Another interesting idea was FlexSC https://www.usenix.org/legacy/events/osdi10/tech/full_papers...

Those articles (and syslets) all seem to have this strong async focus. That could simply reflect pengaru's bias/interest. So, I do not know for sure, but I suspect the TL;DR is "async & scheduling in the mix makes it hard to get right". That complexity may also relate to the missing syscalls.

My approach has both the virtue and curse of being too simple to worry about all that, but it _does_ remove basic syscall overhead.