Background: Recent developments have made it possible to accelerate neural networks training significantly using large batch sizes and data parallelism. Training in an asynchronous fashion, where delay occurs, can make training even more scalable. However, asynchronous training has its pitfalls, mainly a degradation in generalization, even after convergence of the algorithm. This gap remains not well understood, as theoretical analysis so far mainly focused on the convergence rate of asynchronous methods. Contributions: We examine asynchronous training from the perspective of dynamical stability. We find that the degree of delay interacts with the learning rate, to change the set of minima accessible by an asynchronous stochastic gradient descent algorithm. We derive closed-form rules on how the learning rate could be changed, while keeping the accessible set the same. Specifically, for high delay values, we find that the learning rate should be kept inversely proportional to the delay. We then extend this analysis to include momentum. We find momentum should be either turned off, or modified to improve training stability. We provide empirical experiments to validate our theoretical findings.
The authors model A-SGD as a dynamical system, where parameters are updated with delayed gradients. The authors analyze the stability of this system, and they first derive that the learning rate must scale linearly with the inverse of the delay around a minimum to remain stable. Using a similar analysis they show that the standard way of incorporating momentum into A-SGD requires small learning rates for high momentum values, and they propose "shifted momentum," which allows for stability under higher momentum values. Experimentally, the authors show that around minima the learning rate needed to retain stability scales linearly with the inverse of the delay, that there appears to be an analogous threshold when training models from scratch, that shifted momentum allows for higher momentum values, and finally that on several datasets A-SGD with an appropriate learning rate is able to generalize at least as well as large batch synchronous training.
This is a nice paper with a large number of interesting theoretical and experimental results, and I believe it should be accepted. I think there are some largely presentational issues that should be addressed, however:
- I think the authors should attempt to make a stronger case for the practical implications of their analysis: in particular, in the most practical setting (where we don't have a minimum obtained from synchronous training), what does the provided analysis allow us to do? Part of this might involve being more explicit about the results in Table 1: what exactly was the procedure for selecting the learning rates? Is it meaningfully different than just lowering the learning rate?
- Equation (3) is rather obscure without the Appendix, especially since unbolded x hasn't been introduced anywhere. I think the authors should try to convey more of what's going on in this equation in the main text.
Minor: 'looses' should be 'loses' throughout, and it might be good to include a conclusion section.