Process Control Block & Threads and Their Management

Process Control Block
Threads & Their Management
By Ujjwal Kumar
1149210055
CS – V – 2013

Process Control
Block
or
PCB
Presentation by Ujjwal
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Introduction
• Each & every process in the Operating System is
represented by a Process Control Block.
• Process Control Block ~ Task Control Block or
Task Struct.
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What exactly is PCB?
• It is a Data Structure in the operating system
kernel containing the information needed to
manage a particular process.
• The PCB is "the manifestation(Visible) of a
process in an operating system”
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Source: http://lmgtfy.com/?q=define%3Amanifestation/

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Fig: Process Control Block (PCB)

What exactly does this
diagram mean?
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• Pointers: It contains the address of another
process which is present in the ready queue.
• Process State: It contains information about
the state of the process i.e.,
• New
• Ready
• Running
• Waiting
• Halted
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• Program Counter: The address of the next
instruction to be executed.
• CPU Registers: The registers vary in no. and
type depending on the type of computer
architecture, they include:
• Accumulators, index registers, stack pointers and
general purpose registers
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• CPU scheduling information: This
information includes
▫ Process priority
▫ Pointer to scheduling queues
▫ Other scheduling parameters.
• Memory management information: This
information includes
▫ Value of base and limit registers
▫ Page tables
▫ Segment tables.
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• Accounting information: This information
includes
• Amount of CPU and real time used
• Time limits
• Account numbers
• Job or Process numbers etc.
• I/O status information: This information
includes
• List of I/O devices allocated to the process
• List of open files etc .
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Now we know what exactly the
Process Control Block is. But
do we know where the location
of PCB is?
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• Contains critical information of processes and
hence it must be placed where it is protected
from normal user access.
• In many Operating systems, it is placed in the
beginning of the kernel stack of the process since
that is a convenient protected location.
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Interleaving
Execution of
Processes
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Interleaving Execution of Processes
• Perception of normal end user:
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Process
A Process
B
Process
C
Process D
Time

Interleaving Execution of Processes
• In reality, this is how the processes are executed:
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Process 1
Process 2
Process 1
Process 2

Threads &
their Management
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Threads
• Definition: In simple words we can say that
Threads are light weight processes.
• It is a sequence and not a program because it
cannot run on its own so it returns within the
program.
• A process is divided into smaller tasks and these
tasks are known as threads.
• A basic unit of CPU Utilization.
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Advantages of Threads
• If a process has multiple threads then it will be
able to perform more than one single task at the
same time.
• Better Throughput.
• Makes the program Responsive.
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Types of Threading
•Single Threading
•Multi Threading
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Multithreading
• Sometimes several similar tasks have to be
performed by the application in order to get the
word done.
• This is where the Multithreading comes in.
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Real life examples of Multithreading - I
• Web browsers: They have different threads to
do different work.
▫ One thread will retrieve data from the remote
servers.
▫ Another one will render the data received into
visual web pages.
• Example: Google Chrome, Opera, Firefox etc.
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Real life examples of Multithreading -
II
• Word Processors: Different threads in word
processor will do different jobs.
▫ One will render the text based materials.
▫ Another one will render graphic based materials
like the images etc.
• Example: Microsoft Office
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Benefits of Multithreading
• Responsiveness: The process responds much
better while using multithreading as compared
to single threaded processes.
• Speed Up: The process is speeded up by using
multithreading hence giving a much better
experience to the end user.
• Efficient Communication: The threads of a
specific process can communicate with each
other efficiently because of the shared address
space.
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• Utilization in multiprocessor systems:
▫ Multithreading can take advantages of a
multiprocessor system.
▫ Multiple threads can be processed on multiple
cores of the system.
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Implementation of Threads
• Threads are implemented in a system in three
different ways.
▫ Kernel Level Threads
▫ User Level Threads
▫ Hybrid Level Threads
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Kernel Level Threads
• Implemented by the kernel itself.
• Management of threads is also done by the
kernel itself.
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Process
Threads
Process
Table

User Level Thread
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Kernel Space
Thread
Table
Thread
Process
User
Space

User Level Thread
• These type of threads are loaded in the user
space and the kernel doesn’t have anything to do
with this.
• Threads are managed in user space so each
process must have its own private thread table.
• This table consists the information of:
▫ Program Counter
▫ Stack Pointer
▫ Register.
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Acknowledgements
• Sir. Mukesh Kumar Singh
• Google.com
• Book: OS by Vijay Shukla
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Questions
You can ask any questions
you might have in your
mind related to my
presentation.
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Process Control Block & Threads and Their Management

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