Paper:
Efficient Output Kernel Learning for Multiple Tasks
The paradigm of multi-task learning is that one can achieve better generalization by learning tasks jointly and thus exploiting the similarity between the tasks rather than learning them independently of each other. While previously the relationship between tasks had to be user-defined in the form of an output kernel, recent approaches jointly learn the tasks and the output kernel. As the output kernel is a positive semidefinite matrix, the resulting optimization problems are not scalable in the number of tasks as an eigendecomposition is required in each step. Using the theory of positive semidefinite kernels we show in this paper that for a certain class of regularizers on the output kernel, the constraint of being positive semidefinite can be dropped as it is automatically satisfied for the relaxed problem. This leads to an unconstrained dual problem which can be solved efficiently. Experiments on several multi-task and multi-class data sets illustrate the efficacy of our approach in terms of computational efficiency as well as generalization performance.

Review:
The present paper focus on kernel-based multi-task learning. More specifically, it deals with learning the output kernel which defined over multiple tasks. Starting from the vector-valued RKHS formulation of the multi-task learning problem, the authors propose an output kernel learning algorithm that are able to learn both the multi-task learning function and task dependencies. The authors first provide an optimization formulation of the problem which is jointly convex. Then they show that this optimization can be solved without the positive-semidefiniteness constraintof the output kernel, which is computationally costly, and propose the use of a stochastic dual coordinate ascent method to solve it.Experiments on multi-task data sets and comparison in terms of performance and running time with previous multi-task learning algorithms are provided. The paper builds upon recent studies on multitask learning and output kernel learning. In particular, it is closely related to: [1] Dinuzzo & al., Learning output kernels with block coordinate descent., ICML 2011. [2] Zhang & Yeung, A convex formulation for learning task relationships in multi-task learning, UAI 2010.While these two references are clearly mentioned in the paper, there are other closely related papers which are not discussed and cited. For example [3] Kadri & al., Multiple operator-valued kernel learning, NIPS 2012. [4] Scalable matrix-valued kernel learning for high-dimensional nonlinear multivariate regression and Granger causality, UAI 2013. [5] Ciliberto & al., Convex learning of multiple tasks and their structure, ICML 2015.These papers, in particular [4,5], needs to be cited and discussed. The proposed approach needs to be compared to them from a theoretical and experimental perspectives.The contribution is somewhat limited. The main novelty is the dual formulation of the optimization problem which does not need the positive-semidefiniteness constraint of the output kernel. However, it should be noted that even in the previous works [1] and [2] where an alternating optimization scheme is adopted, positive-semidefiniteness constraint is not needed. Incorporating the solution of the minimization over the multi-task learning function in the optimization problem of the output kernel learning (without the positive-semidefiniteness constraint) provides a positive semi-definite matrix over tasks. This is clearly mentioned in [1] (see Eq. 7) and [2]. The way this paper is written gives the impression that it is the first that avoid the optimization over S_+^\top.Also, the result about the joint convexity of the output kernel learning optimization problem is not new. This is largely discussed in [5] and also in [2]. It is true that [5] is a newly published paper but the contribution of the present workwill be more visible by taking into account recent results in this field. In the experiments, it is not clear why the proposed approach is compared to OKL in terms of running time and not in terms of performance. The paper is about output kernel learning, so the performance of the proposed method needs to be compared to those of [1, 4, 5].update: The authors did a good job in answering my questions. My main concern is about the presentation of the results and their novelty. I think that is technical novelty and originality in the proposed approach, however the authors should better explain this and make it clear what the novel contributions of the paper are in comparison to what is known.