Efficiency and expressiveness in UW-OpenMP

Abstract

OpenMP uses the efficient ‘team of workers’ model, where workers are given chunks of tasks (iterations of a parallel-for-loop, or sections in a parallel-sections block) to execute, and worker (not tasks) can be synchronized using barriers. Thus, OpenMP restricts the invocation of barriers in these tasks; as otherwise, the behavior of the program would be dependent on the number of runtime workers. To address such a restriction which can adversely impact programmability and readability, Aloor and Nandivada proposed UW-OpenMP by taking inspiration from the more intuitive interaction of tasks and barriers in newer task parallel languages like X10, HJ, Chapel and so on. UW-OpenMP gives the programmer an impression that each parallel task is executed by a unique worker, and importantly these parallel tasks can be synchronized using a barrier construct. Though UW-OpenMP is a useful extension of OpenMP (more expressive and efficient), it does not admit barriers within recursive functions invoked from parallel-for-loops, because of the inherent challenges in handing them. In this paper, we extend UW-OpenMP (we call it UWOmp++) to address this challenging limitation and in the process also realize more efficient programs.