And (for the moment) the last one - why should anyone use ordinary
Linux (kernel) if one can have, for example, Xenomai and that way
Linux + real time in addition ?
A generalization but for most the only real time applications used are video / music processing and playback. Typically the stock kernel is good enough and the occasional flicker etc, is only a minor annoyance. The default kernel is optimized for total system throughput vs real time applications. Real time applications are robotics, process control etc where timing is extremely critical.
I am not familiar with Xenomai. Yes basically RTAI is two parts i.e. the kernel patch and the programming API.