PhD Oral Exam - Martin Khannouz, Computer Science

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Supervised learning algorithms generally assume the availability of enough memory to store data models during the training and test phases. However, this assumption is unrealistic when data comes in the form of infinite data streams, or when learning algorithms are deployed on devices with reduced amounts of memory. In this manuscript, we investigate the use of data stream classification methods under memory constraints. Our investigation consists of three steps: a benchmark of models, an update of a model, and an optimization of a trade-off. We evaluate data stream classification models with different criteria such as classification performance or resource usage. The benchmark reveals that the Mondrian forest, despite having state-of-the-art classification performance with unlimited memory, is impacted by a low memory limit. We then adapt the online Mondrian forest classification algorithm to work with memory constraints on data streams. In particular, we design five out-of-memory strategies to update Mondrian trees with new data points when the memory limit is reached. We evaluate our algorithms on a variety of real and simulated datasets, and we conclude with recommendations on their use in different situations: the Extend Node strategy appears as the best out-of-memory strategy in all configurations. We identify that the memory-constrained brings a trade-off between the Mondrian forest size and its tree depth. We design an adjusting algorithm to optimize the forest size to the data stream and the memory limit and we evaluate this algorithm on similar datasets. All our methods are implemented in the OrpailleCC open-source library and are ready to be used on embedded systems and connected objects. Overall, the contributions significantly improve the performance of the Mondrian forest under memory constraints.