Paper ID: | 7759 |
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Title: | Primal-Dual Block Generalized Frank-Wolfe |

== POST-REBUTTAL == Overall, I reckon that the authors have made a strong rebuttal. The contribution of the paper is now clearer in my mind and I reckon that both the theoretical result and experimental result of the paper look solid. As pointed out by other reviewers, I strongly encourage the authors to clarify the overall presentation based on our comments (highlight better the contributions, improve the presentation of experiments and algorithms...). For these reasons, I am ok to change my score to 7. == REVIEW BEFORE REBUTTAL == The paper is relatively well written and the proposed algorithms are technically sound. The experimental results on the 6 different datasets seem to indicate the performance of the approach. For these reasons I am leaning towards weak accept. However, as a disclaimer, I am not an expert in the domain and was not able to proofread all the proofs of the paper nor appreciate the complete significance of the contribution (hence my low confidence score). Below are some remarks/questions that could improve the paper: - Highlight better the contribution and significance of the paper: it would be important to add a paragraph in the introduction to highlight better why the proposed algorithm is a significant contribution for machine learning/optimization. - Equation (8) in Algorithm 1, sparse update: I was not able to understand exactly what was the procedure to obtain \tilde{x}. Do you have an exact algorithm? If yes can you give more details on that procedure (the section 7 in appendix was hard to understand). - How do you choose s in practice? - In the experimental section, please provide test accuracy curves as well. As this algorithm has a vocation to be used in Machine Learning one does not really care about getting to machine precision on the training set but we rather care about having good performance on the test set, hence it makes to also report performance on the test set. In particular, it is important to show that the algorithm is also better in the regimes of regularization that performs well in terms of final performance. - Related work: The authors should discuss the following related work: Frank-Wolfe algorithms for saddle point problems, Gauthier Gidel, Tony Jebara, Simon Lacoste-Julien - L92-93, for the three line proofs, I would still add details for the last line (using the \mu strong convexity) as you are in the constrained case and therefore the gradient at the optimum is not necessarily zero, this is not just a trivial application of the \mu strong convexity of f to me). - How does the method would compare to FW variants that also enjoy linear convergence rate (e.g. away FW (AFW) or pairwise FW)? In particular I think that it should be possible to apply the Block coordinate version of AFW presented in the following paper that also enjoys a linear convergence rate theoretically (under specific conditions) and in practice: Minding the Gaps for Block Frank-Wolfe Optimization of Structured SVMs, Osokin et al, ICML2016 It would be interesting to compare to that paper and discuss the advantages of your method compared to this work.

=== Update after reading rebuttal and reviews === We thank the authors for their detailed and convincing rebuttal. They have adequately addressed my concerns about both the correctness of the experiments, and the novelty of their theoretical analysis. However, I agree with reviewer 4 that the presentation of the results could be improved. In particular a discussion about DGPDC and details about the experiments should be added to the final version. Further, I agree with R4 that the title is misleading---it is not really a FW method, and is applicable in very specific settings (l1 and trace norm balls) and urge the authors to reconsider the current title. Given the above, I will increase my score from a 4 to a 6. # Summary This work deals with generalized least squares problems with a regularizer, and an additional constraint which can either be an l1 ball or a trace norm ball. The main result of the authors is a primal-dual algorithm where they show that if the optimal dual solution is always s-sparse, then all of the primal updates can be relaxed to also be s-sparse thereby resulting in significant savings. # Significance/Originality My main concern is that this work seems to nearly directly follow from [1]. The current results follow from combining the proof techniques from [1] with the observation from [2] for the trace norm constraint. Otherwise, the experimental results demonstrate a superior performance of the proposed method but I have some major concerns about their validity. # Concerns Major: 1. Comparison with [1] 2. It is never explicitly stated in the paper that the dual optimal is assumed to be sparse. This is quite a strong assumption and needs to be made more clear. 3. For the rcv1, news20, and mnist experiments (Fig 1), Acc projected gradient descent is faster than SVRG. This seems suspicious since typically the latter outperforms the former. I suspect this may be because multiple threads (i.e. parallelism) is being used which gives an unfair advantage to more expensive methods (including the proposed primal-dual FW). I would re-run the experiments with a single thread to have a fairer outcome and believe the current results are misleading. Minor 4. In Algorithms 1 and 2, step 6 involves greedily minimizing a quadratic function. This is no more a Frank-Wolfe method and is probably misleading to call it so. [1]: Lei, Q., Yen, I.E., Wu, C., Dhillon, I.S. & Ravikumar, P.. (2017). Doubly Greedy Primal-Dual Coordinate Descent for Sparse Empirical Risk Minimization. Proceedings of the 34th International Conference on Machine Learning, in PMLR 70:2034-2042 [2]: Allen-Zhu, Zeyuan, et al. "Linear convergence of a frank-wolfe type algorithm over trace-norm balls." Advances in Neural Information Processing Systems. 2017.

This paper proposes a novel method based on a combination of the Frank-Wolfe-like method in [1] (with an extension to sparsity problems) with a greedy coordinate ascent technique. Technical results are adequately sound: I did not encounter a major flaw in the analysis (note that I did not go thoroughly over the proofs in the supplements, but I did skim it). There are a few issues with the complexity analysis of the methods (listed at the end of this review), but these do not change the main advantage of the proposed method (that is avoiding the O(nd) cost of full matrix-vector multiplication with A). I would recommend emphasize, however, that these improved complexity guarantees come at the cost of knowing the sparsity (or rank) of the solution a priori. I am not particularly impressed by the clarity of this paper. It took me a few reads to understand some of the main contributions. One of the very first questions one would ask when reading the manuscript is "Why do we consider a primal-dual formulation while the template is completely smooth and strongly convex?". This should be clarified in an earlier stage of the paper. Some concerns: - Line 143: Reported O(d) and O(n) cost of quick selection holds only if you need to choose a single coordinate. As the proposed algorithm requires "s" and "k" coordinates, the costs should be O(d*log(s)) and O(n*log(k)) respectively, and this is using heap. - Line 144: It is not clear to me why the primal variable updates cost is O(s). A naive implementation requires O(d), as you also have a scaling factor (1-eta). - An extension of [1] for the sparse problems (but not to the primal-dual setup) should be included in the baseline methods in the numerical experiments. This would also clarify the significance of your approach. - Line 102: I guess one iteration of PGD costs O(nd) instead of O(d^2). - Line 135: How does "elastic net" fall into your template? It is a non-smooth regularizer (although strongly convex). Minor concerns: - The title is too generic, the template covered in the paper is kind of specific. It is restricted to ell-1 or trace norm-ball constraints. The optimization templates have strong convexity assumption on both terms. I would recommend using a more descriptive title. - Please consider defining problem parameters like "d" and "n" in the first equation where you introduce the problem template. I would also list the structural assumptions right here at the beginning, such as smoothness and the strong duality of "f" and "g", and that "C" is either ell-1 or trace norm-ball. This would be much more convenient for the readers. - Please avoid using contractions in lines 62 and 340 (doesn't -> does not) - Is "Delta x" in Eq(9) "x_tilde" ? - Eq (6): There are some Frank-Wolfe variants that can solve saddle point problems, although with sublinear rates. I would recommend the authors to have a look at [R1] and [R2, Section 5.4], as these methods might apply for solving (6). A completely subjective comment: I would not call this algorithm a Frank-Wolfe variant (of course the same goes for the algorithm developed in [1]), despite the similar per-iteration cost. The original Frank-Wolfe algorithm is a successive linear approximation approach. The main geometric intuition behind the original is minimizing a linear approximation of the function at the current estimate at each iteration. This in contrast with the classical first-order methods where we minimize a quadratic approximation. The method introduced in [1] (and used in this submission) minimizes a quadratic approximation with sparsity/rank constraint. This method does not recover the classical Frank-Wolfe algorithm even when we choose the sparsity/rank parameter 1. [R1] Gidel, Jebara, Lacoste-Julien, "Frank-Wolfe Algorithms for Saddle Point Problems", 2016. [R2] Yurtsever, Fercoq, Locatello, Cevher, "A Conditional Gradient Framework for Composite Convex Minimization with Applications to Semidefinite Programming", 2018. ========= After author feedback ========= I increase my score to 6 upon the author feedback and the discussions. I insist, however, that the manuscript requires a major update in terms of the presentation. I also respectfully disagree with the authors' feedback, that the proposed approach is the first primal-dual method for constrained problems. This is a vague claim that needs further specifications and clarifications. This is not even true for the line-up of Frank-Wolfe type algorithms. There is a mass of works that consider a primal-dual formulation of a constrained problem for various reasons. I write down a nonexhaustive list of examples: - Tran-Dinh, Cevher "A Primal-Dual Algorithmic Framework for Constrained Convex Minimization" - Liu, Liu, Ma, "On the Nonergodic Convergence Rate of an Inexact Augmented Lagrangian Framework for Composite Convex Programming" - Gidel, Pedregosa, Lacoste-Julien "Frank-Wolfe Splitting via Augmented Lagrangian Method" - Yurtsever, Fercoq, Cevher "A Conditional Gradient-Based Augmented Lagrangian Framework" - Silveti-Falls, Molinari, Fadili "Generalized Conditional Gradient with Augmented Lagrangian for Composite Minimization" ... I am not telling that these works are direct competitors for your approach, but these papers design and analyze primal-dual methods for constrained problems (or that also apply for constrained problems as a special case).