any high-utilization workload with a chatty protocol dominated by small IOs such as: * distributed filesystems such as MooseFS, Ceph, Gluster used for hyperconverged infrastructure. * SANs hosting VMs with busy OLTP databases * OLTP replication * CXL memory expansion where remote memory needs to be as close to inter-NUMA node latency as possible

High Frequency Trading is one.

Anything else? Because that's the only one I can think of.

I'd expect HPC would be another, since a lot of algorithms that run on those clusters are bottlenecked by latency or throughput in communication.

HPC?

HPC = High-Performance Computing

https://en.wikipedia.org/wiki/High-performance_computing

For the parent: and not only bottlenecked at single hops but also hampered by the propagation of latency as the hops increase, depending on the complexity of the distributed system design.

> […] by the propagation of latency as the hops increase […]

Which is why you get network topologies other than 'just' fat tree in HPC networks:

* https://www.hpcwire.com/2019/07/15/super-connecting-the-supe...

* https://en.wikipedia.org/wiki/Torus_interconnect

High-frequency trading is the primary application, where 5ns can represent millions in profit as firms compete to execute trades first, but you'll also see benefits in distributed database synchronization, real-time financial risk calculations, and some specialized scientific computing workloads.