OpenMP Loop Scheduling Revisited: Making a Case for More Schedules

TL;DR

This paper reviews the limitations of current OpenMP loop scheduling techniques and demonstrates that incorporating more diverse self-scheduling methods can improve performance across various applications and hardware platforms.

Contribution

It highlights the need for additional OpenMP scheduling options and evaluates the potential of self-scheduling techniques to address performance degradation.

Findings

Self-scheduling techniques can reduce load imbalance.

Existing schedules are insufficient for all application types.

More diverse schedules improve performance on heterogeneous platforms.

Abstract

In light of continued advances in loop scheduling, this work revisits the OpenMP loop scheduling by outlining the current state of the art in loop scheduling and presenting evidence that the existing OpenMP schedules are insufficient for all combinations of applications, systems, and their characteristics. A review of the state of the art shows that due to the specifics of the parallel applications, the variety of computing platforms, and the numerous performance degradation factors, no single loop scheduling technique can be a 'one-fits-all' solution to effectively optimize the performance of all parallel applications in all situations. The impact of irregularity in computational workloads and hardware systems, including operating system noise, on the performance of parallel applications, results in performance loss and has often been neglected in loop scheduling research, in particular, the context of OpenMP schedules. Existing dynamic loop self-scheduling techniques, such as trapezoid self-scheduling, factoring, and weighted factoring, offer an unexplored potential to alleviate this degradation in OpenMP due to the fact that they explicitly target the minimization of load imbalance and scheduling overhead. Through theoretical and experimental evaluation, this work shows that these loop self-scheduling methods provide a benefit in the context of OpenMP. In conclusion, OpenMP must include more schedules to offer a broader performance coverage of applications executing on an increasing variety of heterogeneous shared memory computing platforms.