Figure 66 Example of an OpenMP program that includes a critical section in Java

Generating Data Matrix 2d barcode in Java Figure 66 Example of an OpenMP program that includes a critical section
Figure 66 Example of an OpenMP program that includes a critical section
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#include <omph> #include <stdioh> #define N 1000 extern double big_computation(int, int); extern void consume_results(int, double, double *); int main() { double global_result[N]; #pragma omp parallel shared (global_result) { double local_result; int I; int ID = omp_get_thread_num(); // set a thread ID #pragma omp for for(i=0;i<N;i++){ local_result = big_computation(ID, i); // carry out the UE's work #pragma omp critical { consume_results(ID, local_result, global_result); } } } return 0; }
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OpenMP: mutual exclusion
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In OpenMP, mutual exclusion is most easily accomplished using a construct called critical section An example of using the critical section construct for OpenMP is shown in Fig 66 In this program, a team of threads is created to cooperatively carry out a series of N calls to big_computation() The pragma
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#pragma omp for
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is the OpenMP construct that tells the compiler to distribute the iterations of the loop among a team of threads After big_computation() is complete, the results need to be combined into the global data structure that will hold the result While we don't show the code, assume the update within consume_results() can be done in any order, but the update by one thread must complete before another thread can execute an update The critical pragma accomplishes this for us The first thread to finish its big_computation() enters the enclosed block of code and calls consume_results() If a thread arrives at the top of the critical section block while another thread is processing the block, it waits until the prior thread is finished The critical section is an expensive synchronization operation Upon entry to a critical section, a
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thread flushes all visible variables to ensure that a consistent view of the memory is seen inside the critical section At the end of the critical section, we need any memory updates occurring within the critical section to be visible to the other threads in the team, so a second flush of all thread visible variables is required The critical section construct is not only expensive, it is not very general It cannot be used among subsets of threads within a team or to provide mutual exclusion between different blocks of code Thus, the OpenMP API provides a lower level and more flexible construct for mutual exclusion called a lock Locks in different shared memory APIs tend to be similar The programmer declares the lock and initializes it Only one thread at a time is allowed to hold the lock Other threads trying to acquire the lock will block Blocking while waiting for a lock is inefficient, so many lock APIs allow threads to test a lock's availability without trying to acquire it Thus, a thread can opt to do useful work and come back to attempt to acquire the lock later Consider the use of locks in OpenMP The example in Fig 67 shows use of a simple lock to make sure only one thread at a time attempts to write to standard output The program first declares the lock to be of type omp_lock_t This is an opaque object, meaning that as long as the programmer only manipulates lock objects through the OpenMP runtime library, the programmer can safely work with the locks without ever considering the details of the lock type The lock is then initialized with a call to omp_init_lock
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Figure 67 Example of using locks in OpenMP #include <omph> #include <stdioh> int main() { omp_lock_t lock; // declare the lock using the lock // type defined in omph omp_set_num_threads(5); omp_init_lock (&lock); // initialize the lock #pragma omp parallel shared (lock) { int id = omp_get_thread_num(); omp_set_lock (&lock); printf("\n only thread %d can do this print\n",id); omp_unset_lock (&lock); } }
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To be of any use for managing concurrency, a lock must be shared between the individual members of the team of threads Thus, the lock is defined prior to the parallel pragma and declared as a shared variable (which is the default, but we call the shared clause in this example just to emphasize the point) Inside the parallel region, a thread can set the lock, which causes any other threads attempting to set the same lock to block and wait until the lock has been unset Unlike the OpenMP critical section, an OpenMP lock does not define a memory fence If the
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operations carried out by the thread holding the lock depend on any values from other threads, the program might fail because the memory might not be consistent Managing memory consistency can be difficult for many programmers, so most OpenMP programmers opt to avoid locks and use the much safer critical sections
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