Performance Evaluation of OpenMP Applications with Nested Parallelism

Many existing OpenMP systems do not su ciently implement nested parallelism. This is supposedly because nested parallelism is believed to require a signi cant implementation e ort, incur a large overhead, or lack applications. This paper demonstrates Omni/ST, a simple and e cient implementation of OpenMP nested parallelism using StackThreads/MP, which is a ne-grain thread library. Thanks to StackThreads/MP, OpenMP parallel constructs are simply mapped onto thread creation primitives of StackThreads/MP, yet they are e ciently managed with a xed number of threads in the underlying thread package (e.g., Pthreads). Experimental results on Sun Ultra Enterprise 10000 with up to 60 processors show that overhead imposed by nested parallelism is very small (1-3% in ve out of six applications, and 8% for the other), and there is a signi cant scalability bene t for applications with nested parallelism.