Why join 7% faster than reduce?

code review
1

I use to use reduce to concatenate string parts with some separator. However I recently found that join works faster.

use std::error::Error;
use std::time::Instant;
use std::collections::HashMap;

fn use_join(arr: &[&str]) -> String {
    arr.into_iter().map(|curr| "\"".to_owned() +
            &json_encode(&curr) + "\"").collect::<Vec<_>>().join(", ")
}

fn use_reduce(arr: &[&str]) -> String {
    arr.into_iter().map(|curr| "\"".to_owned() +
            &json_encode(&curr) + "\"").reduce(|a,e| a + ", " + &e).unwrap_or_else(String::new)
}

fn main() -> Result<(), Box<dyn Error>> {
    let start = Instant::now();
    let mut sum = 0;
    for _ in 0..10_000_000 {
        let res = use_reduce(&opts_val);
        sum += res.len()
    }
    let duration = start.elapsed();
    println!("Time elapsed: {:?} :{sum}", duration);
    let start = Instant::now();
    let mut sum = 0;
    for _ in 0..10_000_000 {
        let res = use_join(&opts_val);
        sum += res.len()
    }
    let duration = start.elapsed();
    println!("Time elapsed for join: {:?} :{sum}", duration);

    Ok(())
}

My theory is that a closure gives overhead. What could be other reasons? Which technique do you use?

2

in the reduce() version, you repeated use the string + operator, which is inefficient because it needs to grow/reallocate every iteration. the join() version can calculate the final length before hand and do a single allocation.

relevant implementation:

3

I presume that json_encode returns String, so this approach is in general inefficient. If you had json_encode that writes to &mut String, or writes to impl std::io::Write, or returns something that implements Display, you could stream the output without allocating temporary strings.

4

Note that this is also because of the linear structure of the reductions done by reduce. If you instead use Itertools::tree_reduce that cost will be less because you'll be combining smaller strings on average -- the documentation for the method even shows an example of that.

Of course, it's better to use https://doc.rust-lang.org/std/primitive.slice.html#method.concat/https://doc.rust-lang.org/std/primitive.slice.html#method.join if you can, but tree_reduce is a nice trick to keep in your back pocket for times where the basic things aren't enough.

5

Is tree_reduce 3rd party code? Unfortunately it isn't allowed in my company unless it passes a long process of code review. So I stick with join for now. It's good knowledge the Rust performance depends on memory reallocation requests, so I will try to reduce such requests.

6

Not just Rust, it's one of common bottlenecks.

7

It's hard to generalize yet, because I am working on some language where no reallocation happens at all. So, we can say better: it's the bottleneck of most modern languages.

8

Hmmm, that code is suspicious in how it handles result sizes isize::MAX < Size < usize::MAX

9

It's why some companies switch to Ada. You clearly define ranges there.