Example of Semaphore Use in DS

Now consider a distributed system
    with three running processes

• Process 1
• Process 2
• Process 3

Each of the processes has access
    to a shared semaphore, S

Each process has a critical section
    controlled by S

Assume the semaphore/lock is controlled
    by a centralized lock manager

Initially S = 1
    with no processes are in their critical section

Steps:

1. Process 3 requests its critical section
       Since S = 1, S !<= 0
           So S is decremented by 1
       making S = 0
   and
2. Process 3 is granted access to its critical section

3. Process 1 requests its critical section
       But S <= 0, as S = 0
   so
4. So Process 1 is is placed on a queue
       until S > 0

5. Process 2 requests its critical section
       But S <= 0, as S = 0
   so
6. Process 2 is queued
       until S > 0

7. Process 3 finishes its critical section
       So S is incremented by 1
           making S = 1
       releasing access to the critical section

8. The central manager checks the semaphore value S
       Since S = 1, S !<= 0
           So S is decremented by 1
       making S = 0
   and Process 1 is granted access to its critical section
       by the central manager

9. Process 1 finishes its critical section
       So S is incremented by 1
           making S = 1
       releasing access to the critical section

10. The central manager checks the semaphore value S
        Since S = 1, S !<= 0
            So S is decremented by 1
        making S = 0
    and Process 2 is granted access to its critical section
        by the central manager

11. Process 2 finishes its critical section
        So S is incremented by 1
            making S = 1
        releasing access to the critical section

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Comments:
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