All the BSDs and Solaris etc. went through a process of having OS threads and N-to-M threading in user-land only to eventually end up with 1-to-1 threading in u

All the BSDs and Solaris etc. went through a process of having OS threads and N-to-M threading in user-land only to eventually end up with 1-to-1 threading in user-land. N-to-M means that you could have N (with N>M) threads in user-land and a smaller number of OS threads to run them, and then the C library had to manage part of the scheduling by switching contexts between the N user-land threads as they blocked on I/O and as I/Os completed, and also maybe in other cases. N-to-M threading came to be considered harmful, at least in C, but then nowadays green threads are super popular in Java and other languages, and green threads are just more N-to-M threading.