> That time is approaching 20 years old at this point, too. Native threads haven't been "expensive" for a very, very long time now.

It depends on the context, but yes. I worked on stuff throughout the 2000's where we ran into scaling problems with thread based concurrency models. At the time, running 100,000 threads was... challenging. But yeah, by 2010 we were talking about the C10M problem, because the C10K problem wasn't a problem any more. There are some cases where you really do need to handle 10's or 100's of millions of threads, but there aren't a lot of them.

> Maybe if you're in the camp of disabling overcommit it matters, but otherwise the application of green threads is definitely a specialized niche, not generally useful.

Yup, but everyone is still stuck on the old mental model of "threads are bad", partly driven by the assumption that whatever is being done to handle those extreme cases is what one should be doing to address their own problem space. :-(

> I'd go even further and say it'll be a net-loss in most cases, especially with modern complications like heterogeneous compute.

Even more so if you're doing polling based I/O rather than a reactive model. The look on people's faces when I point out to them that there's good reason to think that for the scale they are working at, they'll likely get better performance if they just use threads to scale...

It's so weird how we talk about the context switching costs between threads without recognizing that the thread does the poll is not the same thread that processed the IO request in the kernel.

> I'd go even further and say it'll be a net-loss in most cases, especially with modern complications like heterogeneous compute. If you're use case is specifically spinning up thousands of threads for IO (aka, you're a server & nothing else), then sure. But if you aren't there's no win here, just complications (like times when you need native thread isolation for FFI reasons, like using OpenGL)

Virtual threads are going to be an /option/, not a requirement. Threads have to explicitly created as virtual threads. If this is not done, nothing will change.