I want to make an important point on 3) If you have 600K concurrent connections they are concurrent. You could have them on a single core, limited severely on

I want to make an important point on 3) If you have 600K concurrent connections they are concurrent. You could have them on a single core, limited severely on bandwidth and disk I/O. What you don't have is parallelism, since they are not executing at the same time. I feel that this distinction is important to make. That most of them are waiting on a given event to happen does not make it any less concurrent, but it does limit the amount of parallelism which is possible. On a single core machine, the operating system is concurrently executing processes, perhaps a hundred of those. But only one process can be on the CPU at a time, so the parallelism count is always either 0 or 1.