Private Zeroth-Order Optimization with Public Data
This work addresses efficiency bottlenecks in deploying differentially private machine learning, offering a practical solution for domains like vision and text processing, though it is incremental as it builds on existing zeroth-order approaches.
The paper tackles the high computational cost of differentially private first-order optimization by proposing a zeroth-order method that leverages public data to improve gradient approximation, achieving superior privacy/utility tradeoffs and up to 16x runtime speedup in vision and text tasks.
One of the major bottlenecks for deploying popular first-order differentially private (DP) machine learning algorithms (e.g., DP-SGD) lies in their high computation and memory cost, despite the existence of optimized implementations. Zeroth-order methods have promise in mitigating the overhead, as they leverage function evaluations to approximate the gradients, hence significantly easier to privatize. While recent works have explored zeroth-order approaches in both private and non-private settings, they still suffer from relatively low utilities compared with DP-SGD, and have only been evaluated in limited application domains. In this work, we propose to leverage public information to guide and improve gradient approximation of private zeroth-order algorithms. We explore a suite of public-data-assisted zeroth-order optimizers (PAZO) with minimal overhead. We provide theoretical analyses of the PAZO framework under an assumption of the similarity between public and private data. Empirically, we demonstrate that PAZO achieves superior privacy/utility tradeoffs across vision and text tasks in both pre-training and fine-tuning settings, outperforming the best first-order baselines (with public data) especially in highly private regimes, while offering up to $16\times$ runtime speedup.