The increasing demand for high-performance query processing on large in-memory datasets has driven the adoption of GPU-based B+-trees for handling high-concurrency query (HCQ) workloads. Existing approaches, by randomly assigning queries to GPU threads, suffer from inefficiencies related to memory access patterns, cache utilization, and thread divergence. This paper introduces a novel query grouping strategy that assigns queries with similar search keys to the same CUDA block, thereby improving query throughput. We formalize the optimal key assignment (OKA) problem as a variation of the K-means problem, establishing its theoretical foundations and proposing an efficient algorithm with proven optimality. We implement this algorithm using highly optimized CUDA code and extend our approach to support range queries, a common but understudied workload in GPU-based HCQ systems. Experimental evaluations demonstrate that our query grouping strategy significantly outperforms prior work, achieving up to 10.6X lower latency and 32.2X higher throughput, while also improving GPU resource utilization (e.g., cache hit rate and memory throughput).

Robustness againstword substitution attacks iscrucial for text classifiers and fundamental to broader NLP robustness. Such attacks use semantically similar word substitutions. Existing certified defenses often compute loose outer bounds for the convex hull of word embeddings, including irrelevant words, degrading performance on clean and adversarial tests. Additionally, convex hull-based methods also struggle to emulate worst-case scenarios accurately. This paper proposes theMasked Adversarial Convex Combination(MACC) method, which models the solution space as a convex hull of word vectors and uses Variational Information Bottleneck theory to mask unnecessary words. We also introduce an empirical masking method based on the volume of the convex hull to enhance the performance. By reducing the number of words explored within the convex hull, MACC enables more precise mimicry of worst-case attacks. Experiments across models and datasets show MACC outperforms existing methods in clean and adversarial accuracy against word-level attacks.

To address the power system hardening problem, traditional approaches often adopt robust optimization (RO) that considers a fixed set of concerned contingencies, regardless of the fact that hardening some components actually renders relevant contingencies impractical. In this paper, we directly adopt a dynamic uncertainty set that explicitly incorporates the impact of hardening decisions on the worst-case contingencies, which leads to a decision-dependent uncertainty (DDU) set. Then, a DDU-based robust-stochastic optimization (DDU-RSO) model is proposed to support the hardening decisions on distribution lines and distributed generators (DGs). Also, the randomness of load variations and available storage levels is considered through stochastic programming (SP) in the innermost level problem. Various corrective measures (e.g., the joint scheduling of DGs and energy storage) are included, coupling with a finite support of stochastic scenarios, for resilience enhancement. To relieve the computation burden of this new hardening formulation, an enhanced customization of parametric column-and-constraint generation (P-C&CG) algorithm is developed. By leveraging the network structural information, the enhancement strategies based onresilience importance indicesare designed to improve the convergence performance. Numerical results on 33-bus and 118-bus test distribution networks have demonstrated the effectiveness of DDU-RSO aided hardening scheme. Furthermore, in comparison to existing solution methods, the enhanced P-C&CG has achieved a superior performance by reducing the solution time by a few orders of magnitude.

Extensive investigations have revealed that the gait recognition system is always vulnerable to impersonation attacks, which pose significant threats to the identity access security. Previous impersonation strategies have primarily focused on mimicking the victim’s walking style or probing the similar gait features to merely manipulate the input samples, without concurrently undermining the built-in model of the gait recognizer, thereby failing to achieve cost-effective attacks. In contrast to these existing heuristic approaches, we propose an optimal adversarial complicity strategy, called collusion attack, which leverages the tight collaboration between an external attacker and an internal spy to tie up into the close colluder, simultaneously enabling the input-&model-corrupted tampering modes and misleading the gait recognizer more powerfully and stealthily for misidentifying the illegitimate Alice as legitimate Bob. Specifically, we formulate a bilevel optimization problem to model such a leader-follower Stackelberg game with sequentially adversarial interaction process between the colluders and gait recognizer. Further, to solve this challenging bilevel problem efficiently, we absorb the Lagrangian dual theory and linearization representation method to reformulate a tractable mixed integer program. Finally, we perform comparison and ablation experiments with the state-of-the-art attack modes on single-&multi-source gait datasets to verify the validity of our collusion strategy in inducing the mistaken identity with great success rate, high confidence, and low cost. Empirical results also shed light on key insights in mitigating the collusion attacks and enhancing the gait recognition robustness to safeguard the identity access applications.

We propose a new nonconvex regularization scheme to improve the performance of the stochastic dual dynamic programming (SDDP) algorithm for solving large-scale multistage stochastic programs. Specifically, we use a class of nonconvex regularization functions, namely folded concave penalty functions, to improve solution quality and the convergence rate of the SDDP procedure. We develop a strategy based on mixed-integer programming to guarantee global optimality of the nonconvex regularization problem. Moreover, we establish provable convergence guarantees for our customized SDDP algorithm. The benefits of our regularization scheme are demonstrated by solving large-scale instances of two multistage stochastic optimization problems. History: Pascal Van Hentenryck, Area Editor for Computational Modeling: Methods & Analysis. Supplemental Material: The software that supports the findings of this study is available within the paper and its Supplemental Information ( https://pubsonline.informs.org/doi/suppl/10.1287/ijoc.2021.0255 ) as well as from the IJOC GitHub software repository ( https://github.com/INFORMSJoC/2021.0255 ). The complete IJOC Software and Data Repository is available at https://informsjoc.github.io/ .

Networked microgrids that integrate the hydrogen fueling stations (HFSs) with the on-site renewable energy sources (RES), power-to-hydrogen (P2H) facilities, and hydrogen storage could help decarbonize the energy and transportation sectors. In this paper, to support the hydrogen-based networked microgrids planning subject to multiple uncertainties (e.g., RES generation, electric loads, and the refueling demands of hydrogen vehicles), we propose a two-stage stochastic formulation with mixed integer conic program (MICP) recourse decisions. Our formulation involves the holistic investment and operation modeling to optimally site and configure the microgrids with HFSs. The MICP problems appearing in the second-stage capture the nonlinear power flow of networked microgrids system with binary decisions on storage charging/discharging status and energy transactions (including the trading of electricity, hydrogen, and carbon credits to recover the capital expenditures). To handle the computational challenges associated with the stochastic program with MICP recourse, an augmented Benders decomposition algorithm (ABD) is developed. Numerical studies on 33- and 47-bus exemplary networks demonstrate the economics viability of electricity-hydrogen coordination on microgrids level, as well as the benefits of stochastic modeling. Also, our augmented algorithm significantly outperforms existing methods, e.g., the progressive hedging algorithm (PHA) and the direct use of a professional MIP solver, which has largely improved the solution quality and reduced the computation time by orders of magnitude. Note to Practitioners—This paper proposes an optimal planning model for electricity-hydrogen microgrids with the renewable hydrogen production, storage, and refueling infrastructures. Our planning model is extended under a two-stage stochastic framework to address the multi-energy-sector uncertainties, e.g., RES generation, electric loads, and the refueling demands of hydrogen vehicles. The first-stage problem is to optimize the siting and sizing plan of microgrids. Then, in the second-stage problem, the coordinated scheduling of electricity and hydrogen supply systems is modeled as second-order conic programs (SOCPs) to accurately capture the power flow representation under stochastic scenarios. Also, the logical constraints with binary variables are introduced to describe the energy transactions and storage operations, which results in an MICP recourse structure. Note that the stochastic MICP formulation could be very challenging to compute even with a moderate number of scenarios. One challenge certainly comes from integer variables that cause the problem nonconvex. Another challenge follows from the fact that the strong duality of SOCPs might not hold in general. To mitigate those two challenges, we prove that the continuous relaxation of our recourse problem has strong duality, and make use of that continuous relaxation and other enhancements to design an augmented decomposition algorithm. As revealed by our numerical tests, the proposed decomposition method outperforms PHA in both the solution quality and computational efficiency. Comparing to the PHA, our ABD method often achieves tighter bounds with trivial optimality gaps. Also, it could reduce the computation time by orders of magnitude. With the help of advanced analytical tool, the proposed planning framework can be readily implemented in real-world applications.

Maintenance optimization has been extensively studied in the past decades. However, most of the existing maintenance models focus on single-component systems and are not applicable to complex systems consisting of multiple components, due to various interactions among the components. The multicomponent maintenance optimization problem, which joins the stochastic processes regarding the failures of components with the combinatorial problems regarding the grouping of maintenance activities, is challenging in both modeling and solution techniques, and has remained an open issue in the literature. In this paper, we study the multicomponent maintenance problem over a finite planning horizon and formulate the problem as a multistage stochastic integer program with decision-dependent uncertainty. There is a lack of general efficient methods to solve this type of problem. To address this challenge, we use an alternative approach to model the underlying failure process and develop a novel two-stage model without decision-dependent uncertainty. Structural properties of the two-stage problem are investigated, and a progressive-hedging-based heuristic is developed based on the structural properties. Our heuristic algorithm demonstrates a significantly improved capacity to handle large-size two-stage problems comparing to three conventional methods for stochastic integer programming, and solving the two-stage model by our heuristic in a rolling horizon provides a good approximation of the multistage problem. The heuristic is further benchmarked with a dynamic programming approach and a structural policy, which are two commonly adopted approaches in the literature. Numerical results show that our heuristic can lead to significant cost savings compared with the benchmark approaches.

In this paper, we present a new computation scheme for the pessimistic bilevel optimization problem, which so far does not have any computational methods generally applicable. We first develop a ti...

The block relocation problem (BRP) is a fundamental operational issue in modern warehouse and yard management, which, however, is very challenging to solve. In this article, to advance our understanding of this problem and to provide substantial assistance to practice, we adopt the following: 1) introduce a classification scheme and present a rather comprehensive review on all 16 BRP variants; 2) develop a general framework to derive lower bounds on the number of necessary relocations and demonstrate its connection to existing lower bounds on the unrestricted BRP variants; 3) propose and employ a couple of new critical substructure concepts to analyze the BRP and obtain a lower bound that dominates all existing ones; 4) build a new and strong mixed integer programming (MIP) formulation that is adaptable to compute eight BRP variants, and design a novel MIP-formulation-based iterative procedure to compute exact BRP solutions; and 5) extend the MIP formulation to address four typical industrial considerations. Computational results on standard and practical test instances show that the new lower bound is significantly stronger, and our new MIP computational methods have superior performances over the state-of-the-art formulation and a heuristic adopted in a steel plant.

Abstract We study a class of bilevel spanning tree (BST) problems that involve two independent decision‐makers (DMs), the leader and the follower with different objectives, who jointly construct a spanning tree in a graph. The leader, who acts first, selects an initial subset of edges that do not contain a cycle, from the set under her control. The follower then selects the remaining edges to complete the construction of a spanning tree, but optimizes his own objective function. If there exist multiple optimal solutions for the follower that result in different objective function values for the leader, then the follower may choose either the one that is the most (optimistic version) or least (pessimistic version) favorable to the leader. We study BST problems with the sum‐ and bottleneck‐type objective functions for the DMs under both the optimistic and pessimistic settings. The polynomial‐time algorithms are then proposed in both optimistic and pessimistic settings for BST problems in which at least one of the DMs has the bottleneck‐type objective function. For BST problem with the sum‐type objective functions for both the leader and the follower, we provide an equivalent single‐level linear mixed‐integer programming formulation. A computational study is then presented to explore the efficacy of our reformulation.

This paper investigates the deployment issue of an emergency medical service (EMS) system to maintain the preferred service coverages under different considerations. Specifically, two coverage levels are introduced to reflect the requirements under the regular situation and the situation with ambulance unavailable. We propose the two-stage robust optimization (RO) models to design a reliable ambulance system subject to unavailability of the ambulances, with and without the ambulance relocation. For the RO problem with mixed-integer recourse for relocation, we customize the column and constraint generation method with an approximation strategy to handle the computational challenge. Our numerical study: 1) demonstrates that our RO formulations have a strong modeling capacity on designing the EMS system; 2) shows that our approximation algorithm performs very well; and 3) provides a quantitative evaluation of, including, relocation operations on the system performance.

Bayesian network (BN) has been a popular tool for gaining mechanistic understanding of variables by revealing how the variables influence each other. It has been found very effective in a few studies in quality control and process monitoring. However, for complex problems where the structure of a BN is unknown, a common approach is to learn the BN structure from observational data. A fundamental bottleneck of this approach is that observational data can only be used to discover part of the influential relationships among variables. To overcome this problem, we propose to combine observational data and expert knowledge. To the best of the author's knowledge, our approach is the first of its kind that formulates an experimental design framework to automate the expert elicitation process and collect the most informative expert knowledge, optimally matched to the observational data, to learn the BN structure.

Complex diseases such as various types of cancer and diabetes are conjectured to be triggered and influenced by a combination of genetic and environmental factors. To integrate potential effects from interplay among underlying candidate factors, we propose a new network-based framework to identify effective biomarkers by searching for groups of synergistic risk factors with high predictive power to disease outcome. An interaction network is constructed with node weights representing individual predictive power of candidate factors and edge weights capturing pairwise synergistic interactions among factors. We then formulate this network-based biomarker identification problem as a novel graph optimization model to search for multiple cliques with maximum overall weight, which we denote as the Maximum Weighted Multiple Clique Problem (MWMCP). To achieve optimal or near optimal solutions, both an analytical algorithm based on column generation method and a fast heuristic for large-scale networks have been derived. Our algorithms for MWMCP have been implemented to analyze two biomedical data sets: a Type 1 Diabetes (T1D) data set from the Diabetes Prevention Trial-Type 1 (DPT-1) study, and a breast cancer genomics data set for metastasis prognosis. The results demonstrate that our network-based methods can identify important biomarkers with better prediction accuracy compared to the conventional feature selection that only considers individual effects.

BACKGROUND: Optimization procedures to identify gene knockouts for targeted biochemical overproduction have been widely in use in modern metabolic engineering. Flux balance analysis (FBA) framework has provided conceptual simplifications for genome-scale dynamic analysis at steady states. Based on FBA, many current optimization methods for targeted bio-productions have been developed under the maximum cell growth assumption. The optimization problem to derive gene knockout strategies recently has been formulated as a bi-level programming problem in OptKnock for maximum targeted bio-productions with maximum growth rates. However, it has been shown that knockout mutants in fact reach the steady states with the minimization of metabolic adjustment (MOMA) from the corresponding wild-type strains instead of having maximal growth rates after genetic or metabolic intervention. In this work, we propose a new bi-level computational framework--MOMAKnock--which can derive robust knockout strategies under the MOMA flux distribution approximation. METHODS: In this new bi-level optimization framework, we aim to maximize the production of targeted chemicals by identifying candidate knockout genes or reactions under phenotypic constraints approximated by the MOMA assumption. Hence, the targeted chemical production is the primary objective of MOMAKnock while the MOMA assumption is formulated as the inner problem of constraining the knockout metabolic flux to be as close as possible to the steady-state phenotypes of wide-type strains. As this new inner problem becomes a quadratic programming problem, a novel adaptive piecewise linearization algorithm is developed in this paper to obtain the exact optimal solution to this new bi-level integer quadratic programming problem for MOMAKnock. RESULTS: Our new MOMAKnock model and the adaptive piecewise linearization solution algorithm are tested with a small E. coli core metabolic network and a large-scale iAF1260 E. coli metabolic network. The derived knockout strategies are compared with those from OptKnock. Our preliminary experimental results show that MOMAKnock can provide improved targeted productions with more robust knockout strategies.