Semaphore
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This document discusses semaphores, which are integer variables that coordinate access to shared resources. It describes counting semaphores, which allow multiple processes to access a critical section simultaneously up to a set limit, and binary semaphores, which only permit one process at a time. Key differences are that counting semaphores can have any integer value while binary semaphores are limited to 0 or 1, and counting semaphores allow multiple slots while binary semaphores provide strict mutual exclusion. Limitations of semaphores include potential priority inversion issues and deadlocks if not used properly.
Semaphore
- 2. Contents • What is Semaphore • Types of Semaphore • Counting Semaphore • Binary Semaphore • Counting Semaphore vs Binary Semaphore • Limitations of Semaphore 2
- 3. What is Semaphore? • Semaphores are integer variables, stores the entire wake up calls • On which read, modify, update happens automatically in kernel mode • Needs support from OS to be implemented 3
- 4. Continue P(Semaphore s) { s = s - 1; if (s <= 0) { // add process to queue block(); } } V(Semaphore s) { s = s + 1; if (s <= 0) { // remove process p from queue wakeup(p); } } 4
- 5. Types Of Semaphore According to the demand of the situation • Counting Semaphore • Binary Semaphore or Mutex 5
- 6. Counting Semaphore • When we need to execute more than one process in critical section at the same time. • The value of counting semaphore at any point of time indicates that the maximum number of processes that can enter the critical section at the same time. 6
- 7. Example: • A counting semaphore was initialized to 12. then 10P(wait) and 4V signal operation was computed on this semaphore. 1. S= 12 (initially) 2. 10P wait 3. SS= S-P=2 4. Then signal 4V 5. So Total SS = 4+2 = 6 7
- 8. Binary Semaphore • Strictly provides mutual exclusion. • Rather than having more than 1 slot, we can only have one process in the critical section. • Have only two value 0 and 1 • Extended version of counting semaphore. 8
- 9. Counting Semaphore vs Binary Semaphore Counting Binary 1. No mutual exclusion. 1. Mutual exclusion 2. Any integer value 2. Value is 0 and 1 3. More than one slot 3. Only one slot 4. Provide set of Processes 4. Have mutual exclusion mechanism. 9
- 10. Limitations of Semaphore • Priority Inversion is a big limitation of semaphores. • Their use is not enforced, but is by convention only. • With improper use, a process may block indefinitely. Such a situation is called Deadlock. 10
- 11. Thank You 11