Why is this Rust program so slow? Did I miss something?

Question

I read Minimal distance in Manhattan metric and rewrote the author's "naive" implementation in Rust . The C++ variant is:

#include <utility>
#include <cstdio>
#include <cstdlib>

std::pair<int, int> pointsA[1000001];
std::pair<int, int> pointsB[1000001];

int main() {
    int n, t;
    unsigned long long dist;

    scanf("%d", &t);

    while(t-->0) {
        dist = 4000000000LL;
        scanf("%d", &n);

        for(int i = 0; i < n; i++) {
            scanf("%d%d", &pointsA[i].first, &pointsA[i].second);
        }

        for(int i = 0; i < n; i++) {
            scanf("%d%d", &pointsB[i].first, &pointsB[i].second);
        }

        for(int i = 0; i < n ;i++) {
            for(int j = 0; j < n ; j++) {
                if(abs(pointsA[i].first - pointsB[j].first) + abs(pointsA[i].second - pointsB[j].second) < dist)
                    dist = abs(pointsA[i].first - pointsB[j].first) + abs(pointsA[i].second - pointsB[j].second);
            }
        }
        printf("%lld\n", dist);
    }
}

The Rust variant is:

use std::io;
use std::io::BufReader;
use std::io::BufRead;

fn read_array(stdin: &mut BufReader<io::Stdin>, array_len: usize, points: &mut Vec<(i32, i32)>) {
    let mut line = String::new();
    for _ in 0..array_len {
        line.clear();
        stdin.read_line(&mut line).unwrap();
        let mut item = line.split_whitespace();
        let x = item.next().unwrap().parse().unwrap();
        let y = item.next().unwrap().parse().unwrap();
        points.push((x, y));
    }
}

fn manhattan_dist(a: &(i32, i32), b: &(i32, i32)) -> u32 {
    ((a.0 - b.0).abs() + (a.1 - b.1).abs()) as u32
}

fn main() {
    let mut line = String::new();
    let mut stdin = BufReader::new(io::stdin());
    stdin.read_line(&mut line).unwrap();
    let n_iters = line.trim_right().parse::<usize>().unwrap();
    let mut points_a = Vec::with_capacity(10000);
    let mut points_b = Vec::with_capacity(10000);
    for _ in 0..n_iters {
        line.clear();
        stdin.read_line(&mut line).unwrap();
        let set_len = line.trim_right().parse::<usize>().unwrap();
        points_a.clear();
        points_b.clear();
        read_array(&mut stdin, set_len, &mut points_a);
        read_array(&mut stdin, set_len, &mut points_b);
        let mut dist = u32::max_value();
        for i in points_a.iter() {
            for j in points_b.iter() {
                dist = std::cmp::min(manhattan_dist(i, j), dist);
            }
        }
        println!("{}", dist);
    }
}

Then, I generated data with a Python script:

import random

ITER = 100
N = 10000
MAX_INT = 1000000

print("%d" % ITER)

for _ in range(0, ITER):
    print("%d" % N)
    for _ in range(0, N):
        print(random.randrange(-MAX_INT, MAX_INT + 1), random.randrange(1, MAX_INT + 1))
    for _ in range(0, N):
        print(random.randrange(-MAX_INT, MAX_INT + 1), random.randrange(-MAX_INT, 0))

And compiled both variants with g++ -Ofast -march=native and rustc -C opt-level=3 respectively. The timings are:

C++

real    0m7.789s
user    0m7.760s
sys     0m0.020s

Rust

real    0m28.589s
user    0m28.570s
sys     0m0.010s

Why is my Rust code four times slower than the C++ variant? I am using Rust 1.12.0-beta.1.

I added time measurements:

let now = SystemTime::now();
line.clear();
stdin.read_line(&mut line).unwrap();
let set_len = line.trim_right().parse::<usize>().unwrap();
points_a.clear();
points_b.clear();
read_array(&mut stdin, set_len, &mut points_a);
read_array(&mut stdin, set_len, &mut points_b);
io_time += now.elapsed().unwrap();

let now = SystemTime::now();
let mut dist = u32::max_value();
for i in points_a.iter() {
    for j in points_b.iter() {
        dist = std::cmp::min(manhattan_dist(i, j), dist);
    }
}
calc_time += now.elapsed().unwrap();

And writeln!(&mut std::io::stderr(), "io_time: {}, calc_time: {}", io_time.as_secs(), calc_time.as_secs()).unwrap(); prints io_time: 0, calc_time: 27 .

I tried nightly rustc 1.13.0-nightly (e9bc1bac8 2016-08-24) :

$ time ./test_rust < data.txt  > test3_res
io_time: 0, calc_time: 19

real    0m19.592s
user    0m19.560s
sys     0m0.020s
$ time ./test1 < data.txt  > test1_res

real    0m7.797s
user    0m7.780s
sys     0m0.010s

So it is at now ~ 2.7x difference on my Core i7 .

Answer 1

The difference is of course -march=native ... kind of. Rust has this through -C target_cpu=native , but this doesn't give the same speed benefit. This is because LLVM is unwilling to vectorize in this context, whereas GCC does. You may note that using Clang , a C++ compiler that also uses LLVM, also produces relatively slow code.

To encourage LLVM to vectorize, you can move the main loop into a separate function. Alternatively, you can use a local block. If you write the code carefully as

let dist = {
    let mut dist = i32::max_value();
    for &(a, b) in &points_a[..n] {
        for &(c, d) in &points_b[..n] {
            dist = std::cmp::min(((a - c).abs() + (b - d).abs()), dist);
        }
    }
    dist
} as u32;

the difference between Rust and C++ is then near-negligible (~4%).

Answer 2

The vast majority of the performance you're seeing in C++ is due to the flag -march=native .

This flag is not the equivalent flag to Rust's --release . It uses CPU instructions specific to the CPU it is compiled on, so math in particular is going to be way faster.

Removing that flag puts the C++ code at 19 seconds.

Then there's the unsafety present in the C++ code. None of the input is checked. The Rust code does check it, you use .unwrap() – unwrap has a performance cost, there's an assertion, then the code necessary for unwinding, etc.

Using if let s instead of raw unwrap s, or ignoring results where possible, brings the Rust code down again.

Rust: 22 seconds

C++: 19 seconds

Where's the 3 seconds coming from? A bit of playing around leads me to believe it's println! vs. printf , but I don't have hard numbers for the C++ code. What I can say is that the Rust code drops to 13 seconds when I perform the printing outside of the benchmark.

TLDR: Your compiler flags are different, and your C++ code is not safe.

Answer 3

I'm definitely not seeing any difference in execution time. On my machine,

C++:

real    0m19.672s
user    0m19.636s
sys     0m0.060s

Rust:

real    0m19.047s
user    0m19.028s
sys     0m0.040s

I compiled the Rust code with rustc -O test.rs -o ./test and the C++ code with g++ -Ofast test.cpp -o test .

I'm running Ubuntu 16.04 with Linux Kernel 4.6.3-040603-generic. The laptop I ran this on has an Intel(R) Core(TM) i5-6200U CPU and 8GB of RAM, nothing special.