A Context-Aware Primitive for Nested Recursive Parallelism

Nested recursive parallel applications constitute an important super-class of conventional, flat parallel codes. For this class, parallel libraries utilizing the concept of tasks have been widely adapted. However, the provided abstract task creation and synchronization interfaces force corresponding implementations to focus their attention to individual task creation and synchronization points – unaware of their relation to each other – thereby losing optimization potential. Within this paper, we present a novel interface for task level parallelism, enabling implementations to grasp and manipulate the context of task creation and synchronization points – in particular for nested recursive parallelism. Furthermore, as a concrete application, we demonstrate the interface’s capability to reduce parallel overhead within applications based on a reference implementation utilizing C++14 template meta programming techniques to synthesize multiple versions of a parallel task during the compilation process. To demonstrate its effectiveness, we evaluate the impact of our approach on the performance of a series of eight task parallel benchmarks. For those, our approach achieves substantial speed-ups over state of the art solutions, in particular for use cases exhibiting fine grained tasks.