Operating System Process Synchronization

1. ACKNOWLEDGEMENT
We have no words to express our deepest gratitude to ALLAH who
blessed and enabled us to complete this project. Its great privilege to
mention the feelings of obligations towards our affectionate parents
and department, who prayed, encouraged and inspired us for higher
education and are supporting financially and morally throughout our
studies.
We find it very difficult to select the appropriate words to express our
gratitude to our teacher Prof. Umar and his useful encouragement and
dynamic supervision.
Course Instructor:
-----------------------------------

2. DECLARATION
We hereby declare that this project report entitled “Process Synchronization” is written by us
and is our own effort and that no part has been copied and taken from any other source.
Submitted by Signature with Date
Haziq Naeem (107) ----------------------------------------
Muhammad Adeel (83) ----------------------------------------

3. TABLE OF CONTENTS
Sr# Contents Page#
1 Introduction 1
2 Synchronization 2
2.1 Critical Section Problem 2
2.2 Locks 4
2.3 Petersons Solution 5
2.4 Semaphores 5
2.5 Major Problems of Synchronization 6
3 Windows vs Linux Synchronization 8
4 Activity Chart 9
5 Conclusion 9
6 References 9

4. FIGURES TABLE
Fig# Contents Page#
1 Synchronization 1
2 Synchronization 2
3 Critical Section Problem 3
4 Locks 4
5 Petersons Solution 5
6 Major Problems of Synchronization 7
7 Major Problems of Synchronization 7
8 Major Problems of Synchronization 8
9 Activity Chart 9

5. 1. Introduction
Synchronization is the mutual understanding between two processes or more.
In simple words synchronization means sharing system resources. i.e. Track of a train, they are
mutually synchronized with each other.
2. Synchronization
Access to two or more processes to share data can result in data inconsistency. for this a
mechanism is used called “Synchronization”.
Synchronization is the mutual understanding between two processes or more.
In simple words synchronization means sharing system resources. i.e. Track of a train, they are
mutually synchronized with each other.

6. (Figure 1)
Example:
Synchronizing Traffic
In a one way tunnel the tunnel can only be used when no oncoming traffic is present and when
the traffic is already flowing in the right way.

7. (Figure 2)
Problems:
 Deadlock: Two or more than two process waiting for an event
 Starvation: Indefinite blocking. Process that required resource which is never
allocated.
2.1. Critical SectionProblem
Only one process executed at a time in critical section. Only one process is allowed to start or
execute in its critical section. So every process 1st request permission to entering in critical
section.
Three section involved in critical section:
i. Entering Section
ii. Exit Section
iii. Remainder Section
If one process started working, other processes will have to wait until first will finished.
(Figure 3)
We discuss the critical section problem now we will discuss the solution of critical section
problem.
Solutions of Critical Section Problem:

8. Three kinds of solutions involved in critical section problem
i. Mutual Exclusion
ii. Progress
iii. Bounded Waiting
i. Mutual Exclusion
One processes execute no other process will interfere in it.
For example, if a process named A1 and other Process named A2. If A1 executes in its critical
section, then A2 not allowed to interfere in it.
ii. Progress
If no process executed in its critical section, then those process only requested to execute in
its critical section which are in remainder section and the rest are placed in queue.
iii. Bounded Waiting
When a process makes request or a process request granted to enter in a critical section then a
limit or bound exists on the number of times that the other processes are allowed to enter its
critical section.
For example, a process AB requested to enter in its critical section bounded waiting restricted
that this process enter only two times in its critical section after request made and before request
granted to its critical section
Another example of bounded waiting is the limits of downloading on websites
2.2. Locks
Lock means to prevent someone to do something. Any solution to critical section problem
requires some form of lock.
Acquire: Before entering its critical section
Release: After exits the critical section
A software tool is built for this known as “Mutex Lock”. Also known as spin lock.

9. (Figure 4)
Synchronization Hardware:
Many systems provide the support of hardware for critical section.
 Uniprocessors can disable interrupts:
System clock kept updated by interrupts
 Modern machines are non-interrupt able:
Allow us to test and modify content of a word or to swap contents of two words.
2.3. Peterson’s Solution
It is a software based solution to critical section problem.
 Provides good algorithm description of solving.
 Describe the complexities in designing software (three critical section solutions).
 Could not be work on modern non-interrupt able machines.
The solution is for two processes. Load and store instructions are non-interrupt able.

10. (Figure 5)
2.4. Semaphores
Semaphores is a variable that is used to manage or control access to a common resource by
multiple processes in a similar or concurrent system.
It is basically a synchronization tool. Two operations are involved in it. Wait and Signal
represented as P() and V().
Wait: increasing the value of its arguments as soon as it will become non minus
Signal: decreasing the value of its arguments as an individual operation
Properties:
 It is very simple
 Works with many processes
 Each critical section has different semaphores
 Can have same critical section with same semaphores
Types of semaphores:
i. Counting Semaphores
ii. Binary Semaphores
i. Binary Semaphores
It is a type of semaphores used to implement bounded concurrency
The value of binary semaphores can only range from 0 to 1.
ii. Counting Semaphores
A type of semaphores which is also known as mutex. It is used to implement mutual exclusion
value of counting semaphores over an unrestricted range.
For example Bank account
2.5. MajorProblems of Synchronization
There are three major problems of synchronization

11. i. Bound Buffer Problems
ii. Reader-Writer Problem
iii. Dinning Philosophers Problem
i. Bound Buffer Problem
In bound buffer there are two concepts involved. Producer and consumer
This problem explains these two processes which share common buffer used as a queue.
Producer produce the data take it into the buffer and again started Whereas Consumer delete
or remove data one by one If buffer is full then more data can’t be stored and consumer not
able to remove data from an empty buffer.
The solution for producer is to discard data or go to sleep if the buffer is full. If consumer
removes something into the buffer, then notifies the producer will start again to fill the buffer.
Now for consumer, if consumer finds buffer empty then it can go to sleep. When producer puts
any data in buffer the consumer awakes.
The solution of this problem is semaphores and improper solution will result as a deadlock.
ii. Readers-Writers Problem
Readers: Don’t perform any updates only read the data
Writers: Performing both reading and writing
Problem is that multiple readers to read the data but only one writer can access data at that
time.
iii. Dining Philosophers Problems
Invented by E.W. Dijkstra.
Problem:
Imagine there are five philosophers who only eat and think. They are sitting on a circular table
to eat spaghetti and have only five chopsticks to eat. Each philosopher thins and when he felt
hungry he picks up chopsticks and eat spaghetti, after finish eating he puts down the chopsticks
and starts to think again.

12. (Figure 6)
Analysis of problem:
(Figure 7)
Solution:

13. (Figure 8)
3. Windows vs Linux Synchronization
Windows Linux
Uses interrupts to protect access to
global resources on unipolar systems
Disables interrupts to implement
short critical sections
Provides dispatcher objects which
may act as semaphores
Provides semaphores
Uses spin locks on multiprocessor
systems
Provides spin locks
4. Activity Chart

14. (Figure 9)
5. Conclusion
Synchronization is a mechanism for the mutual understanding between two or more processes.
It Includes critical section problems, locks, Peterson’s solution, semaphores and major
problems of synchronization.
As very thing has positive and negative sights, synchronization has more positive ones.
It is a major technique / mechanism used in the operating systems. It gives access to prevent
from deadlocks and starvation, due to this mechanism the data integrity and consistency
increases, it collaborates data and improve the efficiency of the work.
6. References
Critical Section:
http://web.stanford.edu/class/cs140/cgi-bin/lecture.php?topic=locks
https://cseweb.ucsd.edu/classes/fa05/cse120/lectures/120-l5.pdf
Semaphores:
https://en.wikipedia.org/wiki/Semaphore_(programming)
http://www.personal.kent.edu/~rmuhamma/OpSystems/Myos/semaphore.htm
https://en.wikibooks.org/wiki/Operating_System_Design/Processes/Semaphores
Dining Philosophers Problems:
https://www.cs.mtu.edu/~shene/NSF-3/e-Book/MUTEX/TM-example-philos-1.html
Locks:
http://cse.stfx.ca/~mlin/cs375/lectures/syn3.ppt
Imp of Sync:
http://users.csc.calpoly.edu/~fkurfess/Courses/COMP-346/W00/Slides/05-ProcSync.ppt

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