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Parallel training of DNNs with Natural Gradient and Parameter Averaging (1410.7455v8)

Published 27 Oct 2014 in cs.NE, cs.LG, and stat.ML

Abstract: We describe the neural-network training framework used in the Kaldi speech recognition toolkit, which is geared towards training DNNs with large amounts of training data using multiple GPU-equipped or multi-core machines. In order to be as hardware-agnostic as possible, we needed a way to use multiple machines without generating excessive network traffic. Our method is to average the neural network parameters periodically (typically every minute or two), and redistribute the averaged parameters to the machines for further training. Each machine sees different data. By itself, this method does not work very well. However, we have another method, an approximate and efficient implementation of Natural Gradient for Stochastic Gradient Descent (NG-SGD), which seems to allow our periodic-averaging method to work well, as well as substantially improving the convergence of SGD on a single machine.

Citations (254)

Summary

  • The paper introduces a distributed DNN training method that integrates parameter averaging with NG-SGD to enhance scalability and reduce inter-machine communication.
  • The paper employs an approximate inverse Fisher matrix in NG-SGD to optimize learning rates, resulting in faster and more stable convergence.
  • The paper demonstrates practical benefits on speech recognition tasks, achieving linear speedup and improved Word Error Rates across multiple GPUs.

Overview of "Parallel Training of DNNs with Natural Gradient and Parameter Averaging"

The paper outlines an efficient framework for training Deep Neural Networks (DNNs) in a distributed manner, using the Kaldi speech recognition toolkit. The methodology focuses on large-scale data and leverages distributed resources, specifically multiple GPU-equipped or multi-core machines. The approach aims to be hardware-agnostic by reducing the necessity for constant communication among machines.

Key Concepts and Methods

The framework employs parameter averaging combined with Natural Gradient Stochastic Gradient Descent (NG-SGD) to enhance the typical stochastic gradient descent (SGD) used in DNN training. This combination facilitates effective parallel data processing without requiring frequent data exchanges between units.

  1. Parameter Averaging:
    • Multiple training processes run on separate machines.
    • Each process periodically averages the neural network parameters across machines.
    • This method significantly reduces network traffic during training.
  2. Natural Gradient Stochastic Gradient Descent:
    • The paper introduces an implementation of a natural gradient modified SGD.
    • It optimizes the learning rate matrix using an approximate inverse Fisher matrix, improving convergence efficiency.
    • Two variants are proposed: a simple method and an online method, both efficiently managing the Fisher matrix.

Results and Empirical Findings

The paper demonstrates that the proposed framework enables a linear speedup in training, scaling efficiently up to 4 or 8 GPUs. The use of NG-SGD yields consistently better convergence compared to traditional plain SGD. The experiments showed that the learning efficiency saturates beyond a certain number of parallel units, indicating a sub-linear speedup in those scenarios.

Key results from the experiments on the Fisher English speech recognition task highlight the efficacy of NG-SGD in real-use cases. The paper reports improvements in convergence speed and modest gains in final performance measures such as Word Error Rates.

Practical and Theoretical Implications

The implementation becomes notably beneficial when dealing with DNNs used for speech recognition on extensive data, as hardware constraints are efficiently managed. The approach is applicable beyond speech recognition, making it a potentially valuable technique in other domains that require scalable training on distributed hardware.

While the paper does not attempt to formalize the theoretical underpinnings of parameter averaging effectiveness within non-convex spaces, or rigorously prove why NG-SGD is advantageous, the empirical success hints at significant theoretical implications for understanding DNN optimization landscapes.

Speculations on Future Developments

Future research might further explore the application of the natural gradient approach to a broader spectrum of neural architectures, potentially adapting it for real-time systems where latency and power efficiency are critical. Another avenue may involve theoretical advancements in understanding why NG-SGD allows for this kind of effective parameter averaging, alongside extending the limits of parallel scalability.

The insights from this paper pave the way for more robust solutions in distributed DNN training protocols and potentially influence the architecture design of both hardware and software for deep learning applications.