[jacques_chester]

OS scheduling requires you to switch contexts between different modes in the CPU. Essentially, the OS has the power to perform operations that regular code does not. Threads and processes are usually built on top of these additional permissions.

Context switching is relatively expensive. The CPU needs to push a lot of application state out of the way to clear the path for the OS code to run, then after the OS is done, push the OS out of the way and retrieve the application's state and code again.

Whereas a fiber remains entirely inside the application code. It never requires a context switch. But a fiber loses some of the powers of the OS: for example, it can't draw hard memory boundaries between fibers that will be enforced by the CPU.

For some things you want processes, for some threads, for some fibers.

[wahern]

Context switching doesn't need to be as expensive as it is in these cases, it's just that Linux doesn't provide the mechanisms needed to make it more efficient. See, e.g., https://blog.linuxplumbersconf.org/2013/ocw/system/presentat... which implemented a syscall for a process-directed context switch directly to another thread.