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arXiv 1601.04737math.OCcs.LGstat.ML
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子采样牛顿法I:全局收敛算法

Sub-Sampled Newton Methods I: Globally Convergent Algorithms

International Computer Science Institute(国际计算机科学研究所);University of California at Berkeley(加州大学伯克利分校)

Farbod Roosta-Khorasani, Michael W. Mahoney

摘要:

大规模最优化问题在机器学习和数据分析中普遍存在,并且存在大量用于解决此类问题的算法。这些算法中的许多采用子采样,作为加速计算和/或隐式实现统计正则化形式的一种方式。在本文中,我们考虑二阶迭代优化算法,我们提供的范围上的收敛的变种的牛顿法,将均匀子采样作为一种手段来估计梯度和/或海森。我们的界限是非渐近的和定量的。我们的算法是全球性的,并保证从任何初始收敛。使用随机矩阵浓度不等式,可以对Hessian进行子采样以保持曲率信息。我们的第一个算法结合了海森子采样,同时使用全梯度。我们还给出了额外的收敛结果时,通过修改其频谱或脊型正则化的子采样海森正则化。接下来,除了Hessian子采样之外,我们还考虑对梯度进行子采样,以进一步降低每次迭代的计算复杂性。我们使用随机数值线性代数的近似矩阵乘法结果来获得适当的采样策略。在所有这些算法中,计算更新归结为求解大规模线性系,这在计算上是昂贵的。作为一种补救措施,对于我们所有的算法,我们也给出了整体收敛结果的情况下,不精确的更新,这样的线性系只解决近似。本文有一个更先进的配套文件,[42],其中我们证明,通过做更细粒度的分析,我们可以得到这些算法的局部收敛的问题无关的界限,并探索权衡,以改善本文的基本结果。

英文摘要:

Large scale optimization problems are ubiquitous in machine learning and data analysis and there is a plethora of algorithms for solving such problems. Many of these algorithms employ sub-sampling, as a way to either speed up the computations and/or to implicitly implement a form of statistical regularization. In this paper, we consider second-order iterative optimization algorithms and we provide bounds on the convergence of the variants of Newton's method that incorporate uniform sub-sampling as a means to estimate the gradient and/or Hessian. Our bounds are non-asymptotic and quantitative. Our algorithms are global and are guaranteed to converge from any initial iterate. Using random matrix concentration inequalities, one can sub-sample the Hessian to preserve the curvature information. Our first algorithm incorporates Hessian sub-sampling while using the full gradient. We also give additional convergence results for when the sub-sampled Hessian is regularized by modifying its spectrum or ridge-type regularization. Next, in addition to Hessian sub-sampling, we also consider sub-sampling the gradient as a way to further reduce the computational complexity per iteration. We use approximate matrix multiplication results from randomized numerical linear algebra to obtain the proper sampling strategy. In all these algorithms, computing the update boils down to solving a large scale linear system, which can be computationally expensive. As a remedy, for all of our algorithms, we also give global convergence results for the case of inexact updates where such linear system is solved only approximately. This paper has a more advanced companion paper, [42], in which we demonstrate that, by doing a finer-grained analysis, we can get problem-independent bounds for local convergence of these algorithms and explore trade-offs to improve upon the basic results of the present paper.

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