Turn 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.

 

Turn Variable-

 

• Turn variable is a synchronization mechanism that provides synchronization among two processes.
• It uses a turn variable to provide the synchronization.

 

It is implemented as-

 

 

Initially, turn value is set to 0.

• Turn value = 0 means it is the turn of process P₀ to enter the critical section.
• Turn value = 1 means it is the turn of process P₁ to enter the critical section.

 

Working-

 

This synchronization mechanism works as explained in the following scenes-

 

Scene-01:

 

• Process P₀ arrives.
• It executes the turn!=0 instruction.
• Since turn value is set to 0, 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 turn value to 1.

 

Scene-02:

 

• Process P₁ arrives.
• It executes the turn!=1 instruction.
• Since turn value is set to 0, so it returns value 1 to the while loop.
• The returned value 1 does not break the while loop condition.
• The process P₁ is trapped inside an infinite while loop.
• The while loop keeps the process P₁ busy until the turn value becomes 1 and its condition breaks.

 

Scene-03:

 

• Process P₀ comes out of the critical section and sets the turn value to 1.
• 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 turn value to 0.

 

Also Read- Criteria For Synchronization Mechanisms

 

Characteristics-

 

The characteristics of this synchronization mechanism are-

• It ensures mutual exclusion.
• It follows the strict alternation approach.

 

Strict Alternation Approach   In strict alternation approach, Processes have to compulsorily enter the critical section alternately whether they want it or not. This is because if one process does not enter the critical section, then other process will never get a chance to execute again.

 

• It does not guarantee progress since it follows strict alternation approach.
• It ensures bounded waiting since processes are executed turn wise one by one and each process is guaranteed to get a chance.
• It ensures processes does not starve for the CPU.
• It is architectural neutral since it does not require any support from the operating system.
• It is deadlock free.
• It is a busy waiting solution which keeps the CPU busy when the process is actually waiting.

 

To gain better understanding about Turn Variable,

Watch this Video Lecture

 

Next Article- Interest Variable | Synchronization Mechanism

 

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