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Maniezzo, V.; Zhou, T. Learning Individualized Hyperparameter Settings. Encyclopedia. Available online: https://encyclopedia.pub/entry/46216 (accessed on 10 December 2023).

Maniezzo V, Zhou T. Learning Individualized Hyperparameter Settings. Encyclopedia. Available at: https://encyclopedia.pub/entry/46216. Accessed December 10, 2023.

Maniezzo, Vittorio, Tingting Zhou. "Learning Individualized Hyperparameter Settings" *Encyclopedia*, https://encyclopedia.pub/entry/46216 (accessed December 10, 2023).

Maniezzo, V., & Zhou, T.(2023, June 29). Learning Individualized Hyperparameter Settings. In *Encyclopedia*. https://encyclopedia.pub/entry/46216

Maniezzo, Vittorio and Tingting Zhou. "Learning Individualized Hyperparameter Settings." *Encyclopedia*. Web. 29 June, 2023.

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The performance of optimization algorithms, and consequently of AI/machine learning solutions, is strongly influenced by the setting of their hyperparameters. Over the last decades, a rich literature has developed proposing methods to automatically determine the parameter setting for a problem of interest, aiming at either robust or instance-specific settings. Robust setting optimization is already a mature area of research, while instance-level setting is still in its infancy, with contributions mainly dealing with algorithm selection.

optimization algorithms
inductive learning
parameter setting
neural network generalization

Most optimization algorithms, and consequently most AI/machine learning (ML) solutions, have an effectiveness that depends heavily on the values imposed on high-level guiding parameters, usually called hyperparameters. Choosing a bad setting can result in anything from needing more time to reach the solution to being denied any success in the search. Therefore, the identification of an effective hyperparameter setting is configured as a higher-level optimization task, instrumental in obtaining a successful application of the search algorithm of interest.

Over the last few decades, a rich literature has developed proposing methods for automatically estimating parameter settings, using approaches ranging from purely statistical to tailored heuristics. Two main lines of research have emerged, one aiming at identifying a robust setting that makes an algorithm of interest effective on all (most) instances of the problem to be optimized, and one tailoring the setting to each specific instance. In fact, it is a common practical awareness that different instances of the same problem can require very different computational efforts to solve, even if their dimensions are the same. The computational complexity theory justifies such diversity among problems, but it cannot be declined at the instance level, while the “no free lunch” theorem is valid even among instances ^{[1]}. More efficient solution processes could therefore benefit from different settings tailored at the individual instance level, but the marginal benefits must be weighed against the costs of achieving them.

This work considers both objectives, building upon automatic configurators but taking into account the large diversity among instances of the same problem. The approach researchers follow differs from that usually proposed in the literature of individual settings in that researchers adapt prelearned settings to each specific instance, relying on neural networks, specifically multilayer perceptrons (MLPs), to obtain the adapted settings in real time. Specifically, the methodology researchers present makes use of state-of-the-art automatic algorithm configurators to generate datasets of instance/setting pairs to be processed by the neural module. The generalization and abstraction capabilities of the neural component are used to obtain instance-specific hyperparameter settings for out-of-sample instances.

It is clearly ineffective to propose raw instances as learning bases, so researchers rely on sets of descriptive statistics computed on both in-sample and out-of-sample (i.e., train and test) instances and pass them as input to the neural module. The pipeline starts by computing a set of descriptive statistics on the available instances, possibly filtering them, and applying a state-of-the-art configurator to different subsets of structurally similar instances. This produces a dataset of instance/setting pairs that can be used to instantiate learning. The learning module consists of a multilayer perceptron, whose abstraction and generalization capabilities are well known, and which is used in this case to abstract the mapping between instance statistics and setting, so that when a new instance is proposed, its statistics are computed and fed as input to the MLP, which in turn outputs the individualized setting.

State-of-the-art optimization algorithms typically have a large number of parameters that need to be modified to ensure their performance. Traditionally, the identification of an effective setting has been achieved through a manual, experimental approach, mostly guided by the experience of the algorithm designers. However, manual exploration of the combinatorial space of parameter settings is tedious and tends to produce unsatisfactory results. Expectedly, the manual search can be less efficient than automatic procedures, so automatic configurators have been proposed for use when very high performance is required. Automatic or at least computer-assisted configuration has evolved along a line that tries to identify robust settings to be used on all problem instances, and along a second line that tries to propose individualized settings. There are several surveys that cover these topics ^{[2]}^{[3]}^{[4]}^{[5]}.

Configurators of the first type ^{[6]}^{[7]}^{[8]}^{[9]}^{[10]} typically assume that an instance set of diverse representative instances is available, and specific methods are applied to the set to derive a robust setting that is effective on all of them, and thus hopefully on other instances of the same problem. Widely used systems in this group include ParamILS and irace, besides generic nonlinear optimization algorithms such as the Nelder–Mead simplex, swarm optimization ^{[11]}, Bayesian optimization ^{[12]}, or a random search ^{[13]} applied to this specific task.

ParamILS ^{[9]} is an iterated local search (ILS) algorithm that works in the search space of the parameters. It works on categorical parameters; therefore, real and integer parameters must first be converted to discrete values. After a possibly random initialization, a local search is started. The local search in ParamILS is a first-improvement local search, based on a one-exchange neighborhood, which can only change one parameter at a time. Each time a move finds a new improvement, it takes it and continues the search until all neighborhoods are examined or the budget is exhausted. At that point, the better of the current or previous local minima is kept, and the last solution is perturbed to reinitialize the search. ParamILS can be used for run-time configurations because it implements a procedure called adaptive capping, which prunes the search early to evaluate potentially poor-performing configurations, greatly reducing computation time.

The irace package ^{[10]} implements the so-called iterated race method, which is a generalization of the iterated F-race method for the automatic configuration of optimization algorithms ^{[14]}. irace is based on three successive phases. In the first phase, it generates candidate configurations using an appropriate statistical model. In the second phase, it selects the best candidate configurations using a race method based on either F-race or a paired Student’s t-test. Third, it updates the system’s probability model to give the most promising configurations a higher probability of being used. The general process starts by generating configurations within the user-specified value intervals for each parameter and subjecting them all to a race. In the race, configurations are evaluated on a first instance of the provided instance set, then on the second, and so on. As soon as a configuration is judged to be bad, it is eliminated, and the search continues until a small subset, by default four, of configurations survive, which are proposed in the output.

These algorithms represent the state of the art for robust setting optimization. The literature on individualized configuration uses them as a module of a more complex process, as the approach suggests as well.

Individualized tuning itself has different goals and follows different approaches. One approach is adaptive configuration, where parameter values are adjusted during the optimization process based on the performance of the algorithm ^{[3]}. This was pioneered by evolution strategies ^{[15]}^{[16]} and has received considerable attention since then but falls outside the scope of the research.

Even if researchers narrow the focus to static settings that do not change values during the search, there is a second subdivision to be made, namely between configurators that select the most promising solution algorithm, or part thereof, from a portfolio of candidate algorithms ^{[17]}, and those that work on a single algorithm and optimize its parameters.

Individualized algorithm selection has been more extensively studied and has proven to be able to significantly improve the state of the art in some notable combinatorial problems, including propositional satisfiability ^{[18]} and the traveling salesman problem ^{[19]}. Prominent systems in this area are hydra ^{[20]} or llama ^{[21]}, which are primarily algorithm portfolio selection systems, where the included automatic parameter configurator passes the task to optimizers such as ParamILS. Another interesting effort in the area of algorithm selection is the instance space analysis for algorithm testing ^{[22]}, a proposal to structure the analysis of instance complexity and the degree of coverage of the universe of instances of a given problem guaranteed by the available test sets.

Research on optimizing parameters of a single algorithms, which has been named per-instance algorithm configuration (PIAC) ^{[4]}, has seen less contributions. It typically involves two phases: an offline phase, during which the configurator is trained, and an online phase, in which the configurator is run on given problem instances. The result of the second phase is a parameter configuration determined on the features of the specific instance to be solved. Researchers remark the difference from a standard algorithm configuration, which typically produces a single configuration that is then used on all presented instances.

The state of the art in PIAC is currently represented by ISAC (instance-specific algorithm configuration) ^{[23]}. Its general approach is the same as that described above or in instance space analysis, and as the one researchers adopted in the proposal as well. First, the available benchmark instances are clustered into distinct sets based on the similarity of their feature vectors as computed by a geometric norm. Then, assuming that instances with similar features behave similarly under the same algorithm, some kind of search is used to find good parameters for each cluster of instances. Finally, new instances are presented, and the optimized settings are used to determine the one to use for that instance. The way this is done may vary across systems. ISAC uses a g-means algorithm to cluster the training instances, and when a new instance is shown, it checks if there is a cluster whose center is close enough to the feature vector of the input. If so, the parameters for that cluster are used, otherwise a robust setting optimized for each problem instance is used. Researchers' proposal differs in that it relies on the generalization ability of neural networks, which allows the adaptation of the learned settings to any new instance.

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