I was only addressing that which I quoted:
...the OS has to allow the individual process itself manage its own threads in a very very lightweight way.
However, if the machine has multiple cpus, then separate threads can run concurrently on separate cpus. Any or all of those threads can become an association of fibers. The scheduling of the threads will be managed by the system, but within any given thread, control of which fiber of the set of fibers associated with that thread is cooperative under control of the program logic.
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