如何有条件地终止OpenMP中的并行区域？

Question

I have an OpenMP with C++ program. There are parallel regions that contain #pragma omp task inside a parallel region. Now, I would like to know how to terminate the parallel region depending on a condition that any of the running threads meet.

#pragma omp parallel
{
 #pragma omp task
 {
   //upon reaching a condition i would like to break out of the parallel region. (all threads should exit this parallel region)
 }

}

Answer 1

You can't terminate a parallel construct prematurely. OpenMP has no construct for this and it specifies that parallel regions may have only one exit point (so no branching out of the region...).

I think the only (sane and portable) way to accomplish that is to have a variable which indicates if the work is finished and have the threads check that variable regularly (using atomic instructions and/or flushes to ensure correct visiblity). If the variable indicates that the work is done the threads can skip their remaining work (by putting the remaining work in an if body which isn't branched into if the work is done).

It might be possible to write system specific code, which suspends the other threads and sets them to the end of the block (eg manipulating stack and instructionpointers...) but that doesn't seem very advisable (meaning it's probably very brittle).

If you'd tell us a bit more about what you are trying to do (and why you need this), it might be easier to help you (eg by prosposing a design which doesn't need to do this).