Paper:
Improved Iteration Complexity Bounds of Cyclic Block Coordinate Descent for Convex Problems
The iteration complexity of the block-coordinate descent (BCD) type algorithm has been under extensive investigation. It was recently shown that for convex problems the classical cyclic BCGD (block coordinate gradient descent) achieves an O(1/r) complexity (r is the number of passes of all blocks). However, such bounds are at least linearly depend on K (the number of variable blocks), and are at least K times worse than those of the gradient descent (GD) and proximal gradient (PG) methods. In this paper, we close such theoretical performance gap between cyclic BCD and GD/PG. First we show that for a family of quadratic nonsmooth problems, the complexity bounds for cyclic Block Coordinate Proximal Gradient (BCPG), a popular variant of BCD, can match those of the GD/PG in terms of dependency on K (up to a log 2 (K) factor). Second, we establish an improved complexity bound for Coordinate Gradient Descent (CGD) for general convex problems which can match that of GD in certain scenarios. Our bounds are sharper than the known bounds as they are always at least K times worse than GD. Our analyses do not depend on the update order of block variables inside each cycle, thus our results also apply to BCD methods with random permutation (random sampling without replacement, another popular variant).

Review:
Report for "Improved Iteration Complexity Bounds of Cyclic Block Coordinate Descent for Convex Problems".## SummaryThe authors consider methods of block coordinate types with cyclic block selection rules. The focus is on a composite model involving a non-separable smooth term and a separable non-smooth term. The main methods of interest are block proximal gradient and cyclic minimization. The authors propose a refined analysis of these methods for problems for which the smooth term is quadratic. This leads to new convergence rate estimates for this class of problems. Finally an alternative analysis is presented for the general case with the restriction that blocks should have size one (coordinate proximal gradient). ## Main commentsThe analysis of sections 2 is original, relevant and quite impressive. In some settings, the complexity estimates match those of the usual proximal gradient method which was not known to be possible for such a general class of problems. The analysis removes a multiplicative dependance in the number of blocks compared to previous work. This constitutes a very strong theoretical contribution. Section 3 is also very interesting, but I have one concern with the proof of Theorem 3.1 (see next paragraph). The results of section 4 are a bit less spectacular but still of great interest. Overall, the proof arguments look reasonable and are quite original. As the questions adressed in this paper are strongly related to both machine learning and optimization, I warmly recommend this work for publication after taking into account the following remark:In proof of Theorem 3.1 at thebottom of page 15, I think that the last step (i) is wrong. It is mentioned that (i) can be derived from (49), but I am not sure it is true since Lmin is not the minimal eigenvalue of the matrix of interest in (49), but the minimum of the maximum eigenvalues of its diagonal blocks. I think it is possible to fix this by introducing a factor K, using | | sum b _ i | | ^ 2 leq K sum | | b _ i | | ^2It could also be possible to fix it by using the idea that exact minimization makes more progress than a proximal gradient step, and using arguments from Lemma 2.1, but I am not completely sure about it.In any case, if the authors agree with this remark, important changes have to be made in the proofs and/or in the main text. ## Additional comments and suggestions- An important variant of cyclic block decomposition methods is the random permutation approach where the order of blocks changes at each itteration. In some applications, this additional randomization gives a practical advantage. Most of results usually given for cyclic block decomposition methods still hold deterministically when the order of the blocks changes arbitrarily between iterations. I suspect that this is also the case for the proposed analysis which, if true, would be worth mentioning.- In section 4, the assumption that N=1 is not only for notational simplicity. The analysis falls appart when N > 1. Indeed, the analysis uses the mean valued-theorem in equation (78) which does not hold for vector valued functions. Therefore the analysis cannot be straightforwardly extended to blocks of size more than one without further arguments.- The quantity defined in (3) might very well be infinite, unless compactness of sublevel sets is assumed as in [3] for example. This could be precised.- Notation in (2) constitutes a slight abuse as the order of the arguments of g is not the same as in (1). Without needing to change notations, it could be a good idea to give precisions.- I do not think there is a result as (6) in [3] which is concerned mostly about BCGD and not BCD (only for the two block case). Maybe also give a precise pointer in reference [6].- In (8), all the blocks are assumed to have the same size. I guess the analysis can be extended to cases for which blocks have different sizes. Maybe emphasize that this is not an assumption made for restriction, but for simplicity.- In (7), the constant is independant of problem dimension for fixed ||x^0 - x^*|| and L. From a practical perspective, these values usually depend on the dimension of the problem.- (11) is true, although not very common. Maybe add a sentence explaining that both (9) and (11) are equivalent to the same subdifferential inclusion.- Kroneker - > Kronecker. Maybe precise the notation. I suspect that in the notations of the paper, the order of matrices involved in Kronecker products is reversed compared to other sources (e.g. wikipedia).- Line 661 and 688 in the appendix refer to optimality condition of w_{k+1}^{r+1}. I guess it should be optimality of x^(r)_{k}. Furthermore, this optimality condition could be written explicitely as (11) is for example.- Just before Thorem 4.1, it is mentioned that the results can be established "in the same way as the standard analysis of GD". It would be nicer to have the precise statement, and at least a reference (e.g. [Theorem 2.1.13 10]).- Quantification over r could be more precise in Theorem 4.1 and Corollary 4.1, in particylar, the estimates presented are not well defined for r = 0.- In the discussion of section 4, I think that another relevant feature of the newly proposed bound is that it does not increase with the artificial addition of new variables on which the problem does not depend (with Li = 0). This is not the case for the quantity KL which is problematic since such an artificial introduction does not modify the iterates an we therefore anticipate that it should not modify the complexity estimates.- In the abstract, it is worth precising that the iteration counter is the number of times all blocks have been updated. It would be OK to have a dependance in K if the counter consisted in the number of blocks updated as in Random block methods.- Page 5, line 228, I guess "same dependency on K" should be "same dependency on L".- Remark 3.1 "those derive here" - > "those derived here".- Toplitz - > Toeplitz.- Although transparent, the notations Pmax and Pmin are never defined.- In Lemma 4.1, the logical quantification over xi should be more precise. I think that the correct quantifier is "there exists"- In (36), I guess there are missing indices for A(:,i). This is matlab notation, maybe precise what it means.- In theproof of Lemma 2.1, there are missing indices k in $h_k(x^r)$ on line 504- In (45), there is a use of (11) on the fifth line which could be precised.- Line 637 "the fact" - > "and the fact"- Line 702 "Combining the above to inequalities". There are one inequality and one equality. "to" - > "two".- Section 13 should be "Proof of Corrolary 4.1"?