[brown-dragon]

Isn't it effectively thread-per-socket except that the switches now only happen on an async system call? The difference, I guess, is you can optimize the context switch to be very small (and have smaller thread stacks) at the cost of giving up pre-emptive switching.

[baruch]

It reduces context switches in the kernel which are more expensive and reduces the memory resources needed for kernel threads which are larger.

There are also some hidden gains in terms of TLB caches and other costs of kernel threads switching.

An additional advantage is that between user-space-threads you have fewer locking problem since they implicitly lock out each other between context switch points so you only need locks when you need to protect an area across several context switch points.