The Epochal Sawtooth Effect: Unveiling Training Loss Oscillations in Adam and Other Optimizers

In this paper, we identify and analyze a recurring training loss pattern, which we term the \textit{Epochal Sawtooth Effect (ESE)}, commonly observed during training with adaptive gradient-based optimizers, particularly Adam optimizer. This pattern is characterized by a sharp drop in loss at the beginning of each epoch, followed by a gradual increase, resulting in a sawtooth-shaped loss curve. Through empirical observations, we demonstrate that while this effect is most pronounced with Adam, it persists, although less severely, with other optimizers such as RMSProp. We provide an in-depth explanation of the underlying mechanisms that lead to the Epochal Sawtooth Effect. The influences of factors like $\beta$, batch size, data shuffling on this pattern have been studied. We quantify the influence of $beta_2$ on the shape of the loss curve, showing that higher values of $\beta_2$ result in a nearly linear increase in loss, while lower values create a concave upward trend. Our analysis reveals that this behavior stems from the adaptive learning rate controlled by the second moment estimate, with $\beta_1$ playing a minimal role when $\beta_2$ is large. To support our analysis, we replicate this phenomenon through a controlled quadratic minimization task. By incrementally solving a series of quadratic optimization problems using Adam, we demonstrate that the Epochal Sawtooth Effect can emerge even in simple optimization scenarios, reinforcing the generality of this pattern. This paper provides both theoretical insights and quantitative analysis, offering a comprehensive understanding of this ubiquitous phenomenon in modern optimization techniques.