For managing the concurrency using threads, we have 1. the user-level library to make management in the application's address space. + performant and flexible - less functional 2. kernel modification + high functionality - pool performance, flexibility. Thus we demand a kernel interface combined with a user-level thread package, which can gain the above 2 pros.
User-level Threads are usually lightweight, the current design of the fiber library, can predict the process well and squeeze the context switch to the full.
Cons
The upcall performance drains down the performance (a better message call interface in the kernel using eBPF(XDP for network) or io_uring to asynchronously submit the activation may resolve this problem.