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ML Lunch (March 10): More effective distributed ML via a stale synchronous parallel parameter server

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Published on Mar 17, 2014

Title: More effective distributed ML via a stale synchronous parallel parameter server
Speaker: Qirong Ho

Abstract
Modern applications awaiting next generation machine intelligence systems have posed unprecedented scalability challenges. These scalability needs arise from at least two aspects: 1) massive data volume, such as societal-scale social graphs with up to hundreds of millions of nodes; and 2) massive model size, such as the Google Brain deep neural network containing billions of parameters. Although there exist means and theories to support reductionist approaches like subsampling data or using small models, there is an imperative need for sound and effective distributed ML methodologies for users who cannot be well-served by such shortcuts. To this end, we propose a parameter server system for distributed ML, which follows a Stale Synchronous Parallel (SSP) model of computation that maximizes the time computational workers spend doing useful work on ML algorithms, while still providing correctness guarantees. The parameter server provides an easy-to-use shared interface for read/write access to an ML model's values (parameters and variables), and the SSP model allows distributed workers to read older, stale versions of these values from a local cache, instead of waiting to get them from a central storage. This significantly increases the proportion of time workers spend computing, as opposed to waiting. Furthermore, the SSP model ensures ML algorithm correctness by limiting the maximum age of the stale values. We provide a proof of correctness under SSP, as well as empirical results demonstrating that the SSP model achieves faster algorithm convergence on several different ML problems, compared to fully-synchronous and asynchronous schemes.

For more ML lunch talks, visit http://www.cs.cmu.edu/~learning/

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