Monday, August 01, 2016

Seeing the Forest from the Trees in Two Looks: Matrix Sketching by Cascaded Bilateral Sampling

Here is what I note from the following paper:

(1) Random projection is very accurate but the time consumption can be significant; (2) FJLTleverage CUR has similar time consumption and can be less accurate; (3) Adaptive-sampling CUR is computationally most expensive (in computing the residue of approximation); (4) Two-step Sketch-CUR is the least accurate, which we speculate is due to the instability of Sketch-CUR; (5) Our approach with weighted k-means sampling has a clear computational gain in dense matrices with a good accuracy; (6) Our approach using hard-threshold sampling performs particularly well in sparse matrices
One of the main problem with random projection is the time they take to be performed and that the FJLT which aims to achieve that speed-up is in fact less accurate. The method developed by the authors seem to be doing well. Here is the paper: Seeing the Forest from the Trees in Two Looks: Matrix Sketching by Cascaded Bilateral Sampling by Kai Zhang, Chuanren Liu, Jie Zhang, Hui Xiong, Eric Xing, Jieping Ye
Matrix sketching is aimed at finding close approximations of a matrix by factors of much smaller dimensions, which has important applications in optimization and machine learning. Given a matrix A of size m by n, state-of-the-art randomized algorithms take O(m * n) time and space to obtain its low-rank decomposition. Although quite useful, the need to store or manipulate the entire matrix makes it a computational bottleneck for truly large and dense inputs. Can we sketch an m-by-n matrix in O(m + n) cost by accessing only a small fraction of its rows and columns, without knowing anything about the remaining data? In this paper, we propose the cascaded bilateral sampling (CABS) framework to solve this problem. We start from demonstrating how the approximation quality of bilateral matrix sketching depends on the encoding powers of sampling. In particular, the sampled rows and columns should correspond to the code-vectors in the ground truth decompositions. Motivated by this analysis, we propose to first generate a pilot-sketch using simple random sampling, and then pursue more advanced, "follow-up" sampling on the pilot-sketch factors seeking maximal encoding powers. In this cascading process, the rise of approximation quality is shown to be lower-bounded by the improvement of encoding powers in the follow-up sampling step, thus theoretically guarantees the algorithmic boosting property. Computationally, our framework only takes linear time and space, and at the same time its performance rivals the quality of state-of-the-art algorithms consuming a quadratic amount of resources. Empirical evaluations on benchmark data fully demonstrate the potential of our methods in large scale matrix sketching and related areas.

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