Crate unicycle

A scheduler for driving a large number of futures.

Unicycle provides the Unordered type, which is a futures abstraction that runs a set of futures which may complete in any order. Similarly to FuturesUnordered. But we aim to provide a stronger guarantee of fairness (see below), and better memory locality for the futures being pollled.

Note: This project is experimental. It involves some amount of unsafe and possibly bad assumptions which needs to be either vetted or removed before you should consider putting it in production.

Features

• parking-lot - To enable locking using the parking_lot crate (optional).
• vec-safety - Avoid relying on the assumption that &mut Vec<T> can be safely coerced to &mut Vec<U> if T and U have an identical memory layouts (enabled by default, issue #1).

Examples

use tokio::{stream::StreamExt as _, time};
use std::time::Duration;

#[tokio::main]
async fn main() {
    let mut futures = unicycle::Unordered::new();

    futures.push(time::delay_for(Duration::from_secs(2)));
    futures.push(time::delay_for(Duration::from_secs(3)));
    futures.push(time::delay_for(Duration::from_secs(1)));

    while let Some(_) = futures.next().await {
        println!("tick");
    }

    println!("done!");
}

Fairness

You can think of abstractions like Unicycle as schedulers. They are provided a set of child tasks, and try to do their best to drive them to completion. In this regard, it's interesting to talk about fairness in how the tasks are being driven.

The current implementation of FuturesUnordered maintains a queue of tasks interested in waking up. As a task is woken up, it's added to the head of this queue to signal its interest. When FuturesUnordered is being polled, it checks the head of this queue in a loop. As long as there is a task interested in being woken up, this task will be polled. This procuedure has the side effect of tasks which aggressively signal interest in waking up, will receive priority and be polled more frequently.

This process can lead to an especially unfortunate cases where a small number of tasks can can cause the polling loop of FuturesUnordered to spin abnormally. This issue was reported by Jon Gjengset, and improved on by limiting the amount FuturesUnordered is allowed to spin.

Unicycle addresses this by limiting how frequently a child task may be polled per polling cycle. This is done by keeping two sets of polling interest and atomically swapping it out once we are polling, then taking the swapped out set and use as a basis for what to poll in order, but only once. Additional wakeups are only registered in the swapped in set which will be polled the next cycle. For more details, see the Architecture section below.

Architecture

The Unordered type stores all futures being polled in a PinSlab (Inspired by the slab crate). A slab is capable of utomatically reclaiming storage at low cost, and will maintain decent memory locality. A PinSlab is different from a Slab in how it allocates the memory regions it uses to store objects. While a regular Slab is simply backed by a vector which grows as appropriate, this approach is not viable for pinning, since it would cause the objects to move while being reallocated. Instead PinSlab maintains a growable collection of fixed-size memory regions, allowing it to store and reference immovable objects through the pin API. Each future inserted into the slab is assigned an index, which we will be using below. We now call the inserted future a task, and you can think of this index as a unique task identifier.

Next to the slab we maintain two BitSets, one active and one alternate. When a task registers interest in waking up, the bit associated with its index is set in the active set, and the latest waker passed into Unordered is called to wake it up. Once Unordered is polled, it atomically swaps the active and alternate BitSets, waits until it has exclusive access to the now alternate BitSet, and drains it from all the indexes which have been flagged to determine which tasks to poll. Each task is then polled once in order. If the task is Ready, its result is added to a result queue.

Unicycle now prioritizes draining the result queue above everything else. Once it is empty, we start the cycle over again.

Structs

AtomicBitSet	The same as BitSet, except it provides atomic methods.
BitSet	A sparse, layered bit set.
Drain	A draining iterator of a BitSet.
DrainSnapshot	The snapshot of a drain in progress. This is created using Drain::snapshot.
Iter	An iterator over a BitSet.
PinSlab	Pre-allocated storage for a uniform data type, with slots of immovable memory regions.
Unordered	A container for an unordered collection of Futures.