The paper aims to design an optimal client sampling strategy for federated learning (FL) over wireless networks that addresses both system heterogeneity (diverse computation and communication capacities) and statistical heterogeneity (unbalanced and non-i.i.d. data) to minimize the wall-clock convergence time.
The key contributions are:
The authors obtain a new tractable convergence bound for FL algorithms with arbitrary client sampling probability. This enables them to analytically establish the relationship between the total learning time and sampling probability.
Based on the convergence bound, the authors formulate a non-convex optimization problem to find the optimal client sampling probability. They design an efficient algorithm to approximately solve this non-convex problem.
The solution reveals the impact of system and statistical heterogeneity parameters on the optimal client sampling design. It shows that as the number of sampled clients increases, the total convergence time first decreases and then increases.
Experimental results from both hardware prototype and simulation demonstrate that the proposed sampling scheme significantly reduces the convergence time compared to baseline sampling schemes. For the EMNIST dataset, the proposed scheme in hardware prototype spends 71% less time than the baseline uniform sampling for reaching the same target loss.
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