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Monitors

Monitors. A monitor is a higher level construct for synchronizing multiple threads ’ access to a common code segment Can implement within an object (Monitor Object Pattern) E.g., built with mutexes, condition variables (and threads) At most one thread may be active within a monitor

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Monitors

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  1. Monitors • A monitor is a higher level construct for synchronizing multiple threads’ access to a common code segment • Can implement within an object (Monitor Object Pattern) • E.g., built with mutexes, condition variables (and threads) • At most one thread may be active within a monitor • I.e., all other threads within the monitor must be suspended • Synchronization is based on a common shared lock • Active thread may (atomically) signal, release lock, suspend • Signaled thread then may (atomically) resume, acquire lock • Monitors introduce conditional scopes for concurrency • E.g., a consumer suspends unless a queue is non-empty • E.g., producers suspend if queue reaches “high-water-mark”, until queue again reaches “low-water-mark” condition

  2. Desired Internal Behavior notify Queue empty - wait Queue

  3. (Passive) Monitor Objects • “Monitor Object” Pattern Approach • Methods run in callers’ threads • Condition variables arbitrate use of a common shared lock • E.g., using a std::mutex, a std::unique_lock (must be able to unlock and re-lock it) and a std::condition_variable • Ensures incremental progress while avoiding race conditions • Threads wait on condition • Condition variable performs thread-safe lock/wait and wake/unlock operations • Thread released when it can proceed • E.g., when queue isn’t empty/full • Blocks caller until request can be handled, coordinates callers Client Proxy List (Monitor Object) add() Condition lookup() Lock

  4. Can notify_one() or notify_all() If it doesn’t matter which thread, just wake one up If all need to see if it’s their turn, wake all of them Can limit waiting time Use wait_for() to return after a specified interval Use wait_until() to return at a specified time Can pass a predicate to wait (or wait_*) method(s) Won’t return until predicate is satisfied (or call times out) Helps to avoid spurious wake cases A Few Variations

  5. Today’s Studio Exercises • We’ll code up ideas from Scott Chapter 12.4 to 12.4.3 • Again via C++11 concurrency/synchronization types/features • Looking at higher-level concurrency/synchronization ideas like condition variables being used to implement monitors • Today’s exercises are again in C++ • Please take advantage of the on-line tutorial and reference manual pages that are linked on the course web site • The provided Makefile also may be helpful • As always, please ask us for help as needed • When done send email with your answers to the cse425@seas.wustl.edu account with subject line “Concurrency Studio III”

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