Federated Learning (FL) trains a shared model across distributed devices while keeping the training data on the devices. Most FL schemes are synchronous: they perform a synchronized aggregation of model updates from individual devices. Synchronous training can be slow because of late-arriving devices (stragglers). On the other hand, completely asynchronous training makes FL less private because of incompatibility with secure aggregation. In this work, we propose a model aggregation scheme, FedBuff, that combines the best properties of synchronous and asynchronous FL. Similar to synchronous FL, FedBuff is compatible with secure aggregation. Similar to asynchronous FL, FedBuff is robust to stragglers. In FedBuff, clients trains asynchronously and send updates to the server. The server aggregates client updates in a private buffer until K updates have been received, at which point a server model update is immediately performed. We provide theoretical convergence guarantees for FedBuff in a non-convex setting. Empirically, FedBuff converges up to 3.8× faster than previous proposals for synchronous FL (e.g., FedAvgM), and up to 2.5× faster than previous proposals for asynchronous FL (e.g., FedAsync). We show that FedBuff is robust to different staleness distributions and is more scalable than synchronous FL techniques.Preprint. Under review.

Cross-device Federated Learning (FL) is a distributed learning paradigm with several challenges that differentiate it from traditional distributed learning, variability in the system characteristics on each device, and millions of clients coordinating with a central server being primary ones. Most FL systems described in the literature are synchronous -they perform a synchronized aggregation of model updates from individual clients. Scaling synchronous FL is challenging since increasing the number of clients training in parallel leads to diminishing returns in training speed, analogous to large-batch training. Moreover, stragglers hinder synchronous FL training. In this work, we outline a production asynchronous FL system design. Our work tackles the aforementioned issues, sketches of some of the system design challenges and their solutions, and touches upon principles that emerged from building a production FL system for millions of clients. Empirically, we demonstrate that asynchronous FL converges faster than synchronous FL when training across nearly one hundred million devices. In particular, in high concurrency settings, asynchronous FL is 5× faster and has nearly 8× less communication overhead than synchronous FL.