Accelerating Diffusion Sampling with Optimized Time Steps
Shuchen XueZhaoqiang LiuFei ChenShifeng ZhangTianyang HuEnze XieZhenguo Li
Proposes a training-free optimization framework that computes non-uniform sampling schedules in under 15 seconds to minimize numerical solver approximation errors, substantially improving diffusion generation quality at very low step counts.
Diffusion models have become the leading approach for high-quality image generation, yet their practical deployment is constrained by high computational costs. Generating an image typically requires evaluating complex neural networks over many sequential steps. While modern numerical solvers have reduced the required step count, standard sampling approaches still divide generation time into uniform steps, leaving significant room for performance improvements when operating under extremely small step budgets.
The article establishes a general, training-free optimization framework to find non-uniform time steps tailored to numerical solvers for diffusion models. The primary objective is to formulate and efficiently solve an optimization problem that minimizes the mathematical distance between the exact solution of the generative trajectory and the approximation produced by the numerical solver.
To achieve this, the authors derived theoretical error bounds for generative trajectories and structured an optimization problem using standard score approximation assumptions. This framework accommodates any explicit solver, including algorithms with variable step orders. The authors solved the resulting formulation using the constrained trust region method and evaluated the optimized schedules across standard image benchmarks (such as CIFAR-10, ImageNet, FFHQ, and AFHQv2) using popular diffusion architectures (Score-SDE, ADM, EDM, and DiT) across both pixel- and latent-space models.
The findings show substantial generation quality improvements, measured by lower Fréchet Inception Distance (FID) scores, especially in few-step regimes. When paired with the high-order UniPC solver at five network evaluations, the proposed method reduced FID on CIFAR-10 from 23.22 (using uniform schedules) to 12.11, improved ImageNet 64x64 FID to 10.47, and lowered ImageNet 256x256 FID from 23.48 to 8.66. Across all tested datasets and architectures, optimized schedules consistently outperformed conventional uniform schemes. Furthermore, calculating the optimal schedule requires under 15 seconds on standard central processing units, contrasting sharply with previous reinforcement learning or search-based scheduling techniques that require hours of expensive graphics processing unit compute.
These results demonstrate that optimizing time step intervals delivers dramatic inference speedups without requiring model retraining, architecture modifications, or costly schedule searches. Organizations deploying diffusion models can directly lower computational latency and server infrastructure costs while maintaining or improving output fidelity. The framework integrates seamlessly as a plug-and-play enhancement for existing pre-trained pipelines.
Teams maintaining generative diffusion systems should adopt optimized time step schedules alongside advanced solvers such as UniPC, prioritizing applications where latency is critical. Future engineering and research efforts should explore refining the surrogate error objective for higher precision and extending the optimization framework to implicit solver components.
- Paper: DPM-Solver: A Fast ODE Solver for Diffusion Probabilistic Model Sampling in Around 10 Steps, Cheng Lu et al. (2022). This paper establishes the exact semi-linear diffusion ODE formulation and high-order fast ODE solvers that the source directly builds upon and optimizes time steps for.
- Paper: Score-Based Generative Modeling through Stochastic Differential Equations, Yang Song et al. (2021). This foundational work establishes the continuous-time stochastic differential equation and deterministic probability flow ODE framework essential for understanding ODE-based diffusion sampling.
- Paper: Elucidating the Design Space of Diffusion-Based Generative Models, Tero Karras et al. (2022). This work formulates the standardized design space and empirical time discretization schedules for deterministic ODE diffusion sampling that the source seeks to theoretically optimize.
- Paper: Denoising Diffusion Implicit Models, Jiaming Song et al. (2021). This paper introduces non-Markovian deterministic sampling for diffusion models, laying the initial groundwork for accelerated ODE-like trajectory stepping.
- Paper: Denoising Diffusion Probabilistic Models, Jonathan Ho et al. (2020). This landmark paper defines the fundamental denoising diffusion probabilistic model formulation underpinning the source's sampling acceleration methods.
- Paper: Fast ODE-based Sampling for Diffusion Models in Around 5 Steps, Zhenyu Zhou et al. (2024). This paper pushes few-step ODE diffusion sampling even further down to around five steps by exploiting low-dimensional trajectory geometry and learning optimal intermediate time steps.
