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== Parallel Execution Models ==
In the modern age, parallel programming is an increasingly important topic. Parallel execution models tend to be complex because they involve multiple timelines. Parallel execution models necessarily include the behavior of [[Synchronization_construct| synchronization constructs]]. A synchronization construct has the effect of establishing an ordering between activities in one timeline relative to activities in another timeline.
For example, a common synchronization construct is the lock. Consider one timeline. The timeline has a point at which it executes the "gain ownership of the lock" synchronization construct. In Posix threads this would be pthread_mutex_lock(&myMutex). In Java this would be lock.lock(). In both cases, the timeline is called a thread. The C and Java execution models are sequential, and they state that the timeline has activities that come before the call to "gain ownership of the lock", and activities that come after the call. Likewise there is a "give up ownership of the lock" operation. In C this would be pthread_mutex_unlock(&myMutex). In Java this would be lock.unlock(). Again, the execution models C and Java define that one group of statements is executed before ownership of the lock is given up, and another group of statements is executed after ownership of the lock is given up.
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