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Title: Decentralized Deep Learning with Arbitrary Communication Compression

Paper content:
Decentralized training of deep learning models is a key element for enabling data privacy and on-device learning over networks, as well as for efficient scaling to large compute clusters.
As current approaches are limited by network bandwidth, we propose the use of communication compression in the decentralized training context.
We show that Choco-SGD achieves linear speedup in the number of workers for arbitrary high compression ratios on general non-convex functions, and non-IID training data.
We demonstrate the practical performance of the algorithm in two key scenarios: the training of deep learning models
(i) over decentralized user devices, connected by a peer-to-peer network and
(ii) in a datacenter.
Distributed machine learning-i.e. the training of machine learning models using distributed optimization algorithms-has enabled many recent successful applications in research and industry.
Such methods offer two of the key success factors:
1) computational scalability by leveraging the simultaneous computational power of many devices, and
2) data-locality, the ability to perform joint training while keeping each part of the training data local to each participating device.
Recent theoretical results indicate that decentralized schemes can be as efficient as the centralized approaches, at least when considering convergence of training loss vs. iterations (Scaman et al., 2017; Lian et al., 2017; Tang et al., 2018; Koloskova et al., 2019; Assran et al., 2019) .
Gradient compression techniques have been proposed for the standard distributed training case (Alistarh et al., 2017; Wen et al., 2017; Lin et al., 2018b; Wangni et al., 2018; Stich et al., 2018) , to reduce the amount of data that has to be sent over each communication link in the network.
For decentralized training of deep neural networks, Tang et al. (2018) introduce two algorithms (DCD, ECD) which allow for communication compression.
However, both these algorithms are restrictive with respect to the used compression operators, only allowing for unbiased compressors and-more significantlyso far not supporting arbitrarily high compression ratios.
We here study CHOCO-SGD-recently introduced for convex problems only (Koloskova et al., 2019 )-which overcomes these constraints.
For the evaluation of our algorithm we in particular focus on the generalization performance (on the test-set) on standard machine learning benchmarks, hereby departing from previous work such as e.g. (Tang et al., 2018; Wang et al., 2019; Tang et al., 2019b; Reisizadeh et al., 2019 ) that mostly considered training performance (on the train-set).
We study two different scenarios: firstly,
(i) training on a challenging peer-to-peer setting, where the training data is distributed over the training devices (and not allowed to move), similar to the federated learning setting (McMahan et al., 2017) .
We are again able to show speed-ups for CHOCO-SGD over the decentralized baseline (Lian et al., 2017) with much less communication overhead.
Secondly,
(ii) training in a datacenter setting, where decentralized communication patterns allow better scalability than centralized approaches.
For this setting we show that communication efficient CHOCO-SGD can improve time-to-accuracy on large tasks, such as e.g. ImageNet training.
However, when investigating the scaling of decentralized algorithms to larger number of nodes we observe that (all) decentralized schemes encounter difficulties and often do not reach the same (test and train) performance as centralized schemes.
As these findings do point out some deficiencies of current decentralized training schemes (and are not particular to our scheme) we think that reporting these results is a helpful contribution to the community to spur further research on decentralized training schemes that scale to large number of peers.
We propose the use of CHOCO-SGD (and its momentum version) for enabling decentralized deep learning training in bandwidth-constrained environments.
We provide theoretical convergence guarantees for the non-convex setting and show that the algorithm enjoys the a linear speedup in the number of nodes.
We empirically study the performance of the algorithm in a variety of settings on image classification (ImageNet-1k, Cifar10) and on a language modeling task (WikiText-2).
Whilst previous work successfully demonstrated that decentralized methods can be a competitive alternative to centralized training schemes when no communication constraints are present (Lian et al., 2017; Assran et al., 2019) , our main contribution is to enable training in strongly communication-restricted environments, and while respecting the challenging constraint of locality of the training data.
We theoretically and practically demonstrate the performance of decentralized schemes for arbitrary high communication compression, and under data-locality, and thus significantly expand the reach of potential applications of fully decentralized deep learning.

assistant: text: We propose Choco-SGD---decentralized SGD with compressed communication---for non-convex objectives and show its strong performance in various deep learning applications (on-device learning, datacenter case).