USF++: A Unified Sampling Framework for Solver Searching of Diffusion Probabilistic Models

Recent years have witnessed the rapid progress and broad application of diffusion probabilistic models (DPMs). Sampling from DPMs can be viewed as solving an ordinary differential equation (ODE). Despite the promising performance, the generation of DPMs usually consumes much time due to the large number of function evaluations (NFE). Though recent works have accelerated the sampling to around 20 steps with high-order solvers, the sample quality with less than 10 NFE can still be improved. In this paper, we propose a unified sampling framework (USF++) to study the optional strategies for solver. Under this framework, we further reveal that taking different solving strategies at different timesteps may help further decrease the truncation error, and a carefully designed solver schedule has the potential to improve the sample quality by a large margin. Therefore, we propose a new sampling framework based on the exponential integral formulation that allows free choices of solver strategy at each step and design specific decisions for the framework. Moreover, we apply evolutionary search to find outstanding solver schedules which outperform the state-of-the-art sampling methods on CIFAR-10, ImageNet, and LSUN-Bedroom datasets. Specifically, we achieve 3.89 FID with 5 NFE on CIFAR-10 dataset and 8.62 FID with 3 NFE on LSUN-Bedroom dataset, outperforming the SOTA method significantly. We further apply searching to Stable-Diffusion model and get an acceleration ratio of 2×, showing the feasibility of sampling in very few steps without retraining the neural network.