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Change Backtrace::enabled atomic from SeqCst to Relaxed by dtolnay · Pull Reques...

 3 years ago
source link: https://github.com/rust-lang/rust/pull/92139
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This atomic is not synchronizing anything outside of its own value, so we don't need the Acquire/Release guarantee that all memory operations prior to the store are visible after the subsequent load, nor the SeqCst guarantee of all threads seeing all of the sequentially consistent operations in the same order.

Using Relaxed reduces the overhead of Backtrace::capture() in the case that backtraces are not enabled.

Benchmark

#![feature(backtrace)]

use std::backtrace::Backtrace;
use std::sync::atomic::{AtomicUsize, Ordering};
use std::thread;
use std::time::Instant;

fn main() {
    let begin = Instant::now();
    let mut threads = Vec::new();
    for _ in 0..64 {
        threads.push(thread::spawn(|| {
            for _ in 0..10_000_000 {
                let _ = Backtrace::capture();
                static LOL: AtomicUsize = AtomicUsize::new(0);
                LOL.store(1, Ordering::Release);
            }
        }));
    }
    for thread in threads {
        let _ = thread.join();
    }
    println!("{:?}", begin.elapsed());
}

Before: 6.73 seconds
After: 5.18 seconds


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