That depends. A long-running function is usually just a semantically long running function which consists of multiple (usually suspendable) parts. Of course there are exceptions, such as long running cpu heavy calculations for which a single atomic call takes longer than just milliseconds. But I think this is rather rare.

And for just semantically long running functions, it is not difficult to write a scheduler that tries to be "fair", however you want to interpret that. But of course it requires to fully embrace asynchronous programming, something that will probably difficult in Ruby.