Improving parallel performance of ensemble learners for streaming data through data locality with mini-batching

Machine Learning techniques have been employed in virtually all domains in the past few years. New applications demand the ability to cope with dynamic environments like data streams with transient behavior. Such environments present new requirements like incrementally process incoming data instances in a single pass, under both memory and time constraints. Furthermore, prediction models often need to adapt to concept drifts observed in non-stationary data streams. Ensemble learning comprises a class of stream mining algorithms that achieved remarkable prediction performance in this scenario. Implemented as a set of (several) individual component classifiers whose predictions are combined to predict new incoming instances, ensembles are naturally amendable for task parallelism. Despite its relevance, an efficient implementation of ensemble algorithms is still challenging. For example, dynamic data structures used to model non-stationary data behavior and detect concept drifts cause inefficient memory usage patterns and poor cache memory performance in multi-core environments. In this paper, we propose a minibatching strategy which can significantly reduce cache misses and improve the performance of several ensemble algorithms for stream mining in multi-core environments. We assess our strategy on four different state-of-Art ensemble algorithms applying four widely used machine learning benchmark datasets with varied characteristics. Results from two different hardware show speedups of up to 5X on 8-core processors with ensembles of 100 and 150 learners. The benefits come at the cost of changes in predictive performances.