Quadratic slowdown with many nested futures


Can the futures crate exhibit quadratic performance in the number of .and_then() calls?


That has certainly happened before, but that instance appears to have been fixed. What Rust version are you seeing this on?


Note that the issue above is talking about compile times rather than the efficiency of the actual code.


The problem is that the result has O(n) nested enums. So an O(n) call stack is built and torn down O(n) times. The version using async/await has no such problem, since it uses an O(1) deep call stack.


Ah, you’re talking about runtime performance. Sorry, the compile time was what sprung to mind, and I assumed you’re referring to that.

You’re referring to the Chain enum, right?

I assume that’s because all state is captured in the generator and it yields across successful returns?


Yes. The problem is that deeply nested enums are generated. There would be no problem if they were flattened.

Another problem: if you need to select between N different futures and run them concurrently, you will use O(N^2) time if you use a combinator.

Root cause in both cases is that events propagate top-down, when they should propagate bottom-up, as they do in all other languages. Top-down event propagation is linear in the size of the call stack, whereas bottom-up takes amortized constant time.

With bottom-up propagation, the event loop holds, in addition to the future, a token that tells which future to poll next. It them polls that future directly, without checking its parent. If and only if that future returns Async::Ready does the parent future get polled.

I believe the reason that these problems have not appeared before is that few Rust programs use deep call stacks.