Optimizing Poll Implementation

In this lesson, you will learn how to optimize your custom Future implementations by minimizing redundant state checks and ensuring the poll method remains idempotent.

The Cost of "Lax" Polling

When you implement Future manually, the poll method is the heart of your logic. A common performance pitfall is performing expensive work—like re-checking a buffer or re-acquiring a lock—every time the executor calls poll. Because executors may poll a Future even when it is not yet ready (or due to spurious wakeups), your implementation must be as lean as possible.

State-Machine Efficiency

Instead of re-calculating the state of your resource on every poll, treat your Future as a strict state machine. Store your state in an enum and transition only when progress is actually made.

Consider this optimized approach for a poll method:

fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
    match self.state {
        State::Idle => {
            // Only perform setup logic if we are truly Idle
            self.state = State::Processing(start_heavy_task());
            self.poll(cx) // Recurse or loop to the next state
        }
        State::Processing(ref mut task) => {
            // Check the internal task status once
            match task.poll_unpin(cx) {
                Poll::Ready(result) => {
                    self.state = State::Done;
                    Poll::Ready(result)
                }
                Poll::Pending => Poll::Pending,
            }
        }
        State::Done => panic!("Polled after completion"),
    }
}

Avoiding Unnecessary Re-polling

The key is to avoid "busy-polling." If you know a resource isn't ready, don't perform internal logic that assumes it is. By caching the Waker and transitioning through discrete states, you ensure that the poll implementation returns Poll::Pending as quickly as possible when the work is blocked.

Furthermore, ensure you are not re-registering interest in resources if the state hasn't changed. If you are polling an underlying socket, only invoke the socket's poll_read or poll_write if you are in the specific state that requires it. This reduces the overhead on the event loop and keeps your task from hogging the executor’s threads unnecessarily.

Key Takeaway: To optimize polling, implement a strict state machine that avoids re-calculating work and ensures that every execution of poll is a direct transition toward completion or a fast exit.