Comment by zamalek
9 months ago
A reactor has to move the pending task to some type of work queue. The task has to pulled off the work queue. The work queue is oblivious as to the priority of your tasks. Tasks aren't as expensive as context switching, but they aren't free either: e.g. likely to ruin CPU caches. Less code is fewer instructions is less time.
If you care enough, you generally should be able to outdo the reactor and state machines. Whether you should care enough is debatable.
The cache thing is a thing I think a lot of people with a more... naive... understanding of machine architecture don't clue into.
Even just synchronizing on an atomic can thrash branch prediction and L1 caches both, let alone working your way through a task queue and interrupting program flow to do so.
So yeah, you're thinking about the comparison between async/await and manual state machines management with epoll. But that's not what most people have in mind when you're saying async/await have performance impact, most of them would immediately think you're talking about the difference with threads.