1//! Benchmark the overhead that the synchronization of `OnceCell::get` causes.
2//! We do some other operations that write to memory to get an imprecise but somewhat realistic
3//! measurement.
4
5use once_cell::sync::OnceCell;
6use std::sync::atomic::{AtomicUsize, Ordering};
7
8const N_THREADS: usize = 16;
9const N_ROUNDS: usize = 1_000_000;
10
11static CELL: OnceCell<usize> = OnceCell::new();
12static OTHER: AtomicUsize = AtomicUsize::new(0);
13
14fn main() {
15 let start = std::time::Instant::now();
16 let threads =
17 (0..N_THREADS).map(|i| std::thread::spawn(move || thread_main(i))).collect::<Vec<_>>();
18 for thread in threads {
19 thread.join().unwrap();
20 }
21 println!("{:?}", start.elapsed());
22 println!("{:?}", OTHER.load(Ordering::Relaxed));
23}
24
25#[inline(never)]
26fn thread_main(i: usize) {
27 // The operations we do here don't really matter, as long as we do multiple writes, and
28 // everything is messy enough to prevent the compiler from optimizing the loop away.
29 let mut data = [i; 128];
30 let mut accum = 0usize;
31 for _ in 0..N_ROUNDS {
32 let _value = CELL.get_or_init(|| i + 1);
33 let k = OTHER.fetch_add(data[accum & 0x7F] as usize, Ordering::Relaxed);
34 for j in data.iter_mut() {
35 *j = (*j).wrapping_add(accum);
36 accum = accum.wrapping_add(k);
37 }
38 }
39}
40