We’re spread over 9 servers but only due to ephemeral port and handle exhaustion issues. Each server is fronted by 4 HAProxy frontends that each handle ~18k connections.

Since Nick’s post we’ve moved from StackExchange.NetGain to the managed websocket implementation in .NET Core 3 using Kestrel and libuv. That sits at around 2.3GB RAM and 0.4% CPU. Memory could be better (it used to be < 1GB with NetGain) but improvements would likely come from tweaks to how we configure GC under .NET Core which we haven’t really investigated yet.

We could run on far fewer machines but we have 9 sitting there serving traffic for the sites themselves so no harm in spreading the load a little!