Linux can scale to a machine with hundreds of cores; the largest is 1000+ cores IIRC. It's because it scales horizontally, in a way, and can run kernel threads

Linux can scale to a machine with hundreds of cores; the largest is 1000+ cores IIRC. It's because it scales horizontally, in a way, and can run kernel threads on multiple cores. But a NUMA configuration feels increasingly like a cluster the more cores you add, just because a single-memory-bus architecture can't scale too much; accessing a far node's memory introduces much more latency that accessing local RAM. It's easy to run a thousand independent VMs. It's somehow more challenging to run a thousand VMs that look externally as one service, and can scale down to 500 VMs, or up to 2000 VMs, depending on the load. It's really quite challenging to scale a monolith app to service tens of millions of users on one box, without horizontal scaling. But it definitely can be done, see the architecture of Stackoverflow.com. (Well, this very site is served by a monolith, and all logged-in users are served by a single-threaded monolith, unless they rewrote the engine. Modern computers are absurdly powerful.)