Turn Variable or Strict Alternation Approach

Turn Variable or Strict Alternation Approach is the software mechanism implemented at user mode. It is a busy waiting solution which can be implemented only for two processes. In this approach, A turn variable is used which is actually a lock.

This approach can only be used for only two processes. In general, let the two processes be Pi and Pj. They share a variable called turn variable. The pseudo code of the program can be given as following.

For Process Pi

Non - CS 
while (turn ! = i); 
Critical Section 
turn = j; 
Non - CS

For Process Pj

Non - CS 
while (turn ! = j);
Critical Section 
turn = i ;
Non - CS 

The actual problem of the lock variable approach was the fact that the process was entering in the critical section only when the lock variable is 1. More than one process could see the lock variable as 1 at the same time hence the mutual exclusion was not guaranteed there.

This problem is addressed in the turn variable approach. Now, A process can enter in the critical section only in the case when the value of the turn variable equal to the PID of the process.

There are only two values possible for turn variable, i or j. if its value is not i then it will definitely be j or vice versa.

In the entry section, in general, the process Pi will not enter in the critical section until its value is j or the process Pj will not enter in the critical section until its value is i.

Initially, two processes Pi and Pj are available and want to execute into critical section.

The turn variable is equal to i hence Pi will get the chance to enter into the critical section. The value of Pi remains I until Pi finishes critical section.

Pi finishes its critical section and assigns j to turn variable. Pj will get the chance to enter into the critical section. The value of turn remains j until Pj finishes its critical section.

Analysis of Strict Alternation approach

Let's analyze Strict Alternation approach on the basis of four requirements.

Mutual Exclusion

The strict alternation approach provides mutual exclusion in every case. This procedure works only for two processes. The pseudo code is different for both of the processes. The process will only enter when it sees that the turn variable is equal to its Process ID otherwise not Hence No process can enter in the critical section regardless of its turn.

Progress

Progress is not guaranteed in this mechanism. If Pi doesn't want to get enter into the critical section on its turn then Pj got blocked for infinite time. Pj has to wait for so long for its turn since the turn variable will remain 0 until Pi assigns it to j.

Portability

The solution provides portability. It is a pure software mechanism implemented at user mode and doesn't need any special instruction from the Operating System.