Interest Variable | Process Synchronization

Process Synchronization-

 

Before you go through this article, make sure that you have gone through the previous article on Process Synchronization.

 

We have discussed-

• Process Synchronization provides a synchronization among the processes.
• Synchronization mechanisms allow the processes to access critical section in a synchronized manner.
• This avoids the inconsistent results.

 

Interest Variable-

 

• Interest variable is a synchronization mechanism that provides synchronization among two processes.
• It uses an interest variable to provide the synchronization.

 

It is implemented as-

 

 

Initially, interest [0] and interest [1] are set to False.

• Interest value [0] = False means that process P₀ is not interested to enter the critical section.
• Interest value [0] = True means that process P₀ is interested to enter the critical section.
• Interest value [1] = False means that process P₁ is not interested to enter the critical section.
• Interest value [1] = True means that process P₁ is interested to enter the critical section.

 

Working-

 

This synchronization mechanism works as explained in the following scenes-

 

Scene-01:

 

• Process P₀ arrives.
• It sets interest[0] = True.
• Now, it executes while loop condition- interest [1] == True.
• Since interest [1] is initialized to False, so it returns value 0 to the while loop.
• The while loop condition breaks.
• Process P₀ enters the critical section and executes.
• Now, even if process P₀ gets preempted in the middle, process P₁ can not enter the critical section.
• Process P₁ can not enter unless process P₀ completes and sets the interest [0] = False.

 

Scene-02:

 

• Process P₁ arrives.
• It sets interest[1] = True.
• Now, it executes while loop condition- interest [0] == True.
• Process P₀ has already shown its interest by setting interest [0] = True.
• So, it returns value 1 to the while loop.
• The while loop condition satisfies.
• The process P₁ is trapped inside an infinite while loop.
• The while loop keeps the process P₁ busy until the interest [0] value becomes False and its condition breaks.

 

Scene-03:

 

• Process P₀ comes out of the critical section and sets the interest [0] value to False.
• The while loop condition of process P₁ breaks.
• Now, the process P₁ waiting for the critical section enters the critical section and execute.
• Now, even if process P₁ gets preempted in the middle, process P₀ can not enter the critical section.
• Process P₀ can not enter unless process P₁ completes and sets the interest [1] = False.

 

Also Read- Criteria For Synchronization Mechanisms

 

Characteristics-

 

The characteristics of this synchronization mechanism are-

• It ensures mutual exclusion.
• It does not follow strict alternation approach.
• It ensures progress since if a process is not interested to enter the critical section, it never stops the other process to enter the critical section.
• It is architectural neutral since it does not require any support from the operating system.
• It is a busy waiting solution which keeps the CPU busy when the process is actually waiting.
• It suffers from deadlock.
• Since it suffers from deadlock, it does not guarantee bounded waiting.

 

How it suffers from deadlock?

 

• This synchronization mechanism may cause deadlock between the processes.
• Deadlock may occur through the following sequence of scenes-

 

Scene-01:

 

• Process P₀ arrives.
• It sets interest[0] = True.
• Now, it gets preempted and process P₁ gets scheduled.

 

Scene-02:

 

• Process P₁ arrives.
• It sets interest[1] = True.
• Now, it gets preempted.

 

Scene-03:

 

• Process P₀ gets scheduled again.
• Now, it can not break the while loop condition- interest [1] == True since process P₁ has shown its interest for executing critical section before its arrival.
• It keeps waiting in the infinite while loop for process P₁ to complete its execution first.

 

Scene-04:

 

• Later, Process P₁ gets scheduled again.
• Now, it also can not break the while loop condition- interest [0] == True since process P₀ is also interested for executing critical section.
• It keeps waiting in the infinite while loop for process P₀ to complete its execution first.

Thus, both the processes are deadlocked.

 

To gain better understanding about Interest Variable,

Watch this Video Lecture

 

Next Article- Practice Problems On Synchronization Mechanisms

 

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