How is a clock signal different? It is just another external event userspace wants to be notified about.<p>Note that on Unix you can use signals to get IO readiness notification and in fact for sometime on Linux (before epoll) realtime signals were the preferred method to do get notification over a large amount of fds.<p>Anyway, as a general guideline, user&#x2F;kernel transition is cheaper than a (kernel) thread&#x2F;thread transition which is in turn cheaper than a process&#x2F;process transition.