Lecture 2 Processes in operating systems.pptx

1
IT105
Operating Systems
Process Management: Processes
Harris Chikunya

2
Learning Outcomes
By the end of this Lecture, you should be able to:
• Explain the concepts of process, various states in the process and their scheduling;
• Define a process control block;
• Classify three types of schedulers;
• Explain five types of scheduling algorithms; and
• Compare the performance evaluation of the scheduling algorithms.

3
Concept of Process
• A process is a sequential program in execution
• It defines the fundamental unit of computation for the computer
• Components of process are:
1. Object Program – code to be executed
2. Data – is used for executing the program
3. Resources – while executing the program, it may require some resources
4. Status of the process execution – used for verifying the status of the process execution.

4
Process vs Program
Process
• Is a dynamic entity i.e a program in execution
• Is a sequence of information executions
• Exists in a limited span of time
• Two or more processes could be executing the same program, each using their own data and
resources
Program
• Is a static entity made up of program statements
• Contains the instructions
• A program exist at place in space and continues to exist
• Does not perform the action by itself.

5
Process State
• As a process executes, it changes state
• The state of a process is defined by the current activity of that process
• The process may be in one of the following states
1. New – a process that just been created
2. Ready – processes waiting to have the processor allocated to them by the OS so that they
can run
3. Running – process that is currently being executed
4. Waiting – a process that cannot execute until some event occurs such as the completion of
an I/O operation
5. Terminated – The process has finished execution

7
Process Control Block (PCB)
• Each process is represented on the OS by a PCB
• It is the data structure used by the OS
1. Process state – new, running, waiting etc
2. Program counter – address of next instruction to
be executed for this process
3. CPU registers – indicates general purpose register, index registers and
accumulators etc.
4. Memory management information – include such information as the
base and limit register, page tables or segment tables in memory
5. I/O status information – I/O requests, I/O devices allocated , a list of
open files
6. Accounting information – CPU used, clock time elapsed since start,
time limits

8
Threads
• Is a lightweight process
• Basic unit of CPU utilisation
• Consists of :
 Program counter – keeps track of which instruction to execute next
 Registers – holds its current working variable
 Stack – execution history
• Thread States
1. bornState :A thread is just created
2. readyState :The thread is waiting for CPU
3. Running :System assigns the processor to the thread
4. Sleep: :A sleeping thread becomes ready after the designated
sleep time expires
5. Dead :The execution of the thread is finished

9
Process vs Thead
• Process
 Is heavy weight or resource intensive
 process takes more time to create
 Execution is very slow
 Cant share the same memory area
 Communication between two processes is difficult
• Thread
 Is light weight, taking lesser resources
 Takes less time to create
 Execution is very fast
 Threads can share same memory area
 Communication between two threads is easy

10
Multithreading
• Is when a number of thread execute at the same time within a process
• In a multithreaded program, even when some portion of it is blocked, the whole program is
not blocked
• The rest of the program continues running if multiple CPUs are available
• Multithreading therefore gives the best performance
• Multithreading can simplify code and increase efficiency
• Kernels are generally multithreaded

11
Single vs Multithreaded process
• Code – Contains instruction
• Data – holds global variable
• Files – opening and closing files
• Register – contain information about CPU state
• Stack – parameters, local varibles, functions

12
Types of Threads
1. User Threads
 Thread creation, scheduling, management are done by a thread library at the
user level
 If a user thread performs a system call which blocks it, all the other threads in
that process are also automatically blocked, whole process is blocked
• Advantages
 Thread switching does not require kernel mode privileges
 User level threads can run on any operating system
 User level threads are fast to create and manage
 Is generic and can run on any Os
• Disadvantages
 In a typical operating system, most system calls are blocking
 Multithreaded application cannot take advantage of multiprocessing

13
Types of Threads cont...
2. Kernel threads
 Are created, scheduled, managed by the kernel.
 If one thread in a process is blocked, the process need not be blocked.
• Advantages
 Kernel can simultaneously schedule multiple threads from the same
process on multiple processes.
 If one thread in a process is blocked, the Kernel can schedule another
thread of the same process.
 Kernel routines can be multithreaded
• Disadvantages
 Kernel threads are generally slower to create and manage than the user
threads.
 Is specific to the operating system

15
Multithreading models
• Categories of threading implementation
1. One to One
 One of the earliest implementation of true
multithreading
 One user thread to one kernel thread
 Each user user-level thread created by the
application is known to the kernel and all
threads can access the kernel at the same time.
 Disadvantage of this model is that creating user
thread requires the corresponding Kernel
thread.
 OS/2, windows NT and windows 2000 use one
to one model

16
2. Many to One Model
 Maps many user threads to one kernel thread
 Allow the application to create any number of threads that can execute concurrently.
 All thread activity is restricted to the user space
 However only one thread can access the kernel at a time, so multiple threads are unable to run
in parallel on multiprocessors

17
3. Many to Many Model
 Maps many user-level threads to many kernel-level threads
 Aka the two-level model, minimizes programming effort while reducing the cost and
weight of each thread
 A program can have as many threads as are appropriate without making the process
too heavy.
 This implementation provides a standard interface, a simpler programming model, and
optimal performance for each process

18
Process Scheduling
• refers to a set of policies and mechanisms supported by operating
system that controls the order in which jobs are completed.
• A scheduler is an operating system program (module) that select the
next job to be admitted for execution.
• In this section we will describe:
 Scheduling objectives
 Types of schedulers
 Various scheduling algorithms

19
Scheduling Objectives
1. Maximise throughput – service the largest possible number of processes per unit
time
2. Minimise overhead – scheduling should minimise the waster resources
3. Balance resource use – should keep the resources of the system busy
4. Maximise interactive users – maximise the number of interactive users receiving
acceptable response times
5. Enforce priorities – the scheduling mechanism should favour the higher-priority
processes
6. Avoid indefinite postponement – all processes should be treated the same and no
process can suffer indefinite postponement

20
Types of Schedulers
• Long – term Scheduler
 Also called job scheduler
 Selects processes from the queue and loads them into memory for execution
 It is the change from of state from new to ready
 Its main objectives is to provide a balanced mix of jobs, such as I/O bound and
processor bound
• Short-term Scheduler
 Also called the CPU scheduler/dispatcher
 Selects from among the processes that are ready to execute and allocates the CPU to
one of them
 It is the change of state from ready to running for the process
 Faster than long term scheduler

21
Types of Schedulers cont…
• Medium term Scheduler
 Is part of the swapping function i.e. it is in charge of handling the swapped out-
processes
 It removes process from the memory e.g. suspended processes may be moved to
the secondary storage (swapping) and make space for other processes
 It reduces the degree of multiprogramming

23
Scheduling Algorithms
Scheduling Criteria
1. CPU Utilization – CPU is costly device, it must be kept as busy as possible
2. Throughput – refers to the number of processes completed in unit time
3. Turn around time – the time interval between the submission of the process and time of
completion
4. Waiting time – amount of time a process has been waiting in the ready queue
5. Response time – time duration between the submission and first response

24
• A major division among scheduling algorithms is whether they support
pre-emptive or non-pre-emptive scheduling discipline:
a) Pre-emptive scheduling
 A scheduler may pre-empt a low priority running process anytime when a
high priority process enters into a ready state
 Pre-emptive scheduling is more useful in high priority process which
requires immediate response e.g Real time system.
 Round Robin scheduling, priority based scheduling or event driven
scheduling and SRT
b) Non pre-emptive scheduling
 Once a process enters the running state, it cannot be pre-empted until it
completes its allocated time
 Always processes a scheduled job to its completion.
 First come First Served (FCFS) and Shortest Job First (SJF)

25
1. First-Come First-Serve (FCFS)
2. Shortest-Job First (SJF)
3. Round Robin (RR)
4. Shortest Remaining Time Next (SRTN)
5. Priority Based Scheduling or Event Driven (ED) Scheduling
6. Multilevel Queue Scheduling (Research)
7. Multilevel Feedback Queue Scheduling (Research)

26
First-Come First-Serve (FCFS)
• Jobs are scheduled on the order they are received
• Is non-pre-emptive
• Easy to understand and implement
• Its implementation is based on FIFO queue
• Poor in performance as average wait time is high

27
Exercise
• Calculate the turn around time, waiting time, average turnaround time, average
waiting time, throughput and processor utilisation for the given set of processes
that arrive at a given arrive time shown in the table, with the length of
processing time given in milliseconds:
• If the processes arrives as per the arrival time, the Gantt chart will be:

28
Shortest Job First (SJF)
• Also known shortest job next
• This is a non-pre-emptive type of scheduling
• Best approach to minimize waiting time
• The processor should know in advance how much time the process will take
• Easy to implement in Batch systems where required CPU time is known in
advance
• Impossible to implement in interactive systems where required CPU time is
not known

29
Exercise
• Consider the following set of processes with the following processing time which arrived at
the same time.
• Using SJF scheduling the shortest length of process will first get execution, the Gantt chart
will be:
Process Process Time
P1 06
P2 08
P3 07
P4 03

30
Round Robin (RR)
• Is a pre-emptive process scheduling algorithm
• Each process is provided a fixed time to execute known as a Quantum
• Once a process is executed for a given time period, it is pre-empted
and other process executes for a given time period
• No process can run for more than on quantum while others are waiting
in the ready queue
• Primarily used in time-sharing and multiuser system environment

31
Exercise
• Consider the following Scenario
• Quantum Slice = 4

32
Shortest Remaining Time Next (SRTN)
• This is the pre-emptive version of shortest job first
• This permits a process that enters the ready list to pre-empt the running process, if
the time for the new process is less than the remaining time for running process.

33
Priority Scheduling
• A priority is associated with each process, and the CPU is allocated to the
process with the highest priority.
• Equal priority processes are scheduled in FCFS order.
• Is a non-pre-emptive algorithm
• Priorities can be assigned either internally or externally.
• Internal priorities are assigned by the OS using criteria such as average burst
time, ratio of CPU to I/O activity, system resource use, and other factors
available to the kernel.
• External priorities are assigned by users, based on the importance of the job,
fees paid, politics, etc.

34
Exercise
• P2 has the highest priority.

Lecture 2 Processes in operating systems.pptx

More Related Content

Similar to Lecture 2 Processes in operating systems.pptx (20)

Recently uploaded (20)

Lecture 2 Processes in operating systems.pptx

Editor's Notes