Interested Variable Mechanism for Process Synchronization: Guaranteeing Progress and Mutual Exclusion

Understand the interested variable mechanism, a process synchronization technique that addresses limitations of the simple turn variable approach. This tutorial explains its operation, how it ensures both mutual exclusion and progress, and compares it to other synchronization methods.

Interested Variable Mechanism for Process Synchronization

Addressing the Limitations of the Turn Variable

In operating systems, synchronization mechanisms are needed to ensure that multiple processes access shared resources correctly. The turn variable mechanism, while ensuring mutual exclusion, fails to guarantee progress because a process not wanting to enter the critical section doesn't consider other waiting processes. This can lead to unnecessary waiting even when the critical section is free. The interested variable mechanism is designed to address this limitation.

The Interested Variable Mechanism

The interested variable mechanism uses an additional Boolean variable (interested) for each process to indicate its desire to enter the critical section. This provides a way for a process to check if others are waiting before proceeding.

// For Process Pi
// Non-critical section
interested[i] = TRUE;
while (interested[j] == TRUE); // Wait if other process is interested
// Critical Section
interested[i] = FALSE;


// For Process Pj
// Non-critical section
interested[j] = TRUE;
while (interested[i] == TRUE); // Wait if other process is interested
// Critical Section
interested[j] = FALSE;

Before entering the critical section, a process checks if the other process is interested. It waits until the other process's interest is false. After exiting, it sets its own interest to false to allow the other process to proceed.

Analysis Based on Synchronization Requirements

Mutual Exclusion:

The interested variable mechanism guarantees mutual exclusion because a process will wait if another process is interested in entering the critical section. Only one process can have access at a time.

Progress:

The mechanism ensures progress because a process not interested in entering the critical section won't block a waiting process. If one process is ready, it can proceed.

Bounded Waiting:

Unfortunately, the interested variable mechanism alone does *not* guarantee bounded waiting. A scenario can occur where both processes are waiting for each other to release their interest, leading to a deadlock. Bounded waiting requires a mechanism to prevent such deadlocks (e.g., the turn variable in Peterson's solution).

Architectural Neutrality:

Because the interested variable mechanism is a purely software-based solution implemented in user mode, it is portable and does not depend on specific hardware architectures. This is a significant advantage.