NeurIPS 2019
Sun Dec 8th through Sat the 14th, 2019 at Vancouver Convention Center
Paper ID:7539
Title:Communication trade-offs for Local-SGD with large step size

Reviewer 1

This paper seeks to address the existing ambiguity in distributed SGD and knowing how and when to aggregate the solution. They start by analyzing two extreme aggregate policies (every time and at the end) and show that the more frequent communication is by far better. Their closest work is [21] that assumes uniformly bounded gradients and [40] that also assumes bounded gradients and small step-sizes. They have clear notation and assumptions that make the paper concise and easier to follow. Their main result is in showing they can decompose the results into three terms that they then show bounds for. They bounds help show where communication comes into play to distinguish OSA and MBA. They make the interesting point that though the asymptotic equivalence exists, the pre-asympotic analysis is what distinguish the results in practice. This is interesting and practical and hopefully more papers consider pre-asymptotics.

Reviewer 2

The paper is densely packed with valuable novel analysis of a very popular algorithm: distributed SGD. It makes a substantial contribution to alleviating the communication cost of this algorithm, which is a major bottleneck in its practical application. It is a challenging read, but this is mostly due to the inherent complexity of the subject matter of analysing a distributed stochastic optimisation process. However, a little improvement seems possible by adding a paragraph in the beginning on the underlying optimisation problem and the kind of bounds one is interested in (see also below). Beyond that the paper is breaking down the complexity in an exemplary manner, giving interpretations of all formal results and a systematic listing of all the various assumptions that have been traditionally used in the formal analysis of (distributed) SGD. Some suggestions: - The paper mentions that it is focused on analysing the Malahanobis distance of the parameter vector from the optimum as the “natural quantity in this setting”. However, it also mentions that one “eventually aims to minimise the excess risk”, i.e., the expected difference in function value from the optimum. This is plausible as we are ultimately interested in optimising F. It is mentioned that the two quantities are related but it is also hinted that using straightforward relations does not necessarily lead to a tight analysis. The paper could benefit from a more consistent and explicit discussion of this issue; especially, how translation of parameter distance relates to F differences under the various assumptions listed in Section 2.3. Right now the respective paragraph in Section 3.1 feels more like an afterthought that is not well integrated into the main narrative of the paper. - The paper hints at a couple of places that it would be interesting to investigate adaptive communication schedules (as opposed to statically predefined just based on the number of workers and the properties of F). Such a generalisation has been proposed for the online case in Kamp et al., Communication-Efficient Distributed Online Prediction by Dynamic Model Synchronization, ECMLPKDD, 2013, which uses a form of local SGD combined with actively monitoring an upper bound to the variance of the local parameters. While the focus there is not on the convergence of the Polyak Rupert iterate but on the in-place online loss, the idea seems transferable, and the goals of the papers appear so similar that it might be worth to mention. - Assumption A3 seems more technically than what I would usually expect in an ML conference and, more importantly, could rather be considered a part of the problem definition. Perhaps some valuable space could be reclaimed in the main paper by mentioning in an initial paragraph about the problem that the g’s for a specific weight vector are i.i.d estimates of the gradient (and refer to the supplementary appendix for a full generalisation)? - When mentioning the convexity parameter mu for the first time in Section 2.2 its definition is only implicit (the symbol is defined only later in Assumption A1) - Example 5 should be Theorem or Corollary 5

Reviewer 3

Update: I've revised the overall score based on the response from the authors. This is another step to better understandings to distributed SGD approach for deep learning model training. However, the key limitation is on the strong assumptions over the model, which makes the results not meaningful to any practice. Q1 in Section 2.3 strongly claims that the mapping from weights w to the loss function is a quadratic function. This will never happen in real world. Moreover, batch normalization is commonly applied in real world deep learning tasks. Obviously, the convergence results do not consider it, although it is out of scope of a theoretical study in this submission.