LeWI: A Runtime Balancing Algorithm for Nested Parallelism

We present LeWI: a novel load balancing algorithm, that can balance applications with very different patterns of imbalance. Our algorithm can balance fine grain imbalances, non iterative applications and applications with irregular imbalance. To achieve this LeWI reassigns the computational resources of blocked processes to other processes more loaded. We have implemented LeWI within DLB a Dynamic Load Balancing Library developed by us. DLB helps parallel programming models to make the most of the computational power available with the minimum effort. It solves the imbalance among processes in applications with two levels of parallelism using the malleability of the inner level. The performance evaluation shows that LeWI, the novel balancing algorithm we are presenting in this paper, together with DLB is able to improve the performance of a different range of unbalanced applications and when applied to well balanced applications it does not introduce significant overhead. Therefore we present a mechanism that can be used with any hybrid application without needing a programmer to analyze the application nor modify it.