Difference between Lock and Turn Variable

Purpose

Lock variables are primarily used to provide mutual exclusion, preventing concurrent access by multiple processes or threads to a shared resource.

Turn variables are employed to ensure fair scheduling among multiple processes or threads. They specify the order in which processes will access the resource, promoting fairness.

Core Mechanism

Lock variables use a blocking mechanism. When a process/thread requests a lock and it’s already held, the requesting process is blocked and waits until the lock becomes available.

Turn variables use a queue or waiting list mechanism. Processes are placed in the queue based on a predefined order, and the turn variable determines which process from the queue gets access.

Fairness

Lock variables may lead to potential fairness issues if not used carefully. They may favor one process or thread over others, leading to possible contention problems.

Turn variables are explicitly designed to ensure fairness. They prevent starvation and provide each process with an equal opportunity to access the shared resource.

Complexity

Lock variables are generally simpler to implement and use. They are well-suited for scenarios where the main concern is preventing concurrent access.

Turn variables are more complex to implement. They are often used in conjunction with other synchronization primitives such as semaphores or condition variables to establish a fair scheduling.

Examples

Mutexes, binary semaphores, critical sections, and semaphores with locking mechanisms.

Turn variables are typically used in conjunction with semaphores or condition variables to implement fair resource allocation.