We describe our experience developing what we believe to be the world's first large-scale production deployments of lightwave fabrics used for both datacenter networking and machine-learning (ML) applications. Using optical circuit switches (OCSes) and optical transceivers developed in-house, we employ hardware and software codesign to integrate the fabrics into our network and computing infrastructure. Key to our design is a high degree of multiplexing enabled by new kinds of wavelength-division-multiplexing (WDM) and optical circulators that support high-bandwidth bidirectional traffic on a single strand of optical fiber. The development of the requisite OCS and optical transceiver technologies leads to a synchronous lightwave fabric that is reconfigurable, low latency, rate agnostic, and highly available. These fabrics have provided substantial benefits for long-lived traffic patterns in our datacenter networks and predictable traffic patterns in tightly-coupled machine learning clusters. We report results for a large-scale ML superpod with 4096 tensor processing unit (TPU) V4 chips that has more than one ExaFLOP of computing power. For this use case, the deployment of a lightwave fabric provides up to 3× better system availability and model-dependent performance improvements of up to 3.3× compared to a static fabric, despite constituting less than 6% of the total system cost.

We present our approach for overcoming the cost, operational complexity, and limited scale endemic to datacenter networks a decade ago. Three themes unify the five generations of datacenter networks detailed in this paper. First, multi-stage Clos topologies built from commodity switch silicon can support cost-effective deployment of building-scale networks. Second, much of the general, but complex, decentralized network routing and management protocols supporting arbitrary deployment scenarios were overkill for single-operator, pre-planned datacenter networks. We built a centralized control mechanism based on a global configuration pushed to all datacenter switches. Third, modular hardware design coupled with simple, robust software allowed our design to also support inter-cluster and wide-area networks. Our datacenter networks run at dozens of sites across the planet, scaling in capacity by 100x over ten years to more than 1Pbps of bisection bandwidth.