[beagle3]

It has _little_ to do with the kernel. There, are we friends yet?

Here's a valid explanation, which wasn't explicitly given, although it was hinted at: kernel threads take resources that become significant when you want a very large number of threads (say, one million). The per thread overhead, including user stack, kernel stack and control structures are at a minimum of ~8K. That's 8GB for a million threads before you actually get to do anything useful.

However, lthreads can realistically take as little as 100 bytes per thread, which puts us at a 100MB footprint for the same case. That's a huge difference.

There are tradeoffs, but that's a potentially useful use case which kernel threads do not support (and which, in general, requires async programming, which lthread implements while emulating a thread API).