A program demostrating the impact of cache line bouncing

cache_line_bouncing.rs

//! This is basically the Rust version of the C code from this post
//!
//! https://arighi.blogspot.com/2008/12/cacheline-bouncing.html

use nix::sched::{sched_setaffinity, CpuSet};
use nix::unistd::Pid;
use std::cell::UnsafeCell;
use std::thread::spawn;
use std::time::SystemTime;

const LOOPS_MAX: usize = 2000000000;

struct Foo {
    for_thread1: UnsafeCell<i64>,
    #[cfg(feature = "distinct_cache_line")]
    _padding: [u8; 56],
    for_thread2: UnsafeCell<i64>,
}

impl Foo {
    const fn new() -> Self {
        Self {
            for_thread1: UnsafeCell::new(0),
            #[cfg(feature = "distinct_cache_line")]
            _padding: [0; 56],
            for_thread2: UnsafeCell::new(0),
        }
    }
}

unsafe impl Sync for Foo {}

static FOO: Foo = Foo::new();

fn main() {
    let now = SystemTime::now();
    let mut cpu0 = CpuSet::new();
    cpu0.set(0).unwrap();
    let mut cpu1 = CpuSet::new();
    cpu1.set(1).unwrap();

    let handle = spawn(move || {
        sched_setaffinity(Pid::from_raw(0), &cpu0).unwrap();

        for _ in 0..LOOPS_MAX {
            let ptr = FOO.for_thread2.get();
            unsafe {
                *ptr += 1;
            }
        }
    });

    sched_setaffinity(Pid::from_raw(0), &cpu1).unwrap();
    for _ in 0..LOOPS_MAX {
        let ptr = FOO.for_thread1.get();
        unsafe {
            *ptr += 1;
        }
    }

    handle.join().unwrap();

    let dur = now.elapsed().unwrap();
    println!("{:?}", dur);
}

Test the above code.sh

$ neofetch
       _,met$$$$$gg.          steve@debian
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  ,g$$P"     """Y$$.".        OS: Debian GNU/Linux 12 (bookworm) x86_64
 ,$$P'              `$$$.     Kernel: 6.1.0-15-amd64
',$$P       ,ggs.     `$$b:   Uptime: 1 day, 5 hours, 34 mins
`d$$'     ,$P"'   .    $$$    Packages: 1807 (dpkg), 22 (flatpak)
 $$P      d$'     ,    $$P    Shell: zsh 5.9
 $$:      $$.   -    ,d$$'    Resolution: 3440x1440
 $$;      Y$b._   _,d$P'      DE: GNOME 43.9
 Y$$.    `.`"Y$$$$P"'         WM: Mutter
 `$$b      "-.__              WM Theme: Adwaita
  `Y$$                        Theme: adw-gtk3 [GTK2/3]
   `Y$$.                      Icons: Adwaita [GTK2/3]
     `$$b.                    Terminal: tmux
       `Y$$b.                 CPU: AMD Ryzen 9 7950X (32) @ 4.500GHz
          `"Y$b._             GPU: AMD ATI 0e:00.0 Raphael
              `"""            GPU: AMD ATI Radeon RX 5600 OEM/5600 XT / 5700/5700 XT
                              Memory: 15.45GiB / 30.49GiB (50%)
                              Disk (/): 15G / 28G (57%)

# Don't build it in release mode, or the compiler would not modify the field
$ cargo b -q
$ ./target/debug/rust
18.374641728s

$ cargo b --features distinct_cache_line -q
$ ./target/debug/rust
9.507881497s