpu burst time
given in milli seconds.
burst time is the time, required the cpu to execute that job, it is in milli seconds.
Process Burst time(milliseconds)
P1 5
P2 24
P3 16
P4 10
P5 3
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