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Ti new thread dummy do something with it ti create

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Unformatted text preview: // create N threads S-> P(); P(); for (i=1;i<n: i++) { // got resource! t[i] = new Thread (“dummy”); // do something with it t[i]->create (fn); S->V(); } … … } Locks Locks Locks can be used to ensure mutual exclusion in conflicting critical critical sections. • A lock is an object, a data item in memory. Methods: Lock::Acquire and Lock::Release. • • • • • • • A Thread pair calls to Acquire and Release. A Acquire before entering a critical section. R Release after leaving a critical section. R Between Acquire/Release, the lock is held. Acquire does not return until any previous holder releases. Waiting locks can spin (a spinlock) or block (a mutex). Mutex can be easily implemented by a binary semaphore Synchronization must be programmed carefully …. Condition Condition Variables Condition variables are a synchronization construct that provides provides several simple operations • The operation wait means that the process invoking this operation is suspended until another process invokes signal • The signal operation resumes exactly one suspended process. If no process is suspended, then the signal operation has no effect. • Additional operation is broadcast which wakes up all suspended processes/threads Condition Condition Variables Condition variables allow explicit event notification. Different interface from the book …. • associated with a mutex to prevent races on event conditions Condition::Wait(Lock*) Called with lock held: sleep, atomically releasing lock. Atomically reacquire lock before returning. Condition:: Signal(Lock*) Wake up one waiter, if any. Condition::Broadcast(Lock*) Wake up all waiters, if any. Semaphores Semaphores vs. Condition Variables Fundamental difference implicit vs. explicit synchronization conditions CV: lock arbitrary code if arbitrary condition … wait or signal … unlock Semaphore: if if arbitrary condition P() or V()? lock if arbitrary condition P() P() or V()? unlock Semaphores Semaphores vs. Condition Variables 1. V differs from Signal in that: • Signal has no effect if no thread is waiting on the condition. Condition variables are not variables! They have no value! • V has the same effect whether or not a thread is waiting. Semaphores retain a “memory” of calls to V. 2. P differs from Wait in that: • P checks the condition and blocks only if necessary. no need to recheck the condition after returning from P wait condition is defined internally, but is limited to a counter • Wait is explicit: it does not check the condition, ever. condition is defined externally and protected by integrated mutex BREAK BREAK CSci CSci 5103 Operating Systems Advanced Synchronization Barriers Barriers This is an important...
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