Question

What are Semaphores and Mutex? What are they used for?

Answer 1

SEMAPHORE

Semaphore is simply a variable that is non-negative and shared between processes. A semaphore is a signaling mechanism, and a process that is waiting on a semaphore, can be signaled by another process so that at a time only one process can enter into critical section (shared resource) so that we can have mutual exclusion.

It uses two atomic operations:

1) Wait(P): decrement the value of semaphore

2) Signal(V): Increment the value of semaphore.

It is of two types :

1) Binary Semaphore: In this, semaphore have 0 and 1 value. 0 (zero) is when wait operation is used and 1 is when signal operation is used. Initially value of S is 1.

2) Counting Semaphore: In this, semaphore have non-negative integers (for eg: 0,1,2,3..) as a value.

Use of Semaphore

• Non-binary semaphores are used in resource allocation. A semaphore might hold the count of the number of a particular resource.
• If you have a pool of connections, such as a web browser might use, then an individual thread might reserve a member of the pool by waiting on the semaphore to get a connection, uses the connection, then releases the connection by releasing the semaphore.

MUTEX

• The full form of Mutex is Mutual Exclusion Object. It is a special type of binary semaphore which also used for controlling access to the shared resource (critical section).
• It includes a priority inheritance mechanism to avoid extended priority inversion problems. It allows current higher priority tasks to be kept in the blocked state for the shortest time possible.
• However, priority inheritance does not correct priority- inversion but only minimizes its effect.

Use of Mutex

A mutex provides mutual exclusion, which can be either producer or consumer that can have the key (mutex) and proceed with their work. As long as producer fills buffer, the user needs to wait, and vice versa. In Mutex lock, all the time, only a single thread can work with the entire buffer.