Yep. Signals were literally the original async model on Unix. They were a userspace abstraction over hardware interrupts, much as they are today, but the abstraction didn't turn out to be as fruitful as it might have been, perhaps because it was too thin. (Signaling queueing, i.e. real-time signals, meant to make signals more useful for application events, never went mainstream.) Back in the 1970s and 1980s the big arguments regarding async were about interrupt-driven (aka signals) and readiness-driven (aka polling), and relatedly edge-triggered vs level-triggered events. BSD added the select syscall along with the sockets API and that's when the readiness-driven, level-triggered model began to dominate. Though, before kqueue and then epoll came along there were some attempts at scaling async I/O using the interrupt-driven model--a signal delivered along with the associated descriptor. I think there's a vestige of this still in Linux, SIGIO.

It's not always either/or, though. From the perspective of userspace APIs it's usually one or the other, but further down the stack one model might be implemented in terms of the other, sometimes with multiple transitions between the two, especially around software/hardware boundaries. Basically, it's turtles all the way down.

Similarly, the debates regarding cancellation, stack management, etc, still persist; the fundamental dilemmas haven't changed.

It's still a context switch per event.

Like other people here wrote nowadays one can push some processing into kernel context, but that sort of defeats the purpose of kernel/userland border. One can just write a kmod and be done with it then (and lose isolation).

After decades of gleefully using signal handlers to handle all sorts of contingencies, systems programmers were solemnly informed that signal handler functions were very dangerous indeed, because a bunch of other stuff was on the stack and undefined while they were being run, and therefore, handler functions couldn't call anything that was unsafe or non-reentrant.

Systems programmers were told that the best signal handler function was one that set a flag, a volatile int, and then exited immediately without doing or touching anything else.

Sort of defeats the purpose of the elaborate signal-handler-callback-pointer-to-function system we had in place, but them's the breaks.