Distributed PCA

Problem Formulation

Principal Component Analysis (PCA) is an unsupervised, non-parametric statistical technique primarily used for dimensionality reduction in machine learning. In conventional PCA, the goal is to recover a low rank matrix from measurements $\boldsymbol{y}_{ik} = \langle \boldsymbol{X}^*, \boldsymbol{A}_{ik}\rangle$, $i =1,\ldots, m$, and $k = 1,\ldots, q$. Here $\boldsymbol{X}^*$ is the ground truth matrix and $\boldsymbol{X}^* = \boldsymbol{U}^*\boldsymbol{B}^*$, where $\boldsymbol{U}^*$ and $\boldsymbol{B}^*$ are low dimensional matrices of size $n\times r$ and $r \times q$ respectively. Also, matrices $\boldsymbol{A}_{ik}$'s of size $n \times q$ are the design matrices.

A main issue with the conventional PCA is that recovering matrix $\boldsymbol{X}^*$ is a centralized algorithm. Thus, it faces memory and computation complexity problems. To overcome these issues, we present a distributed version of the problem where \[ \boldsymbol{y}_{ik} = \langle \boldsymbol{x}^*_k, \boldsymbol{a}_{ik} \rangle. \] The design vectors, $\boldsymbol{a}_{i,k}$'s, are mutually independent. Still the goal is to recover $\boldsymbol{U}^*$ and $\boldsymbol{B}^*$, and hence $\boldsymbol{X}^*$. In this system there is a master server and $q$ computing nodes.

The idea is that once we have a good estimate of $\boldsymbol{U}^*$, say $\hat{\boldsymbol{U}}$, then $k$-th node can use it to get an estimate of $\hat{\boldsymbol{b}}_k$. Next, each node will send $\hat{\boldsymbol{b}}_k$ to the master server and the server will update $\hat{\boldsymbol{U}}$. The process iterates untill a stop criteria hits. More preciesly, at each iteration the server sends $\boldsymbol{y}_{ik}$ and $\hat{\boldsymbol{U}}'\boldsymbol{a}_{ik}$, for $i=1,\ldots,m$, to the $k$-th node and the nodes estimate $\hat{\boldsymbol{b}}_k$. Next, the nodes send $\hat{\boldsymbol{b}}_k$'s to the server and the server use it to update $\hat{\boldsymbol{U}}$. Thus, we introduced a decentralized algorithm for PCA that is efficient both in memory and computation point of views. We present a Gradient Based algorithm and showed that if \[ mq \geq C(n+q)r^2, \] then with high probability the algorithm converges. Note that since $mq \geq (n+q)r$ thus our algorithm needs only $r$ times more measurements than the minimum required. Also, each node might enter or leave the party at anytime.

S.Nayer and N.Vaswani, "Fast and Sample-Efficient Federated Low Rank Matrix Recovery from Column-wise Linear and Quadratic Projections", .