[mike_hearn]

I don't quite follow your argument.

Saying "Linux cannot handle 5M connections with one thread per connection" isn't a reasonable statement because no operating system can do that, they can't even get close. The resource usage of a kernel thread is defined by pretty fundamental limits in operating system architecture, namely, that the kernel doesn't know anything about the software using the thread. Any general purpose kernel will be unable to provision userspace with that many threads without consuming infeasible quantities of RAM.

The reason JVM virtual threads can do this is because the JVM has deep control and understanding of the stack and the heap (it compiled all the code). The reason Loom scalability gets worse if you call into native code is that then you're back to not controlling the stack.

Getting to 10M is therefore very much a question for the JVM as well as the operating system. It'll be heavily affected by GC performance with huge heaps, which luckily modern G1 excels at, it'll be affected by the performance of the JVM's userspace schedulers (ForkJoinPool etc), it'll be affected by the JVM's internal book-keeping logic and many other things. It stresses every level of the stack.