The extensible scheduler class

Hmm, I think I'd call Google fibers 1:1, at least in the way most people would mean it. That is, it assigns every userspace thread to a dedicated kernel thread, from the first time the thread function runs, until that function returns.

The magic seems all about having userspace code choose which threads are runnable at any given time -- ensuring that there's only ever 1 "runnable" thread per CPU the process is allowed to use. The most tricky part of that being the ability to cause a new thread to be made chosen to be runnable if another blocks in the kernel.

By ensuring this, the kernel scheduler's decisions are made effectively trivial. ("N CPUs are available, and N threads are currently runnable? Ok I know what to do here!")