Balancing Average and Worst-case Accuracy in Multitask Learning
This work addresses the challenge of ensuring fair performance across all tasks in multitask learning for practitioners in fields like computer vision and NLP, though it is incremental as it builds on existing DRO techniques.
The paper tackled the problem of improving worst-case accuracy in multitask learning, which can be biased by easy tasks, by proposing Lookahead-DRO (L-DRO), a method that dynamically re-weights task losses to anticipate interactions, and it achieved a better trade-off between average and worst-case accuracy on benchmarks like CIFAR-100 and multilingual language modeling with little computational overhead.
When training and evaluating machine learning models on a large number of tasks, it is important to not only look at average task accuracy -- which may be biased by easy or redundant tasks -- but also worst-case accuracy (i.e. the performance on the task with the lowest accuracy). In this work, we show how to use techniques from the distributionally robust optimization (DRO) literature to improve worst-case performance in multitask learning. We highlight several failure cases of DRO when applied off-the-shelf and present an improved method, Lookahead-DRO (L-DRO), which mitigates these issues. The core idea of L-DRO is to anticipate the interaction between tasks during training in order to choose a dynamic re-weighting of the various task losses, which will (i) lead to minimal worst-case loss and (ii) train on as many tasks as possible. After demonstrating the efficacy of L-DRO on a small controlled synthetic setting, we evaluate it on two realistic benchmarks: a multitask version of the CIFAR-100 image classification dataset and a large-scale multilingual language modeling experiment. Our empirical results show that L-DRO achieves a better trade-off between average and worst-case accuracy with little computational overhead compared to several strong baselines.