# FatPaths: Routing in Supercomputers and Data Centers when Shortest Paths Fall Short

## TL;DR

FatPaths is a robust routing architecture that leverages diverse path options and a redesigned transport layer to significantly improve performance in supercomputers and data centers using low-diameter topologies.

## Contribution

It introduces a generic routing architecture that exploits path diversity and a purified transport layer to enhance performance of low-diameter topologies.

## Key findings

- Achieves 15% higher throughput compared to Clos networks.
- Reduces latency by a factor of two.
- Enables low-diameter topologies to outperform traditional designs.

## Abstract

We introduce FatPaths: a simple, generic, and robust routing architecture that enables state-of-the-art low-diameter topologies such as Slim Fly to achieve unprecedented performance. FatPaths targets Ethernet stacks in both HPC supercomputers as well as cloud data centers and clusters. FatPaths exposes and exploits the rich ("fat") diversity of both minimal and non-minimal paths for high-performance multi-pathing. Moreover, FatPaths uses a redesigned "purified" transport layer that removes virtually all TCP performance issues (e.g., the slow start), and incorporates flowlet switching, a technique used to prevent packet reordering in TCP networks, to enable very simple and effective load balancing. Our design enables recent low-diameter topologies to outperform powerful Clos designs, achieving 15% higher net throughput at 2x lower latency for comparable cost. FatPaths will significantly accelerate Ethernet clusters that form more than 50% of the Top500 list and it may become a standard routing scheme for modern topologies.