The 2nd type of process is just an ordinary Unix-style process. It refers to the way that on older hardware process switches were cheaper on account of the smal

The 2nd type of process is just an ordinary Unix-style process. It refers to the way that on older hardware process switches were cheaper on account of the smaller amount of state that needed to be shuffled about and cheaper system calls. So no need to (as you might today) prefer green threads over OS threads, and/or OS threads over processes, on account of the overheads. This is mentioned in part 1 near the bottom: When UNIX was still young ... you were supposed to fork a new instance of the process for each TCP connection I guess it made sense from performance point of view back then. All the stuff that makes process switching slow today haven't yet existed. ... In such an environment process switching was closer to what we call green threads today.