![]() The wait() and signal() operation modifies the value of the semaphore S indivisibly. The value of semaphore S can be modified only by two functions wait() and signal() apart from initialization. ![]() This integer variable S is initialized to the number of resources present in the system. Semaphore does not have condition variables.īeing a process synchronization tool, Semaphore is an integer variable S. ![]() When any process wants to access the shared variables in the monitor, it needs to access it through the procedures. When any process access the shared resources it perform wait() operation on S and when it releases the shared resources it performs signal() operation on S. The Monitor type contains shared variables and the set of procedures that operate on the shared variable. The value of Semaphore S indicates the number of shared resources availabe in the system In this article, we will discuss the differences between semaphore and monitor with the help of comparison chart shown below. On the other hand, the Monitor type is an abstract data type whose construct allow one process to get activate at one time. Dijkstra to thank to Semaphore safety property: the semaphore value is always greater than or equal to 0 Ding Yuan, ECE344 Operating SystemĪssociated with each semaphore is a queue of waiting processes/threads When P() is called by a thread: If semaphore is open (> 0), thread continues If semaphore is closed, thread blocks on queue Then V() opens the semaphore: If a thread is waiting on the queue, the thread is unblocked What if multiple threads are waiting on the queue? If no threads are waiting on the queue, the signal is remembered for the next thread In other words, V() has “history” (c.f., condition vars later) This “history” is a counter Ding Yuan, ECE344 Operating SystemĦ Semaphores in OS161 P(sem) Ding Yuan, ECE344 Operating Systemġ7 Problem: Starvation What if a writer is waiting, but readers keep coming, the writer is starved If you are interested, think how to solve this problem Ding Yuan, ECE344 Operating Systemġ8 Semaphore Questions Are there any problems that can be solved with counting semaphores that cannot be solved with mutex semaphores? If a system provides only mutex semaphores, can you use it to implement a counting semaphores? When to use counting semaphore? Problem needs a counter The maximum value is known (bounded) Ding Yuan, ECE344 Operating SystemĮxample: Thread 1: P(S) P(Q). We looked at using locks to provide mutual exclusion Locks work, but they have some drawbacks when critical regions are long Spinlocks – inefficient Disabling interrupts – can miss or delay important events Instead, we want synchronization mechanisms that Block waiters Leave interrupts enabled inside the critical section Look at two common high-level mechanisms Semaphores: binary (mutex) and counting Monitors: mutexes and condition variables Use them to solve common synchronization problems Ding Yuan, ECE344 Operating SystemĤ Semaphores Semaphores are an abstract data type that provide mutual exclusion to critical region Semaphores can also be used as atomic counters More later Semaphores are integers that support two operations: wait(semaphore): decrement, block until semaphore is open Also P(), after the Dutch word for test, or down() signal(semaphore): increment, allow another thread to enter Also V() after the Dutch word for increment, or up() That's it! No other operations – not even just reading its value – exist P and V are probably the most unintuitive names you encounter in this course and you have Edsger W. ![]() Operating Systems ECE344 Lecture 6: Synchronization (II) – Semaphores and Monitors Ding YuanĢ Review of last lecture Goal: Use mutual exclusion to protect critical sections of code that access shared resources Method: Use locks (spinlocks or disable interrupts) Problem: Critical sections can be long Spinlocks: Threads waiting to acquire lock spin in test-and-set loop Wastes CPU cycles Longer the CS, the longer the spin Greater the chance for lock holder to be interrupted Disabling Interrupts: Should not disable interrupts for long periods of time Can miss or delay important events (e.g., timer, I/O) acquire(lock) … Critical section release(lock) Ding Yuan, ECE344 Operating System 1 Lecture 6: Synchronization (II) – Semaphores and Monitors
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