rust 组播/多播 延迟疑问

[EricJia]

最近在测试 低延迟 rust tokio 组播，用 tokio 和 socket2 ，

设备是 aws c7gn.metal arm 架构 64 核心。 1 个 sender ，n 个 receiver 。发送时间戳。 设置 linux route 走 lo 网卡。

当 receiver20 个以内时。延迟大概是 40us 以内可控。 当 receiver 个数增加到 50-100 个以后， 延迟逐渐上升到 100~200us 以上。cpu 仍然占用很低，延迟显著增加，这是为什么呢？

求解 如何能让 50+ receiver 同时接收， 还能保持 50us 以下延迟？

use serde::{Deserialize, Serialize};
use std::net::{Ipv4Addr, SocketAddrV4};
const MULTICAST_ADDR: Ipv4Addr = Ipv4Addr::new(239, 255, 0, 1);
const MULTICAST_PORT: u16 = 3001;
const BIND_ADDR: Ipv4Addr = Ipv4Addr::LOCALHOST;

pub fn timestamp16() -> u128 {
    SystemTime::now()
        .duration_since(UNIX_EPOCH)
        .unwrap()
        .as_micros()
}

/// Networking options.
#[derive(argh::FromArgs)]
struct Args {
    /// multicast address that the socket must join
    #[argh(option, short = 'a', default = "MULTICAST_ADDR")]
    addr: Ipv4Addr,
    /// specific port to bind the socket to
    #[argh(option, short = 'p', default = "MULTICAST_PORT")]
    port: u16,
    /// whether or not to allow the UDP socket
    /// to be reused by another application
    #[argh(switch)]
    is_sender: bool,
}

fn main() -> std::io::Result<()> {
    init_logger();
    use socket2::{Domain, Protocol, Socket, Type};
    let Args {
        addr,
        port,
        is_sender,
    } = argh::from_env();
    println!("{} {} is_sender: {}", addr, port, is_sender);
    let socket = Socket::new(Domain::IPV4, Type::DGRAM, Some(Protocol::UDP))?;
    socket.set_nonblocking(true)?;
    socket.set_reuse_address(true)?;
    socket.set_reuse_port(true)?;
    socket.set_multicast_loop_v4(true)?;
    socket.set_multicast_ttl_v4(1)?;

    socket.join_multicast_v4(&addr, &Ipv4Addr::LOCALHOST)?;
    let fin_addr = SocketAddrV4::new(addr, port);
    if is_sender {
        socket.bind(&SocketAddrV4::new(Ipv4Addr::LOCALHOST, port).into())?;
    } else {
        socket.bind(&SocketAddrV4::new(Ipv4Addr::UNSPECIFIED, port).into())?;
    }

    let runtime = tokio::runtime::Builder::new_current_thread()
        .thread_name("network")
        .enable_all()
        .build()?;
    let udp = {
        let _guard = runtime.enter();
        tokio::net::UdpSocket::from_std(socket.into())?
    };

    runtime.block_on(async move {
        let mut interval = tokio::time::interval(tokio::time::Duration::from_millis(1000));
        if is_sender == false {
            interval = tokio::time::interval(tokio::time::Duration::from_secs(60 * 60 * 24));
        }
        interval.tick().await;

        let mut buf = [0; 16];
        loop {
            tokio::select! {
                recv_res = udp.recv_from(&mut buf) => {
                    let (count, remote_addr) = recv_res.expect("cannot receive from socket");
                    let parsed = u128::from_be_bytes(buf[..count].try_into().unwrap());
                    let cost = timestamp16() - parsed;
                    println!("{:?}", );!("recv {remote_addr} {parsed} {count} {cost}")
                }
                _ = interval.tick() => {
                    let cur = timestamp16();
                    let input = cur.to_be_bytes();
                    udp.send_to(&input, fin_addr).await.expect("cannot send message to socket");
                    println!("{:?}", );!("send: {}", cur);
                }
            }
        }
    });
    Ok(())
}

[lsk569937453]

https://stackoverflow.com/questions/76589659/does-multithreaded-tokio-run-task-on-a-single-os-thread

把 tokio::runtime::Builder::new_current_thread() 换成 new_multi_thread 试试？？

[EricJia]

@lsk569937453 试过， 这个我测试下来结果 new_current_thread 延迟会更低。

[EricJia]

部署方式: supervisor 开了 60 个 receiver 程序， 用 args 指定的 sender receiver
```
[program:sender1]
stderr_logfile = /data/sender1
command = /home/ubuntu/test_udp --is_sender

[program:receiver0]
stderr_logfile = /data/receiver0
command = /home/ubuntu/test_udp
[program:receiver1]
stderr_logfile = /data/receiver1
command = /home/ubuntu/test_udp
...
```

[rainmote]

检查下网卡软中断绑定 cpu 问题，有些机器绑定到 cpu0 可能出现性能瓶颈。

[everfly]

是不是 udp.send_to 导致的延时增加？可以考虑改成 unbound_channel 异步发送看看。

[EricJia]

@everfly https://stackoverflow.com/questions/6866611/linux-multicast-sendto-performance-degrades-with-local-listeners 问题跟这个很像，listeners 增加后， 延迟以每个 2us 成倍增加。 已经设置 no_blocking, 跟 channel 没关系

[EricJia]

@rainmote https://stackoverflow.com/questions/6866611/linux-multicast-sendto-performance-degrades-with-local-listeners 问题跟这个很像，listeners 增加后， 延迟以每个 listener 2us 线性增加。 似乎跟 cpu 没关系？