Mutex vs Semaphore: Difference and Comparison

Managing the integrity of shared information relies heavily on process synchronization. For dealing with critical section problems, both software and hardware solutions are available.

Hardware solutions for the critical section problem, on the other hand, are extremely difficult to execute. Let’s examine and contrast Mutex and Semaphore, two application solutions for dealing with critical section problems.

Key Takeaways

1. Mutex ensures mutual exclusion, allowing only one thread to access a shared resource at a time.
2. Semaphore controls access to a resource by managing a fixed number of permits, allowing multiple threads to access the resource concurrently.
3. Mutexes are simpler to use, while semaphores provide more flexibility in controlling access to shared resources.

Mutex vs Semaphore

A mutex (short for mutual exclusion) is a type of lock that is used to ensure that only one thread or process can access a shared resource at a time. Semaphore is a mechanism that manages access to shared resources through a counter that controls access, with different values representing different states of the resource.

Mutex stands for Mutual Exclusion Object. It’s a type of binary semaphore that’s used to restrict access to a shared resource. To prevent prolonged priority inversion concerns, it features a priority inheritance mechanism.

It permits existing higher-priority jobs to be blocked for the least amount of time possible. Priority inheritance, on the other hand, does not fix priority inversion; rather, it reduces its impact.

Semaphore is a non-negative variable that is maintained between threads. A semaphore is a signalling mechanism, and another thread can signal a thread that is waiting for a semaphore.

For process synchronization, it employs two atomic procedures: () wait and () signal. Depending on how it’s set up, a semaphore either enables or prevents access to the resource.

Comparison Table

Parameters of Comparison	Mutex	Semaphore
Mechanism	It’s a locking system.	It’s a signaling system of some sort.
Purpose	Thread is represented by the mutex.	Processes are represented by semaphore.
Nature	Mutex is atomic and singular.	Semaphore is atomic in nature, but not singular.
Data type	Mutex is nothing more than a piece of software.	A semaphore is a variable with an integer value.
Types	Mutex does not have any subtypes.	Counting and binary semaphores are two types of semaphores.
Modification	Only the process that can request or release a resource can modify it.	A semaphore can be modified using the wait and signal functions.

What is Mutex?

Mutual Exclusion is a term used to describe a situation. Mutex is a short name for the object. We can deduce from the word mutual exclusion that only one program at the moment has access to a particular resource.

The mutex object allows many application threads to access the same resource at the same time, only one at a time.

Whenever a program asks the system for a resource, the system creates a mutex object with a unique identity or ID. As a result, anytime the program wishes to utilize that resource, it takes a lock on the object.

The program then uses the resource before finally releasing the mutex object. The mutex object can then be created and used by other programs in the same way.

By locking the object, that specific resource is assigned to that specific process, and no other process can use it. As a result, no other programs are permitted to use the system resources in the crucial area. A mutex object can be used to accomplish process synchronization in this manner.

A mutex allows for mutual exclusion, either as the producer or the consumer can have the key (mutex) and continue working. The consumer must wait as long as the producer’s buffer is filled, and vice versa.

What is Semaphore?

Semaphore is an integer variable S that is used for process synchronization and is initialized with the number of resources in the system. To modify the value of S, it employs two main functions: wait() and signal ().

Both of these functions are used to change the value of a semaphore, but they only enable one program to do so at a time, therefore no two methods can change the value of a semaphore at the same time. Counting semaphores and Binary semaphores are the two types of semaphores.

The semaphore variable is first initialized with the number of resources available while counting semaphores. The wait() method is then executed anytime a process requires a resource, and the value of the semaphore variable is reduced by one.

The process then uses the resource, after which it calls the signal() method, which increases the value of the semaphore variable by one. Whenever the value of the semaphore variable reaches 0, that is, when the program has used up all of the resources, and none are remaining to use,

Then, if another process needs to use resources, it will have to wait for its time. We establish process synchronization in this manner.

In Binary Semaphores, the value of the semaphore variable is either 0 or 1. When a process wishes to utilize a resource, the wait() method is invoked, and the value of the semaphore is adjusted to 0 from 1.

The process then makes use of the resource, and after it’s done, the signal() method is called, and the value of the semaphore variable is set to 1.

If the value of the semaphore variable is 0 at a given point in time, and another program wants to access the same resource, it must wait for the prior program to free the recourses. Process synchronization can be performed in this manner. It’s comparable to a mutex, but it doesn’t lock anything.

Main Differences Between Mutex and Semaphore

1. Mutex uses a locking mechanism, which means that if a process needs to use a resource, it must first lock it, use it, and then release it. Semaphore, on the other hand, employs a signalling technique in which the wait() and signal() functions are used to indicate whether a process is giving or consuming a resource.
2. A mutex is an object, whereas a semaphore is a variable with an integer value.
3. A mutex object requires various process threads to connect the same shareable resource at the same time. Semaphore, on the other hand, permits several process threads to access the resource’s finite instance until it becomes accessible.
4. In mutex, the lock can be obtained and discharged by the very same process at the same time. However, the valuation of the semaphore variable can be changed by any process that requires a resource, but only one process can change the value at a time.
5. Mutex enables various program threads to access the same shared resource, but only one at a time, whereas on the other hand, Semaphore requires various program threads to connect a limited number of resources at the same time.

References

1. https://dl.acm.org/doi/abs/10.1145/362759.362813
2. https://lib.hpu.edu.vn/handle/123456789/21469