JDK 21 released

Relatedly, O(1) event polling APIs like kqueue, epoll, and Solaris Ports[1] didn't exist back when M:N threading was being debated. Those APIs don't solve pre-emption, but without such APIs the ability to spawn hundreds or thousands of threads in a network server didn't make any sense--a scheduler relying on select/poll had little chance of "C10K" scaling.[2][3] Such O(1) event interfaces were a necessary, if crude, facility for user land scheduling to be meaningfully competitive with 1:1 threads for any real-world work loads. That fact that they composed well (i.e. they're built around a special file descriptor, rather than some global facility or state) also made it easier for user land developers, including language implementors, to experiment.

NetBSD, like Solaris before it, did end up with scheduler activations to support it's M:N threading model, but I assume it was too little, too late. And AFAIU its event reporting was based on signals (similar to SIGIO/SIGPOLL), which effectively meant it was an interface only NetBSD system developers could iterate on and make use of. (I'm also not sure what types of events it reported. Did it even resolve the network connection scaling problem? Was Solaris' /dev/poll interface an offshoot of its scheduler activation framework?)

[1] Solaris 7 had /dev/poll, but M:N threading predated it. /dev/poll wasn't a public API until Solaris 8, yet by Solaris 9 M:N threading was already deprecated.

[2] SIGIO/SIGPOLL was a thing, but POSIX signal semantics are simply too cumbersome. Interrupts vs polling for software interfaces was and remains an entire debate until itself, ongoing since the 1970s (1960s?).

[3] poll hinting, where the kernel optimistically installed persistent event watchers, also existed, at least experimentally (see https://web.archive.org/web/20020226200329/http://www.hum...), but it was more of a band-aid and marginal fix. I think some BSDs effectively ended up implementing this later as part of refactoring their VFS layer and poll/select around the internal kevent (kqueue) system.