> The advantage is that you can context switch between threads without having to syscall, which speeds up things substantially. There's still a syscall, but it

> The advantage is that you can context switch between threads without having to syscall, which speeds up things substantially. There's still a syscall, but it doesn't enter the kernel scheduler or other machinery, which is where most of the time is spent. Linux syscall overhead (switching into ring 0) was and remains (apparently[1]) miniscule, which is why Linux ended up with 1:1 thread scheduling. But everything else--what those syscalls do--has grown in complexity. The original calculus for rejecting userspace scheduler activations and M:N threading for 1:1 threading no longer holds. What's old is new again. [1] Would be interesting to know if Google's kernels employ all the new Spectre mitigations.