Generalization Dynamics of Linear Diffusion Models

Abstract: Diffusion models trained on finite datasets with $N$ samples from a target distribution exhibit a transition from memorisation, where the model reproduces training examples, to generalisation, where it produces novel samples that reflect the underlying data distribution. Understanding this transition is key to characterising the sample efficiency and reliability of generative models, but our theoretical understanding of this transition is incomplete. Here, we analytically study the memorisation-to-generalisation transition in a simple model using linear denoisers, which allow explicit computation of test errors, sampling distributions, and Kullback-Leibler divergences between samples and target distribution. Using these measures, we predict that this transition occurs roughly when $N \asymp d$, the dimension of the inputs. When $N$ is smaller than the dimension of the inputs $d$, so that only a fraction of relevant directions of variation are present in the training data, we demonstrate how both regularization and early stopping help to prevent overfitting. For $N > d$, we find that the sampling distributions of linear diffusion models approach their optimum (measured by the Kullback-Leibler divergence) linearly with $d/N$, independent of the specifics of the data distribution. Our work clarifies how sample complexity governs generalisation in a simple model of diffusion-based generative models and provides insight into the training dynamics of linear denoisers.