Semaphores In Process Synchronization

   ⇰ Semaphores :-

  Semaphore is actually a method or tool to prevent race condition. Race condition can cause loss of data or even deadlock situation. For prevention from these conditions, the semaphore is one of the method. Semaphore was proposed by Dijkstra in 1965. Simaphore  is a very significant technique to manage concurrent processes. Semaphore is useful tool in the prevention of race condition. But the use of semaphore never means a guarantee that a program is free from these problems.

   Semaphore is an integer variable which is used in mutual exclusive manner by various concurrent cooperative processes in order to acheive synchronization. Hence semaphore is one of the way to achieve synchronization. Semaphore is basically  a variable which is non-negative and shared between threads. This variable is used to solve the critical section problem and to achieve process synchronization in the multiprocessing environment.

Semaphore contains some operations as for entry code and for exit code. 
  →  Entry code  :- P(), Down, Wait.
  → Exit code :- V(), Up, Post/Signal

→ Let us watch the codes of entry :-
Down ( semaphore S)
  {
    SValue = Svalue -1;
    if ( SValue < 0)

    {

      put process ( PCB) in suspanded list, sleep ();

    }

     else

       return;

   } 

→ Let us watch the codes of exit :-

Up ( semaphore S)

 {

     SValue = Svalue +1;
      if ( SValue <= 0)

       {

         select a process ( PCB) from suspanded list, wakeup();

       }

   } 

     

In above two codes, there used Down() and Up(). A simple way to understand Down() and Up() operations is :-

  Down() / Wait() :- It decrements the value of semaphore variuable by 1. If the new value of the semaphore variuable is negative, the process executing wait() or Down() is blocked, otherwise, the process continues execution.

Up() / Signal() :- It increments the value of semaphors variable by 1. 

The defination of Down() and Up() semaphore operations described above present the same problem. To overcome the need for busy waiting, we can modify the definition of the Down() or Wait() operation and UP() or Signal() operation as follows :-

   When a process executes the Down() operation and finds that the semaphore value is not positive, it must wait or can block itself. The block operation places a process into waiting queue. The process that is blocked should be restarted when some other process execute Up() operation. The process is restarted  by a wakeup() operation, which changes the process from the waiting state to the ready state. The process is the placed in the ready queue.

   ⇰  Types of Semaphores :-

  Semaphores can be classified into two types :-

 i> Counting Semaphores.

ii> Binary Semaphores.

i> Counting Semaphore :- Semaphores which allows integer value and have an unrestricted value domain. These are used to coordinate the resource access, where the semaphore count is the number of available resources. If the resources are added, semaphore count automatically incremented and if the resources are removed, the count is decremented.

ii> Binary Semaphores :- Semaphores which are similar to counting semaphores but their value is restricted to 0 and 1. The wait operation only works when the semaphore is 1 and the signal operation succeeds when semaphore is 0. 

   ⇰  Advantages of Semaphores :-
i> Semaphores allow only one process into the critical section. 
ii> They follow the mutual exclusion principle strictly.
iii> There is no resource wastage.
iv>  It is machine independent.

⇰  Disadvantages of Semaphores :-
 i> It is complicated so the implementation must in the correct order to prevent from deadlocks.
 ii>  Their use leads to loss of modularity.
iii> Semaphores donot checks about priority of process. Low priority processes may access the critical section first and high priority processes.

 Critical section problems
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