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High-Probability Bound for Non-Smooth Non-Convex Stochastic Optimization with Heavy Tails
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Extends the online-to-non-convex framework with gradient clipping and a sampling-based validation step to find (δ,ϵ)-stationary points of non-smooth non-convex objectives with heavy-tailed stochastic gradients, with high-probability guarantees.
Problem
Finding (δ,ϵ)-stationary points of non-smooth non-convex stochastic objectives when gradients have only finite p-th moments (heavy tails, p in (1,2]), with high-probability rather than in-expectation guarantees.
Approach
Incorporates gradient clipping into the online-to-non-convex framework so OGD updates use clipped stochastic gradients, giving high-probability control of shifting regret, and adds a clipped-gradient validation procedure to select an actual (δ,ϵ)-stationary candidate.
Key results
- Algorithm complexity Õ(ϵ^{-(2p-1)/(p-1)} δ^{-1}) gradient queries for (δ,ϵ)-stationary point with high probability; reduces to Õ(ϵ^{-3}δ^{-1}) when p=2, improving the prior Õ(ϵ^{-4}δ^{-1}) high-probability bound.
- For smooth objectives, finds ϵ-stationary point with Õ(ϵ^{-(3p-2)/(p-1)}) queries, matching the Ω(ϵ^{-(3p-2)/(p-1)}) lower bound up to log factors.
- On CIFAR-10 with injected heavy-tailed gradient noise, CGA outperforms SINGD and ONCCA in training and testing accuracy while SGD fails to converge.
Measured effects reported in the paper
| What was changed | Where | Measured | Effect |
|---|---|---|---|
| apply gradient clipping to stochastic gradients in candidate generation | CGA · CIFAR10 training set | accuracy | Helped |
| apply gradient clipping to stochastic gradients in candidate generation | CGA · CIFAR10 testing set | accuracy | Helped |
| use SGD with momentum instead of SINGD, ONCCA, or CGA | ResNet18 on CIFAR10 with manually added heavy-tailed gradient noise · CIFAR10 | accuracy | Hurt |
Limitations
- Algorithm requires knowledge of problem parameters σ_p and G to set the clipping parameter.
- Preliminary experiment is small-scale and uses synthetic heavy-tailed noise added to gradients.
- Guarantees hold only for differentiable objectives (non-differentiable case via randomized smoothing).
Experimental setup
Datasets: CIFAR-10
Benchmarks: CIFAR-10
Metrics: training accuracy testing accuracy query complexity (theoretical)
Baselines: SGD with momentum SINGD ONCCA Zhang et al. 2020c high-probability method Tian et al. 2022 method
Builds on: online-to-non-convex conversion clipped SGD SINGD Zhang et al. (2020) heavy-tailed smooth non-convex analysis
Closest papers by meaning
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