what happens when omp num_threads (more than 1) and parallel for with only 1 loop is present-CodePudding

l_thread = 4;
max = 1; //4
#pragma omp parallel for num_threads(l_thread) private(i)
 for(i=0;i<max;i  )
   ;//some operation

in this case, 4 threads will be created by omp. I want to know if the for loop which loops only 1 time(in the case), will be taken only by one of 4 threads right? and other threads will be idle state ? and in this case i am seeing cpu usage of 4 threads are nearly same. What might be the reason? only one thread should be high and others must be low right?

CodePudding user response：

Your take on this is correct. If max=1 and you have more than one thread, thread 0 will execute the single loop iteration and the other threads will wait at the end of the parallel region for thread 0 to catch up. The reason you're seeing the n-1 other threads causing load on the system is because they spin-wait at the end of regions, because that is much faster when the threads have to wake up and notice that the parallel work (or in your case: not so parallel work :-)) is completed.

You can change this behavior via the OMP_WAIT_POLICY environment variable. See the OpenMP specification for a full description.