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#89 in Concurrency

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Used in rust-quiz


204 lines

Non-interleaving output queue

github crates.io docs.rs build status

Mechanism to prevent interleaving of output of tasks while still allowing an arbitrary number of tasks to make progress, even ones other than the task currently printing output.

oqueue = "0.1"

Use case

This crate addresses a seemingly narrow use case, but it is one I have hit in a couple different circumstances.

Suppose we have some embarrassingly parallel workload where each piece of work may want to write to stdout/stderr. If we just parallelize this naïvely, output from different tasks will interleave and end up unreadable. If we make each task lock the output stream, do its work, and then unlock, we avoid interleaving but tasks can no longer run in parallel. If we have each task write its output into a local buffer and print it all atomically at the end, all output is unnecessarily delayed and the application can feel lurchy and unresponsive because none of the output is seen in real time.


  • We have an ordered sequence of tasks 0..N.

  • We want to receive all output from task 0, then all output from task 1, etc in order. Task output must not interleave with other tasks and must follow the task order.

  • We want tasks to execute in parallel.

  • We want all output to be printed as soon as possible, meaning real time for exactly one task at a time and deferred until replacement of the realtime task for other tasks.


This example uses oqueue to sequence the output of worker threads on a Rayon threadpool.

use oqueue::{Color::Red, Sequencer, Task};
use rayon::ThreadPoolBuilder;
use std::error::Error;
use std::fs;
use std::path::{Path, PathBuf};
use std::thread;
use std::time::Duration;

type Result<T> = std::result::Result<T, Box<dyn Error>>;

fn main() -> Result<()> {
    // Come up with some work that needs to be performed. Let's pretend to
    // perform work on each file in the current directory.
    let mut files = Vec::new();
    for entry in fs::read_dir(".")? {

    // Build a thread pool with one thread per cpu.
    let cpus = num_cpus::get();
    let pool = ThreadPoolBuilder::new().num_threads(cpus).build()?;

    // Spin up the right number of worker threads. They will write to stderr.
    let oqueue = Sequencer::stderr();
    pool.scope(|scope| {
        for _ in 0..cpus {
            scope.spawn(|_| worker(&oqueue, &files));


fn worker(oqueue: &Sequencer, inputs: &[PathBuf]) {
    // Perform tasks indicated by the sequencer.
    loop {
        let task = oqueue.begin();
        match inputs.get(task.index) {
            Some(path) => work(task, path),
            None => return,

fn work(task: Task, path: &Path) {
    // Produce output by writing to the task.
    write!(task, "evaluating ");
    writeln!(task, "{}", path.display());

    // Do some expensive work...
    let string = path.to_string_lossy();
    thread::sleep(Duration::from_millis(150 * string.len() as u64));

    // ... which may fail or succeed.
    if string.contains('c') {
        write!(task, "  ERROR");
        writeln!(task, ": path contains the letter 'c'");

The output of this program is guaranteed to display tasks in the intended sorted order and non-interleaved. Tasks will make progress in parallel without needing to wait to perform output. All output will appear the earliest possible including one task in real time at all times.

evaluating ./.git
evaluating ./.gitignore
evaluating ./Cargo.lock
  ERROR: path contains the letter 'c'
evaluating ./Cargo.toml
evaluating ./LICENSE-APACHE
evaluating ./LICENSE-MIT
evaluating ./README.md
evaluating ./examples
evaluating ./src
  ERROR: path contains the letter 'c'
evaluating ./target

Further reading

  • The oqueue::Sequencer documentation covers some different techniques for distributing work items across tasks.

  • The oqueue::Task documentation shows the APIs for setting output color and writing output to a task.


Licensed under either of Apache License, Version 2.0 or MIT license at your option.
Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this crate by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


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