A Microbenchmark Study of OpenMP Overheads under Nested Parallelism

In this work we present a microbenchmark methodology for assessing the overheads associated with nested parallelism in OpenMP. Our techniques are based on extensions to the well known EPCC microbenchmark suite that allow measuring the overheads of OpenMP constructs when they are effected in inner levels of parallelism. The methodology is simple but powerful enough and has enabled us to gain interesting insight into problems related to implementing and supporting nested parallelism. We measure and compare a number of commercial and freeware compilation systems. Our general conclusion is that while nested parallelism is fortunately supported by many current implementations, the performance of this support is rather problematic. There seem to exist issues which have not yet been addressed effectively, as most OpenMP systems do not exhibit a graceful reaction when made to execute inner levels of concurrency.