Larry: Practical Network Reconfigurability in the Data Center


Andromachi Chatzieleftheriou, Sergey Legtchenko, Hugh Williams, and Antony Rowstron, Microsoft Research


Modern data center (DC) applications require high cross-rack network bandwidth and ultra-low, predictable end-to-end latency. It is hard to meet these requirements in traditional DC networks where the bandwidth between a Top-of-Rack (ToR) switch and the rest of the DC is typically oversubscribed.

Larry is a network design that allows racks to dynamically adapt their bandwidth to the aggregation switches as a function of the traffic demand. Larry reconfigures the network topology to enable racks with high demand to use underutilized uplinks from their neighbors. Operating at the physical layer, Larry has a predictably low traffic forwarding overhead that is adapted to latency sensitive applications. Larry is effective even when deployed on a small set of racks (e.g., 4) because rack traffic demand is not correlated in many DC workloads. It can be deployed incrementally and transparently co-exist with existing non-reconfigurable racks. Our prototype uses a 40 Gbps electrical circuit switch we have built, with a simply local control plane. Using multiple workloads, we show that Larry improves tail latency by to 2.3x for the same network cost.

@inproceedings {211235,
author = {Andromachi Chatzieleftheriou and Sergey Legtchenko and Hugh Williams and Antony Rowstron},
title = {Larry: Practical Network Reconfigurability in the Data Center},
booktitle = {15th {USENIX} Symposium on Networked Systems Design and Implementation ({NSDI} 18)},
year = {2018},
address = {Renton, WA},
url = {},
publisher = {{USENIX} Association},