Threads

Spawn a short-lived thread

The example uses the crossbeam crate, which provides data structures and functions for concurrent and parallel programming. Scope::spawn spawns a new scoped thread that is guaranteed to terminate before returning from the closure that passed into crossbeam::scope function, meaning that you can reference data from the calling function.

This example splits the array in half and performs the work in separate threads.

fn main() {
    let arr = &[1, 25, -4, 10];
    let max = find_max(arr);
    assert_eq!(max, Some(25));
}

fn find_max(arr: &[i32]) -> Option<i32> {
    const THRESHOLD: usize = 2;
  
    if arr.len() <= THRESHOLD {
        return arr.iter().cloned().max();
    }

    let mid = arr.len() / 2;
    let (left, right) = arr.split_at(mid);
  
    crossbeam::scope(|s| {
        let thread_l = s.spawn(|_| find_max(left));
        let thread_r = s.spawn(|_| find_max(right));
  
        let max_l = thread_l.join().unwrap()?;
        let max_r = thread_r.join().unwrap()?;
  
        Some(max_l.max(max_r))
    }).unwrap()
}

Pass data between two threads

This example demonstrates the use of crossbeam-channel in a single producer, single consumer (SPSC) setting. We build off the ex-crossbeam-spawn example by using crossbeam::scope and Scope::spawn to manage the producer thread. Data is exchanged between the two threads using a crossbeam_channel::unbounded channel, meaning there is no limit to the number of storeable messages. The producer thread sleeps for half a second in between messages.

use std::{thread, time};
use crossbeam_channel::unbounded;

fn main() {
    let (snd, rcv) = unbounded();
    let n_msgs = 5;
    crossbeam::scope(|s| {
        s.spawn(|_| {
            for i in 0..n_msgs {
                snd.send(i).unwrap();
                thread::sleep(time::Duration::from_millis(100));
            }
        });
    }).unwrap();
    for _ in 0..n_msgs {
        let msg = rcv.recv().unwrap();
        println!("Received {}", msg);
    }
}

Maintain global mutable state

Declare global state using lazy_static. lazy_static creates a globally available static ref which requires a Mutex to allow mutation (also see RwLock). The Mutex wrap ensures the state cannot be simultaneously accessed by multiple threads, preventing race conditions. A MutexGuard must be acquired to read or mutate the value stored in a Mutex.

use error_chain::error_chain;
use lazy_static::lazy_static;
use std::sync::Mutex;

error_chain!{ }

lazy_static! {
    static ref FRUIT: Mutex<Vec<String>> = Mutex::new(Vec::new());
}

fn insert(fruit: &str) -> Result<()> {
    let mut db = FRUIT.lock().map_err(|_| "Failed to acquire MutexGuard")?;
    db.push(fruit.to_string());
    Ok(())
}

fn main() -> Result<()> {
    insert("apple")?;
    insert("orange")?;
    insert("peach")?;
    {
        let db = FRUIT.lock().map_err(|_| "Failed to acquire MutexGuard")?;

        db.iter().enumerate().for_each(|(i, item)| println!("{}: {}", i, item));
    }
    insert("grape")?;
    Ok(())
}

Calculate SHA256 sum of iso files concurrently

This example calculates the SHA256 for every file with iso extension in the current directory. A threadpool generates threads equal to the number of cores present in the system found with num_cpus::get. Walkdir::new iterates the current directory and calls execute to perform the operations of reading and computing SHA256 hash.

use walkdir::WalkDir;
use std::fs::File;
use std::io::{BufReader, Read, Error};
use std::path::Path;
use threadpool::ThreadPool;
use std::sync::mpsc::channel;
use ring::digest::{Context, Digest, SHA256};

// Verify the iso extension
fn is_iso(entry: &Path) -> bool {
    match entry.extension() {
        Some(e) if e.to_string_lossy().to_lowercase() == "iso" => true,
        _ => false,
    }
}

fn compute_digest<P: AsRef<Path>>(filepath: P) -> Result<(Digest, P), Error> {
    let mut buf_reader = BufReader::new(File::open(&filepath)?);
    let mut context = Context::new(&SHA256);
    let mut buffer = [0; 1024];

    loop {
        let count = buf_reader.read(&mut buffer)?;
        if count == 0 {
            break;
        }
        context.update(&buffer[..count]);
    }

    Ok((context.finish(), filepath))
}

fn main() -> Result<(), Error> {
    let pool = ThreadPool::new(num_cpus::get());

    let (tx, rx) = channel();

    for entry in WalkDir::new("/home/user/Downloads")
        .follow_links(true)
        .into_iter()
        .filter_map(|e| e.ok())
        .filter(|e| !e.path().is_dir() && is_iso(e.path())) {
            let path = entry.path().to_owned();
            let tx = tx.clone();
            pool.execute(move || {
                let digest = compute_digest(path);
                tx.send(digest).expect("Could not send data!");
            });
        }

    drop(tx);
    for t in rx.iter() {
        let (sha, path) = t?;
        println!("{:?} {:?}", sha, path);
    }
    Ok(())
}

Draw fractal dispatching work to a thread pool

This example generates an image by drawing a fractal from the Julia set with a thread pool for distributed computation.

Allocate memory for output image of given width and height with ImageBuffer::new. Rgb::from_channels calculates RGB pixel values. Create ThreadPool with thread count equal to number of cores with num_cpus::get. ThreadPool::execute receives each pixel as a separate job.

mpsc::channel receives the jobs and Receiver::recv retrieves them. ImageBuffer::put_pixel uses the data to set the pixel color. ImageBuffer::save writes the image to output.png.

use error_chain::error_chain;
use std::sync::mpsc::{channel, RecvError};
use threadpool::ThreadPool;
use num::complex::Complex;
use image::{ImageBuffer, Pixel, Rgb};

error_chain! {
    foreign_links {
        MpscRecv(RecvError);
        Io(std::io::Error);
    }
}

// Function converting intensity values to RGB
// Based on http://www.efg2.com/Lab/ScienceAndEngineering/Spectra.htm
fn wavelength_to_rgb(wavelength: u32) -> Rgb<u8> {
    let wave = wavelength as f32;

    let (r, g, b) = match wavelength {
        380..=439 => ((440. - wave) / (440. - 380.), 0.0, 1.0),
        440..=489 => (0.0, (wave - 440.) / (490. - 440.), 1.0),
        490..=509 => (0.0, 1.0, (510. - wave) / (510. - 490.)),
        510..=579 => ((wave - 510.) / (580. - 510.), 1.0, 0.0),
        580..=644 => (1.0, (645. - wave) / (645. - 580.), 0.0),
        645..=780 => (1.0, 0.0, 0.0),
        _ => (0.0, 0.0, 0.0),
    };

    let factor = match wavelength {
        380..=419 => 0.3 + 0.7 * (wave - 380.) / (420. - 380.),
        701..=780 => 0.3 + 0.7 * (780. - wave) / (780. - 700.),
        _ => 1.0,
    };

    let (r, g, b) = (normalize(r, factor), normalize(g, factor), normalize(b, factor));
    Rgb::from_channels(r, g, b, 0)
}

// Maps Julia set distance estimation to intensity values
fn julia(c: Complex<f32>, x: u32, y: u32, width: u32, height: u32, max_iter: u32) -> u32 {
    let width = width as f32;
    let height = height as f32;

    let mut z = Complex {
        // scale and translate the point to image coordinates
        re: 3.0 * (x as f32 - 0.5 * width) / width,
        im: 2.0 * (y as f32 - 0.5 * height) / height,
    };

    let mut i = 0;
    for t in 0..max_iter {
        if z.norm() >= 2.0 {
            break;
        }
        z = z * z + c;
        i = t;
    }
    i
}

// Normalizes color intensity values within RGB range
fn normalize(color: f32, factor: f32) -> u8 {
    ((color * factor).powf(0.8) * 255.) as u8
}

fn main() -> Result<()> {
    let (width, height) = (1920, 1080);
    let mut img = ImageBuffer::new(width, height);
    let iterations = 300;

    let c = Complex::new(-0.8, 0.156);

    let pool = ThreadPool::new(num_cpus::get());
    let (tx, rx) = channel();

    for y in 0..height {
        let tx = tx.clone();
        pool.execute(move || for x in 0..width {
                         let i = julia(c, x, y, width, height, iterations);
                         let pixel = wavelength_to_rgb(380 + i * 400 / iterations);
                         tx.send((x, y, pixel)).expect("Could not send data!");
                     });
    }

    for _ in 0..(width * height) {
        let (x, y, pixel) = rx.recv()?;
        img.put_pixel(x, y, pixel);
    }
    let _ = img.save("output.png")?;
    Ok(())
}