You want the OMP_SET_LOCK/OMP_UNSET_LOCK functions: https://hpc.llnl.gov/tuts/openMP/#OMP_SET_LOCK Basically: omp_lock_t writelock; omp_init_lock(&writelock); #pragma omp parallel for for ( i = 0; i < x; i++ ) { // some stuff omp_set_lock(&writelock); // one thread at a time stuff omp_unset_lock(&writelock); // some stuff } omp_destroy_lock(&writelock); Most locking routines such as pthreads semaphores and sysv semaphores work on that … Read more
As Ofir already wrote – if you try to read data from an object that some other thread is modyfying – you could get data in some inconsistent state. But – if you are sure the object is not being modified, you can of course read it from multiple threads. In general, the question you … Read more
First, let me introduce some terminology. A critical section (CS) is a sequence of instructions that can be executed by at most one process at the same time. When using critical sections, the code can be broken down into the following sections: // Some arbitrary code (such as initialization). EnterCriticalSection(cs); // The code that constitutes … Read more
Just imagine, how the processor would execute a typical spin-wait loop: 1 Spin_Lock: 2 CMP lockvar, 0 ; Check if lock is free 3 JE Get_Lock 4 JMP Spin_Lock 5 Get_Lock: After a few iterations the branch predictor will predict that the conditional branch (3) will never be taken and the pipeline will fill with … Read more
The effect on g_qCount is the same, but what’s done is different. An OpenMP critical section is completely general – it can surround any arbitrary block of code. You pay for that generality, however, by incurring significant overhead every time a thread enters and exits the critical section (on top of the inherent cost of … Read more