Comparison of OpenMP 3.0 and Other Task Parallel Frameworks on Unbalanced Task Graphs

The UTS benchmark is used to evaluate the expression and performance of task parallelism in OpenMP 3.0 as implemented in a number of recently released compilers and run-time systems. UTS performs parallel search of an irregular and unpredictable search space, as arises, e.g., in combinatorial optimization problems. As such UTS presents a highly unbalanced task graph that challenges scheduling, load balancing, termination detection, and task coarsening strategies. Expressiveness and scalability are compared for OpenMP 3.0, Cilk, Cilk++, Intel Thread Building Blocks, as well as an OpenMP implementation of the benchmark without tasks that performs all scheduling, load balancing, and termination detection explicitly. Current OpenMP 3.0 run time implementations generally exhibit poor behavior on the UTS benchmark. We identify inadequate load balancing strategies and overhead costs as primary factors contributing to poor performance and scalability.