By yuanlin on Jun 29, 2006
Sun's OpenMP implementation supports true nested parallel regions - when nested parallelism is enabled, the inner parallel region can be executed by multiple threads concurrently.
We provide an environment variable called SUNW_MP_MAX_POOL_THREADS for users to control the total number of OpenMP slave threads in a process.
For example, if you have want a maximum of 16 threads to be used for a nest of parallel regions in your program, you can set SUNW_MP_MAX_POOL_THREADS to 15. That's 15 slave threads (some of them may become masters in inner parallel regions) plus one user thread which is the master thread for the out-most parallel region.
Why did we design an environment variable like SUNW_MP_MAX_NUM_THREADS so that a user can set it to 16 in the above example? Intel's implementation has KMP_ALL_THREADS and KMP_MAX_THREADS which do that.
Well, we were trying to have a scheme that works on more general cases, not just pure OpenMP codes. In particular, we think our scheme works better than others for mixed pthread and OpenMP thread code. The pool defines a set of threads that can be used as OpenMP slave threads. If the program has two pthreads and both will create a team, then both will try to grab slave threads from the same pool. The env var SUNW_MP_MAX_POOL_THREADS was NOT designed for users to control the total number of threads in a process. We cannot control that because of the use of pthreads. The env var is designed for users to control the total number of OpenMP slave threads.