We study differentially private stochastic convex optimization (DP-SCO) under user-level privacy, where each user may hold multiple data items. Existing work for user-level DP-SCO either requires super-polynomial runtime [Ghazi et al. (2023)] or requires the number of users to grow polynomially with the dimensionality of the problem with additional strict assumptions [Bassily et al. (2023)]. We develop new algorithms for user-level DP-SCO that obtain optimal rates for both convex and strongly convex functions in polynomial time and require the number of users to grow only logarithmically in the dimension. Moreover, our algorithms are the first to obtain optimal rates for non-smooth functions in polynomial time. These algorithms are based on multiple-pass DP-SGD, combined with a novel private mean estimation procedure for concentrated data, which applies an outlier removal step before estimating the mean of the gradients.

我们研究了具有用户级隐私的差分隐私随机凸优化（DP-SCO），其中每个用户可能拥有多个数据项。我们开发了新算法用于用户级DP-SCO，在多项式时间内获得凸和强凸函数的最优速率，并且在维度上只要求用户数量呈对数增长。此外，我们的算法是第一个在多项式时间内获得非平滑函数最优速率的算法。这些算法基于多遍DP-SGD，结合了一种集中数据的新颖私有均值估计过程，在估计梯度的平均值之前应用了一个异常值移除步骤。

用户级差分隐私随机凸优化：具有最优收敛速率的高效算法