Interested Variable Mechanism

It is a must to provide progress to the process synchronization mechanism.

In variable mechanism, if there is a process that doesn’t need to enter into the critical section, then due to this, the other process which wants to execute in the critical section will not be considered. Due to this, other processes may wait for a long time even though there is no process that is present in the critical section. This problem can be solved by using an additional variable along with the turn variable. And by doing this, we can solve the problem of progress.

In the Interested Variable Mechanism, for progress, we use an additional Boolean variable.

To understand the concept of Interested Variable, the pseudocode of the program is given below:

For Process Pa

Non-CS Int[a] = T; While ( Int[b] = = T ) ; Critical Section Int[I] = F;    

For Process Pb

Non-CS Int [1] = T ; While ( Int[b] = = T ); Critical Section Int[b]= F ;

 In the Interested Variable Mechanism, we use an additional variable, which is known as ‘interested’ variable. The interested variable is a kind of Boolean Variable. Using the Boolean variable, we store the interest of the processes so that the process gets to enter into the critical section.

In the interested variable mechanism, if there is a process that needs to enter into the critical section, then the process checks that in the entry section, another process is interested or not to enter into the critical section. Then the process will wait till the other process is interested in remaining into the critical section.

In the exit section, we set the process value of the interested variable as false so that other process gets the opportunity to enter into the critical section.

The below table provides the values of the interested variable for the processes Pa and Pb and the chance of process to get enter into the critical section in the different scenarios.

Interest [Pa] Interest [Pb] Process which get the chance
           True           True The process that first shows interest
            True            False           Pa
            false             True            Pb
            false            False             x

Now, based on the requirement, we analyze the Interested Variable Mechanism:

  1. Mutual Exclusion: –It provides Mutual Exclusion because if there is a process that is interested in entering into the critical section, then the other process waits until the process becomes uninterested. So in this way, at the same time, no two processes will enter into the critical section.
  2. Progress: – Progress is also provided in the Interested Variable Mechanism because if a process doesn’t want to enter into the critical section, then it doesn’t affect other processes to enter into the critical section.
  3. Bounded Waiting: – We analyze bounded waiting with the help of two processes that is Process Pa and Pb. Both processes are cooperative processes. The Process Pa and Pb are both interested in entering into the critical section.

The below table shows the instruction executed by the process:

 Process Pa   Process Pb   Process Pa    Process Pb
1.Int [Pa] = True 2.while (Int [Pb] = = True); 3. Critical Section   1.Int [Pb] = True 2.while (Int [Pa] = = True); 1.Int [Pa] = False 2.Int [Pa] = True 3.while (Int [Pb] = = True); // waiting for Pb 1.while (Int [Pa] = = True); // waiting for Pb

In starting, the value of the interested variable for process Pa and Pb is false. In the above table, we can see that the Process Pa defines the interest to enter into the critical section.

The process Pa first set the value of interested variable True, and after setting the value, it checks for the Process Pb means it checks Pb is interested or not to enter into the critical section. So, the value of the interested variable for other processes is false; therefore, the Process Pa will enter inside the critical section.

During this, the Pa process gets preempted and schedule the Process Pb. Process Pb is a kind of cooperative process; that’s why it also needs to enter into the critical section. Process Pb shows its interest in entering into the critical section by setting the value of interest variable True.

The process Pb also examine that other processes want to enter inside the critical section or not. As we already know that the process Pa is preempted, and the value of the interested variable is true. It means the Process Pa also again want to execute in the critical section. That’s why the Process Pb will not get the opportunity to enter into the critical section, and it will get trapped in the while loop.

During this, the CPU changes the state of Process Pa from a blocked state to the running state. Process Pa is not completing its critical section, so the process Pa first completes its critical section execution, and after essential completion of the section, it exits from the critical section by setting the value of interest value as False.

There is a possibility of one case, i.e., if a Process Pa further needs to enter into the critical section and set value as true to the interested variable, and after setting the value, it verifies that the value of interest variable for Process Pb is True or not. The value of the interested variable for Process Pb’s is found true. So, the Process Pa will get trapped in the while loop and will wait for Process Pb to become uninterested.

Till now, the Process Pb is trapped in the while loop and waiting for the Process Pa to become make interested variable value as False. Hence in this condition, the Process Pa and Pb are waiting for each other, and in this situation, no Process will get enter into the critical section.

 Hence, a deadlock occurred.

In the case of deadlock, bounded waiting can’t be offered. So, in this way, we can say that in the interested variable mechanism, there is no guarantee of deadlock.

Architectural Neutrality: – Interested Variable Mechanism offers architectural neutrality because it is a software mechanism that is executed at the user mode.

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