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Will Bilevel Optimizers Benefit from Loops
Kaiyi Ji · Mingrui Liu · Yingbin Liang · Lei Ying

Wed Dec 07 05:00 PM -- 07:00 PM (PST) @

Bilevel optimization has arisen as a powerful tool for solving a variety of machine learning problems. Two current popular bilevel optimizers AID-BiO and ITD-BiO naturally involve solving one or two sub-problems, and consequently, whether we solve these problems with loops (that take many iterations) or without loops (that take only a few iterations) can significantly affect the overall computational efficiency. Existing studies in the literature cover only some of those implementation choices, and the complexity bounds available are not refined enough to enable rigorous comparison among different implementations. In this paper, we first establish unified convergence analysis for both AID-BiO and ITD-BiO that are applicable to all implementation choices of loops. We then specialize our results to characterize the computational complexity for all implementations, which enable an explicit comparison among them. Our result indicates that for AID-BiO, the loop for estimating the optimal point of the inner function is beneficial for overall efficiency, although it causes higher complexity for each update step, and the loop for approximating the outer-level Hessian-inverse-vector product reduces the gradient complexity. For ITD-BiO, the two loops always coexist, and our convergence upper and lower bounds show that such loops are necessary to guarantee a vanishing convergence error, whereas the no-loop scheme suffers from an unavoidable non-vanishing convergence error. Our numerical experiments further corroborate our theoretical results.

Author Information

Kaiyi Ji (University at Buffalo)

Kaiyi Ji is now an assistant professor at the Department of Computer Science and Engineering of the University at Buffalo. He was a postdoctoral research fellow at the Electrical Engineering and Computer Science Department of the University of Michigan, Ann Arbor, in 2022, working with Prof. Lei Ying. He received his Ph.D. degree from the Electrical and Computer Engineering Department of The Ohio State University in December, 2021, advised by Prof. Yingbin Liang. He was a visiting student research collaborator at the department of Electrical Engineering, Princeton University working with Prof. H. Vincent Poor. Previously he obtained his B.S. degree from University of Science and Technology of China in 2016.

Mingrui Liu (George Mason University)
Yingbin Liang (The Ohio State University)
Lei Ying (University of Michigan, Ann Arbor)

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