So I've looked around online for some time to no avail. I'm new to using OpenMP and so not sure of the terminology here, but is there a way to figure out a specific machine's mapping from OMPThread (given by omp_get_thread_num();) and the physical cores on which the threads will run?
Also I was interested in how exactly OMP assigned threads, for example is thread 0 always going to run in the same location when the same code is run on the same machine? Thanks.
Typically, the OS takes care of assigning threads to cores, including with OpenMP. This is by design, and a good thing - you normally would want the OS to be able to move a thread across cores (transparently to your application) as required, since it will interrupt your application at times.
Certain operating system APIs will allow thread affinity to be set. For example, on Windows, you can use SetThreadAffinityMask to force a thread onto a specific core.
Most of the time Reed is correct, OpenMP doesn't care about the assignment of threads to cores (or processors). However, because of things like cache reuse and data locality we have found that there are many cases where having the threads assigned to cores increases the performance of OpenMP. Therefore if you look at most OpenMP implementations, you will find that there is usually some environment variable that can be set to "bind" threads to cores. The OpenMP ARB has not yet specified any "standard" way of doing this, so at this time it is left up to an OpenMP implementation to decide if and how this should be done. There has been a great deal of discussion about whether this should be included in the OpenMP spec or not and if so how it could best be done.
