PopSGD: Decentralized Stochastic Gradient Descent in the Population Model
Authors: Giorgi Nadiradze, Amirmojtaba Sabour, Aditya Sharma, Ilia Markov, Vitaly Aksenov, Dan Alistarh.
Keywords: Distributed machine learning, distributed optimization, decentralized parallel SGD, population protocols
TL;DR: We show that large-scale distributed optimization can be performed efficiently in the population model of distributed computing. 
Abstract: The population model is a standard way to represent large-scale decentralized
distributed systems, in which agents with limited computational power interact
in randomly chosen pairs, in order to collectively solve global computational
tasks. In contrast with synchronous gossip models, nodes are anonymous, lack a
common notion of time, and have no control over their scheduling. In this paper,
we examine whether large-scale distributed optimization can be performed in this
extremely restrictive setting. 

We introduce and analyze a natural decentralized variant of stochastic gradient
descent (SGD), called PopSGD, in which every node maintains a local parameter,
and is able to compute stochastic gradients with respect to this parameter. 
Every pair-wise node interaction performs a stochastic gradient step at each
agent, followed by averaging of the two models. We prove that, under standard
assumptions, SGD can converge even in this extremely loose, decentralized
setting, for both convex and non-convex objectives.  Moreover, surprisingly, in
the former case, the algorithm can achieve linear speedup in the number of nodes
n. Our analysis leverages a new technical connection between decentralized SGD
and randomized load balancing, which enables us to tightly bound the
concentration of node parameters. We validate our analysis through experiments,
showing that PopSGD can achieve convergence and speedup for large-scale
distributed learning tasks in a supercomputing environment.