Zero Copy Publish and Subscribe

Source code.

In this chapter, we will introduce how to use zero copy communications by CycloneDDS. To use CycloneDDS, please install it as Eclipse Cyclone DDS.

Create Project

First of all, create a project as follows.

$ cargo new zerocpy

The files we use are as follows. cyclonedds.xml will be created and used later.

Add safe_drive to dependencies section of Cargo.toml as follows.

Cargo.toml

[dependencies]
safe_drive = "0.4"

main.rs

main.rs can be implemented as follows, but almost every lines are same as shown before.

main.rs

use safe_drive::{context::Context, msg::common_interfaces::std_msgs};
use std::{error::Error, time::Duration};

const TOPIC_NAME: &str = "pubsub_loaned";

fn main() -> Result<(), Box<dyn Error + Sync + Send + 'static>> {
    // create a context
    let ctx = Context::new()?;

    // create a subscribe node
    let node_sub = ctx.create_node("loaned_sub_node", None, Default::default())?;

    // create a publish node
    let node_pub = ctx.create_node("loaned_pub_node", None, Default::default())?;

    std::thread::sleep(Duration::from_millis(500));

    // create a publisher and a subscriber
    let subscriber = node_sub.create_subscriber::<std_msgs::msg::Bool>(TOPIC_NAME, None)?;
    let publisher = node_pub.create_publisher::<std_msgs::msg::Bool>(TOPIC_NAME, None)?;

    let mut loaned = publisher.borrow_loaned_message()?;
    *loaned = std_msgs::msg::Bool::new().ok_or("failed to new Bool")?;
    loaned.data = false;
    publisher.send_loaned(loaned)?; // send message

    std::thread::sleep(Duration::from_millis(500));

    // wait messages
    let mut selector = ctx.create_selector()?;
    selector.add_subscriber(
        subscriber,
        Box::new(move |msg| {
            assert!(!msg.data);
        }),
    );
    selector.wait()?;

    Ok(())
}

The important thing is using borrow_loaned_message() and send_loaned() methods as follows.

#![allow(unused)]
fn main() {
let mut loaned = publisher.borrow_loaned_message()?;
*loaned = std_msgs::msg::Bool::new().ok_or("failed to new Bool")?;
loaned.data = false;
publisher.send_loaned(loaned)?; // send message
}

borrow_loaned_message() borrows a memory region from CycloneDDS and send_loaned() sends a message in the borrowed region. safe_drive automatically check whether zero copy is available or not, and it uses conventional copied APIs if zero copy is not available.

Setting-up Zero Copy

To enable zero copy, please prepare cyclonedds.xml, which is a configuration file of CycloneDDS, as follows. You can use arbitrary name for it.

cyclonedds.xml

<?xml version="1.0" encoding="UTF-8" ?>
<CycloneDDS xmlns="https://cdds.io/config" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://cdds.io/config https://raw.githubusercontent.com/eclipse-cyclonedds/cyclonedds/iceoryx/etc/cyclonedds.xsd">
    <Domain id="any">
        <SharedMemory>
            <Enable>true</Enable>
            <LogLevel>info</LogLevel>
        </SharedMemory>
    </Domain>
</CycloneDDS>

<SharedMemory> tag indicates the configuration of zero copy.

To use zero copy enabled CycloneDDS, please set environment arguments as follows.

$ export RMW_IMPLEMENTATION=rmw_cyclonedds_cpp
$ export CYCLONEDDS_URI=file://path/to/cyclonedds.xml

In addition to that, CycloneDDS requires iox-roudi daemon to use zero copy. It can be launched as follows.

$ iox-roudi

Execute

After that, zero copy communications can be performed!

$ cargo run