[toast0]

You need both your operating system and your application environment need to be up to the task. I'd expect most operating systems to be up to the task; although it might need settings set. Some of the settings are things that are statically allocated in non-swappable memory and you don't want to waste memory on being able to to have 5M sockets open if you never go over 10k. Often you'll want to reduce socket buffers from defaults, which will reduce throughput per socket, but target throughput per socket is likely low or you wouldn't want to cram so many connections per client. You may need to increase the size of the connection table and the hash used for it as well; again, it wastes non-swappable ram to have it too big if you won't use it.

For application level, it's going to depend on how you handle concurrency. This post is interesting, because it's a benchmark of a different way to do it in Java. You could probably do 5M connections in regular Java through some explicit event loop structure; but with the Loom preview, you can do it connection per Thread. You would be unlikely to do it with connection per Thread without Loom, since Linux threads are very unlikely to scale so high (but I'd be happy to read a report showing 5M Linux threads)