Tree-Structured Parzen Estimator Can Solve Black-Box Combinatorial Optimization More Efficiently

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Hauptverfasser: Abe, Kenshin, Wang, Yunzhuo, Watanabe, Shuhei
Format: Preprint
Veröffentlicht: 2025
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author Abe, Kenshin
Wang, Yunzhuo
Watanabe, Shuhei
author_facet Abe, Kenshin
Wang, Yunzhuo
Watanabe, Shuhei
contents Tree-structured Parzen estimator (TPE) is a versatile hyperparameter optimization (HPO) method supported by popular HPO tools. Since these HPO tools have been developed in line with the trend of deep learning (DL), the problem setups often used in the DL domain have been discussed for TPE such as multi-objective optimization and multi-fidelity optimization. However, the practical applications of HPO are not limited to DL, and black-box combinatorial optimization is actively utilized in some domains, e.g., chemistry and biology. As combinatorial optimization has been an untouched, yet very important, topic in TPE, we propose an efficient combinatorial optimization algorithm for TPE. In this paper, we first generalize the categorical kernel with the numerical kernel in TPE, enabling us to introduce a distance structure to the categorical kernel. Then we discuss modifications for the newly developed kernel to handle a large combinatorial search space. These modifications reduce the time complexity of the kernel calculation with respect to the size of a combinatorial search space. In the experiments using synthetic problems, we verified that our proposed method identifies better solutions with fewer evaluations than the original TPE. Our algorithm is available in Optuna, an open-source framework for HPO.
format Preprint
id arxiv_https___arxiv_org_abs_2507_08053
institution arXiv
publishDate 2025
record_format arxiv
spellingShingle Tree-Structured Parzen Estimator Can Solve Black-Box Combinatorial Optimization More Efficiently
Abe, Kenshin
Wang, Yunzhuo
Watanabe, Shuhei
Machine Learning
Artificial Intelligence
Tree-structured Parzen estimator (TPE) is a versatile hyperparameter optimization (HPO) method supported by popular HPO tools. Since these HPO tools have been developed in line with the trend of deep learning (DL), the problem setups often used in the DL domain have been discussed for TPE such as multi-objective optimization and multi-fidelity optimization. However, the practical applications of HPO are not limited to DL, and black-box combinatorial optimization is actively utilized in some domains, e.g., chemistry and biology. As combinatorial optimization has been an untouched, yet very important, topic in TPE, we propose an efficient combinatorial optimization algorithm for TPE. In this paper, we first generalize the categorical kernel with the numerical kernel in TPE, enabling us to introduce a distance structure to the categorical kernel. Then we discuss modifications for the newly developed kernel to handle a large combinatorial search space. These modifications reduce the time complexity of the kernel calculation with respect to the size of a combinatorial search space. In the experiments using synthetic problems, we verified that our proposed method identifies better solutions with fewer evaluations than the original TPE. Our algorithm is available in Optuna, an open-source framework for HPO.
title Tree-Structured Parzen Estimator Can Solve Black-Box Combinatorial Optimization More Efficiently
topic Machine Learning
Artificial Intelligence
url https://arxiv.org/abs/2507.08053