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Title: A Non-asymptotic comparison of SVRG and SGD: tradeoffs between compute and speed

Paper content:
Stochastic gradient descent (SGD), which  trades off noisy gradient updates for computational efficiency, is the de-facto optimization algorithm to solve large-scale machine learning problems.
SGD can make rapid learning progress by performing updates using subsampled training data, but the noisy updates also lead to slow asymptotic convergence.
Several variance reduction algorithms, such as SVRG, introduce control variates to obtain a lower variance gradient estimate and faster convergence.
Despite their appealing asymptotic guarantees, SVRG-like algorithms have not been widely adopted in deep learning.
The traditional asymptotic analysis in stochastic optimization provides limited insight into training deep learning models under a fixed number of epochs.
In this paper, we present a non-asymptotic analysis of SVRG under a noisy least squares regression problem.
Our primary focus is to compare the exact loss of SVRG to that of SGD at each iteration t.
We show that the learning dynamics of our regression model closely matches with that of neural networks on MNIST and CIFAR-10 for both the underparameterized and the overparameterized models.
Our analysis and experimental results suggest there is a trade-off between the computational cost and the convergence speed in underparametrized neural networks.
SVRG outperforms SGD after a few epochs in this regime.
However, SGD is shown to always outperform SVRG in the overparameterized regime.
Many large-scale machine learning problems, especially in deep learning, are formulated as minimizing the sum of loss functions on millions of training examples (Krizhevsky et al., 2012; Devlin et al., 2018) .
Computing exact gradient over the entire training set is intractable for these problems.
Instead of using full batch gradients, the variants of stochastic gradient descent (SGD) (Robbins & Monro, 1951; Zhang, 2004; Bottou, 2010; Sutskever et al., 2013; Duchi et al., 2011; Kingma & Ba, 2014) evaluate noisy gradient estimates from small mini-batches of randomly sampled training points at each iteration.
The mini-batch size is often independent of the training set size, which allows SGD to immediately adapt the model parameters before going through the entire training set.
Despite its simplicity, SGD works very well, even in the non-convex non-smooth deep learning problems (He et al., 2016; Vaswani et al., 2017) .
However, the optimization performance of the stochastic algorithm near local optima is significantly limited by the mini-batch sampling noise, controlled by the learning rate and the mini-batch size.
The sampling variance and the slow convergence of SGD have been studied extensively in the past (Chen et al., 2016; Li et al., 2017; Toulis & Airoldi, 2017) .
To ensure convergence, machine learning practitioners have to either increase the mini-batch size or decrease the learning rate toward the end of the training (Smith et al., 2017; Ge et al., 2019) .
The minimum loss achieved in real dataset MNIST (a logistic regression model).
Our theoretical prediction (a) matched with the training dynamics for real datasets, demonstrating tradeoffs between computational cost and convergence speed.
The curves in red are SVRG and curves in blue are SGD.
Different markers refer to different per-iteration computational cost, i.e., the number of backpropagation used per iteration on average.
their strong theoretical guarantees, SVRG-like algorithms have seen limited success in training deep learning models (Defazio & Bottou, 2018) .
Traditional results from stochastic optimization focus on the asymptotic analysis, but in practice, most of deep neural networks are only trained for hundreds of epochs due to the high computational cost.
To address the gap between the asymptotic benefit of SVRG and the practical computational budget of training deep learning models, we provide a non-asymptotic study on the SVRG algorithms under a noisy least squares regression model.
Although optimizing least squares regression is a basic problem, it has been shown to characterize the learning dynamics of many realistic deep learning models (Zhang et al., 2019; Lee et al., 2019) .
Recent works suggest that neural network learning behaves very differently in the underparameterized regime vs the overparameterized regime Vaswani et al., 2019) , characterized by whether the learnt model can achieve zero expected loss.
We account for both training regimes in the analysis by assuming a linear target function and noisy labels.
In the presence of label noise, the loss is lower bounded by the label variance.
In the absence of the noise, the linear predictor can fit each training example perfectly.
We summarize the main contributions as follows:
• We show the exact expected loss of SVRG and SGD along an optimization trajectory as a function of iterations and computational cost.
• Our non-asymptotic analysis provides an insightful comparison of SGD and SVRG by considering their computational cost and learning rate schedule.
We discuss the trade-offs between the total computational cost, i.e. the total number of back-propagations performed, and convergence performance.
• We consider two different training regimes with and without label noise.
Under noisy labels, the analysis suggests SGD only outperforms SVRG under a mild total computational cost.
However, SGD always exhibits a faster convergence compared to SVRG when there is no label noise.
• Numerical experiments validate our theoretical predictions on both MNIST and CIFAR-10 using various neural network architectures.
In particular, we found the comparison of the convergence speed of SGD to that of SVRG in underparameterized neural networks closely matches with our noisy least squares model prediction.
Whereas, the effect of overparameterization is captured by the regression model without label noise.
In this paper, we studied the convergence properties of SGD and SVRG in the underparameterized and overparameterized settings.
We provided a non-asymptotic analysis of both algorithms.
We then investigated the question about which algorithm to use under certain total computational cost.
We performed numerical simulations of dynamics equations for both methods, as well as extensive experiments on the standard machine learning datasets, MNIST and CIFAR-10.
Remarkably, we found in many cases phenomenon predicted by our theory matched with observations in practice.
Our experiments suggested there is a trade-off between the computational cost and the convergence speed for underparameterized neural networks.
SVRG outperformed SGD after the first few epochs in this regime.
In the case of overparameterized model, a setting that matches with modern day neural networks training, SGD strictly dominated SVRG by showing a faster convergence for all computational cost.

assistant: text: Non-asymptotic analysis of SGD and SVRG, showing the strength of each algorithm in convergence speed and computational cost, in both under-parametrized and over-parametrized settings.