Typically it is needed only by programmers building their own low level synchronization primitives in Java

Generation DataMatrix in Java Typically it is needed only by programmers building their own low level synchronization primitives
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critical implements a critical section for mutual exclusion In other words, only one thread at a time will execute the structured block within a critical section The other threads will wait their turn at the top of the construct The syntax of a critical section is
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#pragma omp critical [(name)] { a structured block }
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where name is an identifier that can be used to support disjoint sets of critical sections A critical section implies a call to flush on entry to and on exit from the critical section
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barrier provides a synchronization point at which the threads wait until every member of the team has arrived before any threads continue The syntax of a barrier is
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#pragma omp barrier
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A barrier can be added explicitly, but it is also implied where it makes sense (such as at the end of parallel or worksharing constructs) A barrier implies a flush Critical sections, barriers, and flushes are discussed further in the Implementation Mechanisms design space Returning to our example in Fig A5, we can safely include the call to the combine() routine inside the parallel loop if we enforce mutual exclusion We will do this with the critical construct as shown in Fig A11 Notice that we had to create a private copy of the variable res to prevent conflicts between iterations of the loop
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Figure A11 Parallel version of the program in Fig A5 In this case, however, we assume that the calls to combine() can occur in any order as long as only one thread at a time executes the function This is enforced with the critical construct #include <stdioh> #include <omph> #define N 1000 extern void combine(double,double); extern double big_comp(int); int main() { int i; double answer, res; answer = 00; #pragma omp parallel for private (res) for (i=O;i<N;i++){ res = big_comp(i); #pragma omp critical combine(answer,res); } printf("%f\n", answer); }
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For most programmers, the high level synchronization constructs in OpenMP are sufficient There are cases, however, where the high level synchronization constructs cannot be used Two common cases where this situation comes up are the following
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The synchronization protocols required by a problem cannot be expressed with OpenMP's high level synchronization constructs The parallel overhead incurred by OpenMP's high level synchronization constructs is too large
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To address these problems, the OpenMP runtime library includes low level synchronization functions that provide a lock capability The prototypes for lock functions are included in the omp h include file The locks use an opaque data type omp_lock_t defined in the omph include file The key functions follow
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void omp_init_lock(omp_lock_t *lock) Initialize the lock void omp_destroy_lock(omp_lock_t *lock) Destroy the lock, thereby freeing any memory associated with the lock void omp_set_lock(omp_lock_t *lock) Set or acquire the lock If the lock is free, then the thread calling omp_set_lock() will acquire the lock and continue If the lock is held by another thread, the thread calling omp_set_lock() will wait on the call until the lock is available void omp_unset_lock(omp_lock_t *lock) Unset or release the lock so some other thread can acquire it int omp_test_lock(omp_lock_t *lock) Test or inquire if the lock is available If it is, then the thread calling this function will acquire the lock and continue The test and acquisition of the lock is done atomically If it is not, the function returns false (nonzero) and the calling thread continues This function is used so a thread can do useful work while waiting for a lock
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The lock functions guarantee that the lock variable itself is consistently updated between threads, but do not imply a flush of other variables Therefore, programmers using locks must call flush explicitly as needed An example of a program using OpenMP locks is shown in Fig A12 The program declares and then initializes the lock variables at the beginning of the program Because this occurs prior to the parallel region, the lock variables are shared between the threads Inside the parallel region, the first lock is used to make sure only one thread at a time tries to print a message to the standard output The lock is needed to ensure that the two printf statements are executed together and not interleaved with those of other threads The second lock is used to ensure that only one thread at a time executes the go_for_it() function, but this time, the omp_test_lock() function is used so a thread can do useful work while waiting for the lock After the parallel region completes, the memory associated with the locks is freed by a call to omp_lock_destroy()
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