performance - 零成本抽象 : performance of for-loop vs. 迭代器

Question

阅读Zero-cost abstractions看着 Introduction to rust: a low-level language with high-level abstractions我尝试比较两种计算向量点积的方法:一种使用 for 循环，另一种使用迭代器。

#![feature(test)]

extern crate rand;
extern crate test;

use std::cmp::min;

fn dot_product_1(x: &[f64], y: &[f64]) -> f64 {
    let mut result: f64 = 0.0;
    for i in 0..min(x.len(), y.len()) {
        result += x[i] * y[i];
    }
    return result;
}

fn dot_product_2(x: &[f64], y: &[f64]) -> f64 {
    x.iter().zip(y).map(|(&a, &b)| a * b).sum::<f64>()
}

#[cfg(test)]
mod bench {
    use test::Bencher;
    use rand::{Rng,thread_rng};
    use super::*;

    const LEN: usize = 30;

    #[test]
    fn test_1() {
        let x = [1.0, 2.0, 3.0];
        let y = [2.0, 4.0, 6.0];
        let result = dot_product_1(&x, &y);
        assert_eq!(result, 28.0);
    }

    #[test]
    fn test_2() {
        let x = [1.0, 2.0, 3.0];
        let y = [2.0, 4.0, 6.0];
        let result = dot_product_2(&x, &y);
        assert_eq!(result, 28.0);
    }

    fn rand_array(cnt: u32) -> Vec<f64> {
        let mut rng = thread_rng();
        (0..cnt).map(|_| rng.gen::<f64>()).collect()

    }

    #[bench]
    fn bench_small_1(b: &mut Bencher) {
        let samples = rand_array(2*LEN as u32);
        b.iter(|| {
            dot_product_1(&samples[0..LEN], &samples[LEN..2*LEN])
        })
    }

    #[bench]
    fn bench_small_2(b: &mut Bencher) {
        let samples = rand_array(2*LEN as u32);
        b.iter(|| {
            dot_product_2(&samples[0..LEN], &samples[LEN..2*LEN])
        })
    }
}

以上链接的后一个声称带有迭代器的版本应该具有类似的性能，“而且实际上要快一点”。然而，当对两者进行基准测试时，我得到了截然不同的结果:

running 2 tests
test bench::bench_small_loop ... bench:          20 ns/iter (+/- 1)
test bench::bench_small_iter ... bench:          24 ns/iter (+/- 2)

test result: ok. 0 passed; 0 failed; 0 ignored; 2 measured; 0 filtered out

那么，“零成本抽象”去了哪里？

更新:添加由@wimh 提供的foldr 示例并使用split_at 而不是切片得到以下结果。

running 3 tests
test bench::bench_small_fold ... bench:          18 ns/iter (+/- 1)
test bench::bench_small_iter ... bench:          21 ns/iter (+/- 1)
test bench::bench_small_loop ... bench:          24 ns/iter (+/- 1)

test result: ok. 0 passed; 0 failed; 0 ignored; 3 measured; 0 filtered out

因此，额外的时间似乎直接或间接来自于在测量代码中构建切片。为了检查是否确实如此，我尝试了以下两种方法，结果相同(此处显示的是 foldr 案例并使用 map + sum):

#[bench]
fn bench_small_iter(b: &mut Bencher) {
    let samples = rand_array(2 * LEN);
    let s0 = &samples[0..LEN];
    let s1 = &samples[LEN..2 * LEN];
    b.iter(|| dot_product_iter(s0, s1))
}

#[bench]
fn bench_small_fold(b: &mut Bencher) {
    let samples = rand_array(2 * LEN);
    let (s0, s1) = samples.split_at(LEN);
    b.iter(|| dot_product_fold(s0, s1))
}

Answer 1

对我来说，这似乎是零成本。我写你的代码稍微更习惯一点，对两个测试使用相同的随机值，然后测试多次:

fn dot_product_1(x: &[f64], y: &[f64]) -> f64 {
    let mut result: f64 = 0.0;
    for i in 0..min(x.len(), y.len()) {
        result += x[i] * y[i];
    }
    result
}

fn dot_product_2(x: &[f64], y: &[f64]) -> f64 {
    x.iter().zip(y).map(|(&a, &b)| a * b).sum()
}

fn rand_array(cnt: usize) -> Vec<f64> {
    let mut rng = rand::rngs::StdRng::seed_from_u64(42);
    rng.sample_iter(&rand::distributions::Standard).take(cnt).collect()
}

#[bench]
fn bench_small_1(b: &mut Bencher) {
    let samples = rand_array(2 * LEN);
    let (s0, s1) = samples.split_at(LEN);
    b.iter(|| dot_product_1(s0, s1))
}

#[bench]
fn bench_small_2(b: &mut Bencher) {
    let samples = rand_array(2 * LEN);
    let (s0, s1) = samples.split_at(LEN);
    b.iter(|| dot_product_2(s0, s1))
}

bench_small_1   20 ns/iter (+/- 6)
bench_small_2   17 ns/iter (+/- 1)

bench_small_1   19 ns/iter (+/- 3)
bench_small_2   17 ns/iter (+/- 2)

bench_small_1   19 ns/iter (+/- 5)
bench_small_2   17 ns/iter (+/- 3)

bench_small_1   19 ns/iter (+/- 3)
bench_small_2   24 ns/iter (+/- 7)

bench_small_1   28 ns/iter (+/- 1)
bench_small_2   24 ns/iter (+/- 1)

bench_small_1   27 ns/iter (+/- 1)
bench_small_2   25 ns/iter (+/- 1)

bench_small_1   28 ns/iter (+/- 1)
bench_small_2   25 ns/iter (+/- 1)

bench_small_1   28 ns/iter (+/- 1)
bench_small_2   25 ns/iter (+/- 1)

bench_small_1   28 ns/iter (+/- 0)
bench_small_2   25 ns/iter (+/- 1)

bench_small_1   28 ns/iter (+/- 1)
bench_small_2   17 ns/iter (+/- 1)

在 10 次运行中的 9 次中，惯用代码比 for 循环更快。这是在具有 32 GB RAM 的 2.9 GHz Core i9 (I9-8950HK) 上完成的，使用 rustc 1.31.0-nightly (fc403ad98 2018-09-30) 编译。