Increasing the Scope as You Learn: Adaptive Bayesian Optimization in Nested Subspaces
Leonard PapenmeierLuigi NardiMatthias Poloczek
Proposes BAXUS, a high-dimensional Bayesian optimization algorithm that adaptively expands nested random subspaces to remove the need for prior assumptions about active subspace dimensionality while providing theoretical convergence guarantees and superior empirical performance.
High-dimensional black-box optimization is critical across engineering and science, including chemical engineering, drug discovery, vehicle design, and machine learning tuning. In these domains, evaluating candidate designs is expensive, and optimization problems often involve hundreds or thousands of input parameters. Traditional Bayesian optimization methods, which model unknown functions to guide sampling efficiently, struggle in high dimensions due to the exponential growth of the search space. Existing high-dimensional techniques either degrade rapidly as parameter counts grow or rely on unprovable assumptions, such as guessing the unknown size of an active lower-dimensional subspace beforehand.
The article develops and evaluates BAXUS (Bayesian optimization with adaptively expanding subspaces), an optimization algorithm designed to optimize high-dimensional black-box functions efficiently without requiring users to guess subspace dimensions. The primary objective is to demonstrate that adaptively expanding nested subspaces provides strong theoretical guarantees and outperforms existing state-of-the-art optimization methods on complex, high-dimensional benchmarks.
The authors designed a sparse random linear embedding that starts in a low-dimensional target space and systematically increases dimensionality over time as more data is evaluated. By splitting target dimensions and copying prior observations, the method preserves all previously collected data in newly expanded spaces. The framework also integrates an adaptive trust-region strategy to focus sampling around the best-known candidates while dynamically adjusting failure tolerances to ensure the full input space can be reached within a fixed budget. The authors evaluated the approach across six diverse benchmarks ranging from 124 to 1,000 dimensions—including vehicle design, hyperparameter tuning, synthetic test functions, and LASSO benchmarks with and without observational noise—running 20 repeated trials per method alongside established baselines like TURBO, SAASBO, ALEBO, HESBO, and CMA-ES.
The evaluation revealed several key findings. First, BAXUS achieved the best overall optimization performance across the benchmarks, consistently outperforming standard high-dimensional baselines on 1,000-dimensional and 300-dimensional LASSO tasks as well as on a 388-dimensional support vector machine tuning problem and a 124-dimensional vehicle design problem. Second, the proposed sparse embedding was mathematically proven to provide a larger worst-case guarantee of containing the true global optimum than existing hash-based embeddings like HESBO, achieving optimality among sparse linear embeddings. Third, BAXUS demonstrated strong robustness to observational noise, maintaining steady optimization progress past 1,000 evaluations where competing methods degraded significantly. Finally, while methods like SAASBO converged quickly on specific low-dimensional active spaces (such as the 500-dimensional Hartmann function), they faced severe computational scalability bottlenecks, whereas BAXUS scaled efficiently across large evaluation budgets.
These results show that engineering teams and researchers can optimize complex systems with hundreds of design parameters without manual, risky guesses about problem structure. By removing the risk of subspace misspecification and lowering evaluation waste, the approach reduces the time, computational overhead, and experimental costs required to find high-performing designs in sensitive applications such as drug discovery and manufacturing.
Organizations handling high-dimensional black-box optimization should consider deploying BAXUS as an out-of-the-box solver, utilizing its open-source implementation. In the near term, teams should validate performance on internal pilot problems, particularly those involving noisy experimental data. Future development should explore tailoring subspace expansion strategies using domain knowledge and extending the framework to structured combinatorial search spaces common in materials science.
Confidence in these findings is supported by rigorous convergence proofs and diverse experimental benchmarks with repeated trials. However, users should note that the current implementation focuses primarily on continuous parameter spaces and uses a fixed heuristic schedule for subspace growth, which may benefit from domain-specific tuning on highly specialized tasks.
- Paper: A Tutorial on Bayesian Optimization, Peter I. Frazier (2018). This tutorial provides a comprehensive foundation in Gaussian process surrogates and acquisition functions, which form the core mechanism BAXUS adapts to nested subspaces.
- Paper: A Tutorial on Bayesian Optimization of Expensive Cost Functions, with Application to Active User Modeling and Hierarchical Reinforcement Learning, Eric Brochu et al. (2010). Reading this seminal tutorial establishes the fundamental exploration-exploitation trade-offs and covariance models in Bayesian optimization necessary to understand high-dimensional adaptations.
- Paper: Practical Bayesian Optimization of Machine Learning Algorithms, Jasper Snoek et al. (2012). This foundational work details practical Bayesian optimization workflows and acquisition formulations for machine learning applications that BAXUS scales to high dimensions.
- Paper: Algorithms for Hyper-Parameter Optimization, James Bergstra et al. (2011). This work introduces sequential model-based optimization concepts and probabilistic surrogate techniques used in scaling black-box optimization.
- Paper: Random Search for Hyper-Parameter Optimization, James Bergstra et al. (2012). This paper demonstrates how low effective dimensionality can be exploited in high-dimensional search spaces, a key conceptual motivation behind subspace Bayesian optimization methods like BAXUS.
- Paper: Joint Entropy Search for Multi-Objective Bayesian Optimization, Ben Tu et al. (2022). This paper extends Bayesian optimization principles to vector-valued multi-objective settings using an information-theoretic joint entropy search criterion.
- Paper: Diffusion Models for Black-Box Optimization, Siddarth Krishnamoorthy et al. (2023). This work explores an alternative generative paradigm for high-dimensional black-box optimization by conditioning diffusion models on performance targets.
- Paper: Sample Efficiency Matters: A Benchmark for Practical Molecular Optimization, Wenhao Gao et al. (2022). This benchmark provides a realistic, budget-constrained testbed for evaluating sample-efficient black-box optimizers on real-world high-dimensional design tasks.
