4.4 Some Tips on Using Nested Parallelism

• Nesting parallel regions provides an immediate way to allow more threads to participate in the computation.

  For example, suppose you have a program that contains two levels of parallelism and the degree of parallelism at each level is 2. Also, suppose your system has four cpus and you want use all four CPUs to speed up the execution of this program. Just parallelizing any one level will use only two CPUs. You want to parallelize both levels.

• Creating nested parallel regions adds overhead. If there is enough parallelism at the outer level and the load is balanced, generally it will be more efficient to use all the threads at the outer level of the computation than to create nested parallel regions at the inner levels.

  For example, suppose you have a program that contains two levels of parallelism. The degree of parallelism at the outer level is 4 and the load is balanced. You have a system with four CPUs and want to use all four CPUs to speed up the execution of this program. Then, in general, using all 4 threads for the outer level could yield better performance than using 2 threads for the outer parallel region, and using the other 2 threads as helper threads for the inner parallel regions.

• Nesting parallel regions can easily create too many threads and oversubscribe the system. Set OMP_THREAD_LIMIT and OMP_MAX_ACTIVE_LEVELS appropriately to limit the number of threads in use and prevent runaway oversubscription.