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5
Concurrent Systems/notes/4b - Monitors.md
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Semaphores are hard to use in practice because quite low level Monitors provide an easier definition of concurrent objects at the level of Prog. Lang. Semaphores are hard to use in practice because quite low level Monitors provide an easier definition of concurrent objects at the level of Prog. Lang.
>[!note] MUTEX >[!note] MUTEX
>Every operation in a monitor is done with mutual exc >Monitors provide mutual exclusion. Operations in a monitor are done with mutual exclusion.
Inter-process synchronization is done through *conditions*, which are objects that provide the following operations: Inter-process synchronization is done through *conditions*, which are objects that provide the following operations:
- ***wait*:** the invoking process suspends, enters into the condition's queue, and releases the mutex on the monitor - ***wait*:** the invoking process suspends, enters into the condition's queue, and releases the mutex on the monitor
@ -34,7 +34,8 @@ Of course, `signal()` will have no effect on the last process who calls it.
- A semaphore MUTEX init at 1 (to guarantee mutex in the monitor) - A semaphore MUTEX init at 1 (to guarantee mutex in the monitor)
- For every condition C, a semaphore SEMC init at 0 and an integer $N_{C}$ init at 0 (to store and count the number of suspended processes on the given condition) - For every condition C, a semaphore SEMC init at 0 and an integer $N_{C}$ init at 0 (to store and count the number of suspended processes on the given condition)
- A semaphore PRIO init at 0 and an integer $N_{PR}$ init at 0 (to store and count the number of processes that have performed a signal, and so have priority to re-enter the monitor) - A semaphore PRIO init at 0 and an integer $N_{PR}$ init at 0 (to store and count the number of processes that have performed a signal, and so have priority to re-enter the monitor)
>[!note]Operations
>[!note] Operations
>Every monitor operation starts with `MUTEX.down()` and ends with `if NPR > 0 then PRIO.up() else MUTEX.up()` >Every monitor operation starts with `MUTEX.down()` and ends with `if NPR > 0 then PRIO.up() else MUTEX.up()`
``` ```