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Title: A Copula approach for hyperparameter transfer learning

Paper content:
Bayesian optimization (BO) is a popular methodology to tune the hyperparameters of expensive black-box functions.
Despite its success, standard BO focuses on a single task at a time and is not designed to leverage information from related functions, such as tuning performance metrics of the same algorithm across multiple datasets.
In this work, we introduce a novel approach to achieve transfer learning across different datasets as well as different metrics.
The main idea is to regress the mapping from hyperparameter to metric quantiles with a semi-parametric Gaussian Copula distribution, which provides robustness against different scales or outliers that can occur in different tasks.
We introduce two methods to leverage this estimation: a Thompson sampling strategy as well as a Gaussian Copula process using such quantile estimate as a prior.
We show that these strategies can combine the estimation of multiple metrics such as runtime and accuracy, steering the optimization toward cheaper hyperparameters for the same level of accuracy.
Experiments on an extensive set of hyperparameter tuning tasks demonstrate significant improvements over state-of-the-art methods.
Tuning complex machine learning models such as deep neural networks can be a daunting task.
Object detection or language understanding models often rely on deep neural networks with many tunable hyperparameters, and automatic hyperparameter optimization (HPO) techniques such as Bayesian optimization (BO) are critical to find the good hyperparameters in short time.
BO addresses the black-box optimization problem by placing a probabilistic model on the function to minimize (e.g., the mapping of neural network hyperparameters to a validation loss), and determine which hyperparameters to evaluate next by trading off exploration and exploitation through an acquisition function.
While traditional BO focuses on each problem in isolation, recent years have seen a surge of interest in transfer learning for HPO.
The key idea is to exploit evaluations from previous, related tasks (e.g., the same neural network tuned on multiple datasets) to further speed up the hyperparameter search.
A central challenge of hyperparameter transfer learning is that different tasks typically have different scales, varying noise levels, and possibly contain outliers, making it hard to learn a joint model.
In this work, we show how a semi-parametric Gaussian Copula can be leveraged to learn a joint prior across datasets in such a way that scale issues vanish.
We then demonstrate how such prior estimate can be used to transfer information across tasks and objectives.
We propose two HPO strategies: a Copula Thompson Sampling and a Gaussian Copula Process.
We show that these approaches can jointly model several objectives with potentially different scales, such as validation error and compute time, without requiring processing.
We demonstrate significant speed-ups over a number of baselines in extensive experiments.
The paper is organized as follows.
Section 2 reviews related work on transfer learning for HPO.
Section 3 introduces Copula regression, the building block for the HPO strategies we propose in Section 4.
Specifically, we show how Copula regression can be applied to design two HPO strategies, one based on Thompson sampling and an alternative GP-based approach.
Experimental results are given in Section 5 where we evaluate both approaches against state-of-the-art methods on three algorithms.
Finally, Section 6 outlines conclusions and further developments.
We introduced a new class of methods to accelerate hyperparameter optimization by exploiting evaluations from previous tasks.
The key idea was to leverage a semi-parametric Gaussian Copula prior, using it to account for the different scale and noise levels across tasks.
Experiments showed that we considerably outperform standard approaches to BO, and deal with heterogeneous tasks more robustly compared to a number of transfer learning approaches recently proposed in the literature.
Finally, we showed that our approach can seamlessly combine multiple objectives, such as accuracy and runtime, further speeding up the search of good hyperparameter configurations.
A number of directions for future work are open.
First, we could combine our Copula-based HPO strategies with Hyperband-style optimizers (Li et al., 2016) .
In addition, we could generalize our approach to deal with settings in which related problems are not limited to the same algorithm run over different datasets.
This would allow for different hyperparameter dimensions across tasks, or perform transfer learning across different black-boxes.

assistant: text: We show how using semi-parametric prior estimations can speed up HPO significantly across datasets and metrics.