The post literally mentions that a few paragraphs down in further detail. > The less that doing things with threads crosses the CPU's security boundary into (a

The post literally mentions that a few paragraphs down in further detail. > The less that doing things with threads crosses the CPU's security boundary into (and out of) the kernel, the faster the threads go but the less we can really describe them as 'OS threads' and the harder it is to get things like forced thread preemption. And this applies not just for the 'OS threads' themselves but also to their activities. If you want 'OS threads' that perform 'synchronous IO through simple system calls', those IO operations are also transitioning into and out of the kernel. If you work to get around this purely through software, I suspect that what you wind up with is something that looks a lot like 'green' (user-space) threads with asynchronous IO once you peer behind the scenes of the abstractions that programs are seeing.