Process Synchronization-
When multiple processes execute concurrently sharing system resources, then inconsistent
results might be produced.
● Process Synchronization is a mechanism that deals with the synchronization of processes.
● It controls the execution of processes running concurrently to ensure that consistent results are
produced.
Need of Synchronization-
Process synchronization is needed-
● When multiple processes execute concurrently sharing some system
resources.
● To avoid the inconsistent results.
Critical Section-
Critical section is a section of the program where a process access the shared
resources during its execution.
Problem-
The following two functions P1 and P2 that share a variable B with an initial
value of 2 execute concurrently-
The number of distinct values that B can possibly take after the execution is
Critical Section Problem-
● If multiple processes access the critical section concurrently, then results produced might
be inconsistent.
● This problem is called as critical section problem.
Synchronization Mechanisms-
Synchronization mechanisms allow the processes to access critical section in a
synchronized manner to avoid the inconsistent results.
For every critical section in the program, a synchronization mechanism
● An entry section before the critical section
● An exit section after the critical section
Entry Section-
● It acts as a gateway for a process to enter inside the critical section.
● It ensures that only one process is present inside the critical section at any
time.
● It does not allow any other process to enter inside the critical section if one
process is already present inside it.
Exit Section-
● It acts as an exit gate for a process to leave the critical section.
● When a process takes exit from the critical section, some changes are made so that other processes
can enter inside the critical section.
Criteria For Synchronization Mechanisms-
Any synchronization mechanism proposed to handle the critical section problem should meet
the following criteria-
1. Mutual Exclusion
2. Progress
3. Bounded Wait
1. Mutual Exclusion-
The mechanism must ensure-
● The processes access the critical section in a mutual exclusive manner.
● Only one process is present inside the critical section at any time.
● No other process can enter the critical section until the process already
present inside it completes.
2. Progress-
The mechanism must ensure-
● An entry of a process inside the critical section is not dependent on the entry
of another process inside the critical section.
● A process can freely enter inside the critical section if there is no other process
present inside it.
● A process enters the critical section only if it wants to enter.
● A process is not forced to enter inside the critical section if it does not want to
enter.
3. Bounded Wait-
The mechanism should ensure-
● The wait of a process to enter the critical section is bounded.
● A process gets to enter the critical section before its wait gets over.
Turn Variable-
Initially, turn value is set to 0.
Interest Variable-
● Interest variable is a synchronization mechanism that provides synchronization among two processes.
● It uses an interest variable to provide the synchronization.
Characteristics-
The characteristics of this synchronization mechanism are-
● It ensures mutual exclusion.
● It suffers from deadlock
Peterson’s Solution
Peterson’s Solution is a classical software based solution to the critical section problem.
In Peterson’s solution, we have two shared variables:
● boolean flag[i] :Initialized to FALSE, initially no one is interested in entering the
critical section
● int turn : The process whose turn is to enter the critical section.
Peterson’s Solution
Question
Consider the methods used by processes P1 and P2 for accessing their critical sections whenever needed, as given below. The
initial values of shared Boolean variables S1 and S2 are randomly assigned
Which one of the following statements describes the properties achieved?
1. Mutual exclusion but not progress
2. Progress but not mutual exclusion
3. Neither mutual exclusion nor progress
4. Both mutual exclusion and progress
Method Used by P2
Method Used by P1
while (S1 != S2) ;
while (S1 == S2) ;
Critica1 Section
Critica1 Section
S2 = not (S1);
S1 = S2;
Semaphores in OS-
● A semaphore is a simple integer variable.
● It is used to provide synchronization among multiple processes running concurrently.
Question
A shared variable x, initialized to zero, is operated on by four concurrent processes W, X, Y, Z as follows. Each of the processes
W and X reads x from memory, increments by one, stores it to memory and then terminates. Each of the processes Y and Z
reads x from memory, decrements by two, stores it to memory, and then terminates. Each process before reading x invokes the P
operation (i.e. wait) on a counting semaphore S and invokes the V operation (i.e. signal) on the semaphore S after storing x to
memory. Semaphore S is initialized to two. What is the maximum possible value of x after all processes complete execution?
Binary semaphores
Binary semaphores are mainly used for two purposes-
● To ensure mutual exclusion.
● To implement the order in which the processes must execute.
Wait (S1) Process P2 Wait (S2)
Process P1 Signal (S1) Process P3
Signal (S2)
Two binary semaphores S1 and S2 both initialized with 0 are used.
● The execution order of the processes is P2 → P1 → P3.
