Asynchronous Fully-Decentralized SGD in the Cluster-Based Model

This paper presents fault-tolerant asynchronous Stochastic Gradient Decent (SGD) algorithms. SGD is widely used for approximating the minimum of a cost function Q, a core part of optimization and learning algorithms. Our algorithms are designed for the cluster-based model, which combines message-passing and shared-memory communication layers. Processes may fail by crashing, and the algorithm inside each cluster is wait-free, using only reads and writes. For a strongly convex Q, our algorithm can withstand partitions of the system. It provides convergence rate that is the maximal distributed acceleration over the optimal convergence rate of sequential SGD. For arbitrary smooth functions, the convergence rate has an additional term that depends on the maximal difference between the parameters at the same iteration. (This holds under standard assumptions on Q). In this case, the algorithm obtains the same convergence rate as sequential SGD, up to a logarithmic factor. This is achieved by using, at each iteration, a multidimensional approximate agreement algorithm, tailored for the cluster-based model. The general algorithm communicates with nonfaulty processes belonging to clusters that include a majority of all processes. We prove that this condition is necessary when optimizing some non-convex functions.