Leveraging asynchronous parallel computing to produce simple genetic programming computational models

Traditionally, reducing complexity in Machine Learning promises benefits such as less overfitting. However, complexity control in Genetic Programming (GP) often means reducing the sizes of the evolving expressions, and past literature shows that size reduction does not necessarily reduce overfitting. In fact, whether size consistently represents complexity is itself debatable. Therefore, this paper proposes evaluation time of an evolving model - the computational time required to evaluate a model on data - as the estimate of its complexity. Evaluation time depends upon the size, but crucially also on the composition of an evolving model, and can thus distil its underlying complexity. To discourage complexity, this paper takes an innovative approach that asynchronously evaluates multiple models concurrently. These models race to their completion; thus, those models that finish earlier, join the population earlier to breed further in a steady-state fashion. Thus, the computationally simpler models, even if less accurate, get further chances to evolve before the more accurate yet expensive models join the population. Crucially, since evaluation times vary from one execution to another, this paper also shows how to significantly minimise this variation. The paper compares the proposed method on six challenging symbolic regression problems with both standard GP and GP with an effective bloat control method. The results demonstrated that the proposed asynchronous parallel GP (APGP) indeed produces individuals that are smaller, faster and more accurate than those in standard GP. While GP with bloat control (GP+BC) produced smaller individuals, it did so at the cost of lower accuracy than APGP both on training and test data, thus questioning the overall benefits of bloat control. Also, while APGP took the fewest evaluations to match the training accuracy of GP, GP+BC took the most. These results, and the portability of evaluation time as an estimate of complexity encourage further experimentation and fine-tuning of this hitherto unexplored style of GP.