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54
Dual averaging methods for regularized stochastic learning and online optimization
 In Advances in Neural Information Processing Systems 23
, 2009
"... We consider regularized stochastic learning and online optimization problems, where the objective function is the sum of two convex terms: one is the loss function of the learning task, and the other is a simple regularization term such as ℓ1norm for promoting sparsity. We develop extensions of Nes ..."
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Cited by 60 (3 self)
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We consider regularized stochastic learning and online optimization problems, where the objective function is the sum of two convex terms: one is the loss function of the learning task, and the other is a simple regularization term such as ℓ1norm for promoting sparsity. We develop extensions of Nesterov’s dual averaging method, that can exploit the regularization structure in an online setting. At each iteration of these methods, the learning variables are adjusted by solving a simple minimization problem that involves the running average of all past subgradients of the loss function and the whole regularization term, not just its subgradient. In the case of ℓ1regularization, our method is particularly effective in obtaining sparse solutions. We show that these methods achieve the optimal convergence rates or regret bounds that are standard in the literature on stochastic and online convex optimization. For stochastic learning problems in which the loss functions have Lipschitz continuous gradients, we also present an accelerated version of the dual averaging method.
Adaptive Subgradient Methods for Online Learning and Stochastic Optimization
, 2010
"... Stochastic subgradient methods are widely used, well analyzed, and constitute effective tools for optimization and online learning. Stochastic gradient methods ’ popularity and appeal are largely due to their simplicity, as they largely follow predetermined procedural schemes. However, most common s ..."
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Cited by 42 (0 self)
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Stochastic subgradient methods are widely used, well analyzed, and constitute effective tools for optimization and online learning. Stochastic gradient methods ’ popularity and appeal are largely due to their simplicity, as they largely follow predetermined procedural schemes. However, most common subgradient approaches are oblivious to the characteristics of the data being observed. We present a new family of subgradient methods that dynamically incorporate knowledge of the geometry of the data observed in earlier iterations to perform more informative gradientbased learning. The adaptation, in essence, allows us to find needles in haystacks in the form of very predictive but rarely seenfeatures. Ourparadigmstemsfromrecentadvancesinstochasticoptimizationandonlinelearning which employ proximal functions to control the gradient steps of the algorithm. We describe and analyze an apparatus for adaptively modifying the proximal function, which significantly simplifies setting a learning rate and results in regret guarantees that are provably as good as the best proximal function that can be chosen in hindsight. In a companion paper, we validate experimentally our theoretical analysis and show that the adaptive subgradient approach outperforms stateoftheart, but nonadaptive, subgradient algorithms. 1
Proximal Methods for Hierarchical Sparse Coding
, 2010
"... Sparse coding consists in representing signals as sparse linear combinations of atoms selected from a dictionary. We consider an extension of this framework where the atoms are further assumed to be embedded in a tree. This is achieved using a recently introduced treestructured sparse regularizatio ..."
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Cited by 39 (8 self)
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Sparse coding consists in representing signals as sparse linear combinations of atoms selected from a dictionary. We consider an extension of this framework where the atoms are further assumed to be embedded in a tree. This is achieved using a recently introduced treestructured sparse regularization norm, which has proven useful in several applications. This norm leads to regularized problems that are difficult to optimize, and we propose in this paper efficient algorithms for solving them. More precisely, we show that the proximal operator associated with this norm is computable exactly via a dual approach that can be viewed as the composition of elementary proximal operators. Our procedure has a complexity linear, or close to linear, in the number of atoms, and allows the use of accelerated gradient techniques to solve the treestructured sparse approximation problem at the same computational cost as traditional ones using the ℓ1norm. Our method is efficient and scales gracefully to millions of variables, which we illustrate in two types of applications: first, we consider fixed hierarchical dictionaries of wavelets to denoise natural images. Then, we apply our optimization tools in the context of dictionary learning, where learned dictionary elements naturally organize in a prespecified arborescent structure, leading to a better performance in reconstruction of natural image patches. When applied to text documents, our method learns hierarchies of topics, thus providing a competitive alternative to probabilistic topic models.
Design and Evaluation of a RealTime URL Spam Filtering Service
"... On the heels of the widespread adoption of web services such as social networks and URL shorteners, scams, phishing, and malware have become regular threats. Despite extensive research, emailbased spam filtering techniques generally fall short for protecting other web services. To better address th ..."
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Cited by 34 (6 self)
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On the heels of the widespread adoption of web services such as social networks and URL shorteners, scams, phishing, and malware have become regular threats. Despite extensive research, emailbased spam filtering techniques generally fall short for protecting other web services. To better address this need, we present Monarch, a realtime system that crawls URLs as they are submitted to web services and determines whether the URLs direct to spam. We evaluate the viability of Monarch and the fundamental challenges that arise due to the diversity of web service spam. We show that Monarch can provide accurate, realtime protection, but that the underlying characteristics of spam do not generalize across web services. In particular, we find that spam targeting email qualitatively differs in significant ways from spam campaigns targeting Twitter. We explore the distinctions between email and Twitter spam, including the abuse of public web hosting and redirector services. Finally, we demonstrate Monarch’s scalability, showing our system could protect a service such as Twitter— which needs to process 15 million URLs/day—for a bit under $800/day.
Smoothing Proximal Gradient Method for General Structured Sparse Learning
"... We study the problem of learning high dimensional regression models regularized by a structuredsparsityinducing penalty that encodes prior structural information on either input or output sides. We consider two widely adopted types of such penalties as our motivating examples: 1) overlapping group ..."
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Cited by 23 (5 self)
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We study the problem of learning high dimensional regression models regularized by a structuredsparsityinducing penalty that encodes prior structural information on either input or output sides. We consider two widely adopted types of such penalties as our motivating examples: 1) overlapping group lasso penalty, based on the ℓ1/ℓ2 mixednorm penalty, and 2) graphguided fusion penalty. For both types of penalties, due to their nonseparability, developing an efficient optimization method has remained a challenging problem. In this paper, we propose a general optimization approach, called smoothing proximal gradient method, which can solve the structured sparse regression problems with a smooth convex loss and a wide spectrum of structuredsparsityinducing penalties. Our approach is based on a general smoothing technique of Nesterov [17]. It achieves a convergence rate faster than the standard firstorder method, subgradient method, and is much more scalable than the most widely used interiorpoint method. Numerical results are reported to demonstrate the efficiency and scalability of the proposed method. 1
Convergence Rates of Inexact ProximalGradient Methods for Convex Optimization
 NIPS'11 25 TH ANNUAL CONFERENCE ON NEURAL INFORMATION PROCESSING SYSTEMS
, 2011
"... We consider the problem of optimizing the sum of a smooth convex function and a nonsmooth convex function using proximalgradient methods, where an error is present in the calculation of the gradient of the smooth term or in the proximity operator with respect to the nonsmooth term. We show that b ..."
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Cited by 19 (3 self)
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We consider the problem of optimizing the sum of a smooth convex function and a nonsmooth convex function using proximalgradient methods, where an error is present in the calculation of the gradient of the smooth term or in the proximity operator with respect to the nonsmooth term. We show that both the basic proximalgradient method and the accelerated proximalgradient method achieve the same convergence rate as in the errorfree case, provided that the errors decrease at appropriate rates. Using these rates, we perform as well as or better than a carefully chosen fixed error level on a set of structured sparsity problems.
Optimal distributed online prediction using minibatches
, 2010
"... Online prediction methods are typically presented as serial algorithms running on a single processor. However, in the age of webscale prediction problems, it is increasingly common to encounter situations where a single processor cannot keep up with the high rate at which inputs arrive. In this wor ..."
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Cited by 16 (2 self)
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Online prediction methods are typically presented as serial algorithms running on a single processor. However, in the age of webscale prediction problems, it is increasingly common to encounter situations where a single processor cannot keep up with the high rate at which inputs arrive. In this work, we present the distributed minibatch algorithm, a method of converting many serial gradientbased online prediction algorithms into distributed algorithms. We prove a regret bound for this method that is asymptotically optimal for smooth convex loss functions and stochastic inputs. Moreover, our analysis explicitly takes into account communication latencies between nodes in the distributed environment. We show how our method can be used to solve the closelyrelated distributed stochastic optimization problem, achieving an asymptotically linear speedup over multiple processors. Finally, we demonstrate the merits of our approach on a webscale online prediction problem.
NonAsymptotic Analysis of Stochastic Approximation Algorithms for Machine Learning
"... We consider the minimization of a convex objective function defined on a Hilbert space, which is only available through unbiased estimates of its gradients. This problem includes standard machine learning algorithms such as kernel logistic regression and leastsquares regression, and is commonly ref ..."
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Cited by 15 (6 self)
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We consider the minimization of a convex objective function defined on a Hilbert space, which is only available through unbiased estimates of its gradients. This problem includes standard machine learning algorithms such as kernel logistic regression and leastsquares regression, and is commonly referred to as a stochastic approximation problem in the operations research community. We provide a nonasymptotic analysis of the convergence of two wellknown algorithms, stochastic gradient descent (a.k.a. RobbinsMonro algorithm) as well as a simple modification where iterates are averaged (a.k.a. PolyakRuppert averaging). Our analysis suggests that a learning rate proportional to the inverse of the number of iterations, while leading to the optimal convergence rate in the strongly convex case, is not robust to the lack of strong convexity or the setting of the proportionality constant. This situation is remedied when using slower decays together with averaging, robustly leading to the optimal rate of convergence. We illustrate our theoretical results with simulations on synthetic and standard datasets. 1
Efficient first order methods for linear composite regularizers,” ArXiv preprint:1104.1436
, 2011
"... USA. A wide class of regularization problems in machine learning and statistics employ a regularization term which is obtained by composing a simple convex function ω with a linear transformation. This setting includes Group Lasso methods, the Fused Lasso and other total variation methods, multitas ..."
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Cited by 9 (3 self)
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USA. A wide class of regularization problems in machine learning and statistics employ a regularization term which is obtained by composing a simple convex function ω with a linear transformation. This setting includes Group Lasso methods, the Fused Lasso and other total variation methods, multitask learning methods and many more. In this paper, we present a general approach for computing the proximity operator of this class of regularizers, under the assumption that the proximity operator of the function ω is known in advance. Our approach builds on a recent line of research on optimal first order optimization methods and uses fixed point iterations for numerically computing the proximity operator. It is more general than current approaches and, as we show with numerical simulations, computationally more efficient
Accelerated and inexact forwardbackward algorithms
 Optimization Online, EPrint
"... Abstract. We propose a convergence analysis of accelerated forwardbackward splitting methods for composite function minimization, when the proximity operator is not available in closed form, and can only be computed up to a certain precision. We prove that the 1/k 2 convergence rate for the functio ..."
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Cited by 8 (6 self)
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Abstract. We propose a convergence analysis of accelerated forwardbackward splitting methods for composite function minimization, when the proximity operator is not available in closed form, and can only be computed up to a certain precision. We prove that the 1/k 2 convergence rate for the function values can be achieved if the admissible errors are of a certain type and satisfy a sufficiently fast decay condition. Our analysis is based on the machinery of estimate sequences first introduced by Nesterov for the study of accelerated gradient descent algorithms. Furthermore, we give a global complexity analysis, taking into account the cost of computing admissible approximations of the proximal point. An experimental analysis is also presented.