arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 1908.00633math.NAcs.DScs.NAstat.ML
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预条件子选择的随机算法

A Randomized Algorithm for Preconditioner Selection

Conner DiPaolo, Weiqing Gu

AI总结:

本文提出一种随机算法,通过草图化快速估计预条件子稳定性,从而在候选集中选择最优预条件子,并证明其计算效率与并行性,实验显示在核回归中优于非预条件方法。

摘要:

在使用迭代方法求解线性系统 $\boldsymbol{Ax}=\boldsymbol{b}$ 时,选择预条件子 $\boldsymbol{M}$ 的任务是困难的。例如,即使可以获得候选预条件子集合 $\boldsymbol{M}_1,\boldsymbol{M}_2,\ldots,\boldsymbol{M}_n$,目前仍不清楚如何实际选择能够最小化迭代算法迭代次数以达到 $\boldsymbol{x}$ 的合适近似的 $\boldsymbol{M}_i$。本文在这一子问题上取得了进展,表明预条件子稳定性 $\\|\boldsymbol{I}-\boldsymbol{M}^{-1}\boldsymbol{A}\\|_\mathsf{F}$(已知可预测预条件子质量)可以通过使用草图化方法在运行常数次共轭梯度迭代的时间内计算。尽管有民间传说认为该量难以计算,并且我们给出的证明确保该量不可能由确定性算法在有用时间内近似,但我们的方法仍然可行。利用我们的估计器,我们提供了一种方法,可以在与运行 $n\log n$ 阶共轭梯度算法步骤相称的浮点运算次数内,从 $n$ 个候选中可证明地选择最小稳定性预条件子。我们的方法还可以建议从业者,如果所有候选看起来都无用,则完全不使用预条件子。该算法极易实现且可平凡并行化。在我们的一个实验中,我们使用我们的预条件子选择算法创建了据我们所知第一个预条件核回归方法,该方法在标准测试中报告从不使用比非预条件对应方法更多的迭代次数。

英文摘要:

The task of choosing a preconditioner $\boldsymbol{M}$ to use when solving a linear system $\boldsymbol{Ax}=\boldsymbol{b}$ with iterative methods is difficult. For instance, even if one has access to a collection $\boldsymbol{M}_1,\boldsymbol{M}_2,\ldots,\boldsymbol{M}_n$ of candidate preconditioners, it is currently unclear how to practically choose the $\boldsymbol{M}_i$ which minimizes the number of iterations of an iterative algorithm to achieve a suitable approximation to $\boldsymbol{x}$. This paper makes progress on this sub-problem by showing that the preconditioner stability $\|\boldsymbol{I}-\boldsymbol{M}^{-1}\boldsymbol{A}\|_\mathsf{F}$, known to forecast preconditioner quality, can be computed in the time it takes to run a constant number of iterations of conjugate gradients through use of sketching methods. This is in spite of folklore which suggests the quantity is impractical to compute, and a proof we give that ensures the quantity could not possibly be approximated in a useful amount of time by a deterministic algorithm. Using our estimator, we provide a method which can provably select the minimal stability preconditioner among $n$ candidates using floating point operations commensurate with running on the order of $n\log n$ steps of the conjugate gradients algorithm. Our method can also advise the practitioner to use no preconditioner at all if none of the candidates appears useful. The algorithm is extremely easy to implement and trivially parallelizable. In one of our experiments, we use our preconditioner selection algorithm to create to the best of our knowledge the first preconditioned method for kernel regression reported to never use more iterations than the non-preconditioned analog in standard tests.

机构、总结、中文摘要均由AI分析而来,可能存在错误,请以论文正文为准。

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