How can I improve the performance of my Rust code compared to Python code?

Question

How can I improve the performance of my Rust code as Python takes around 5 seconds to finish and Rust takes 7 seconds.

I am using build --release

Rust code

fn main() {
    let mut n = 0;

    loop {
        n += 1;
        println!("The value of n is {}", &n);
        if n == 100000 {
            break;
        }
    }
}

Python3 code

n = 0
while True:
   n+=1
   print("The value of n is ",n)
   if n == 100000:
       break

`build --release` is not a Rust command. Did you mean **`cargo build --release`**? — Shepmaster, Oct 21 '19 at 17:46
Tangentially, no need for `&n`: [Does println! borrow or own the variable?](https://stackoverflow.com/q/30450399/155423) — Shepmaster, Oct 21 '19 at 17:46
Without printing of any kind, I expect the Rust program to be completely optimized away to the equivalent of `fn main() {}`. — Shepmaster, Oct 21 '19 at 17:57

score 5 · Answer 1 · edited Oct 21 '19 at 20:52

If I recall correctly, println locks stdout. Taken from Rust Performance Pitfalls:

[...] the default print! macros will lock STDOUT for each write operation. So if you have a larger textual output (or input from STDIN), you should lock manually.

This:
let mut out = File::new("test.out");
println!("{}", header);
for line in lines {
    println!("{}", line);
    writeln!(out, "{}", line);
}
println!("{}", footer);
locks and unlocks io::stdout a lot, and does a linear number of (potentially small) writes both to stdout and the file. Speed it up with:
{
    let mut out = File::new("test.out");
    let mut buf = BufWriter::new(out);
    let mut lock = io::stdout().lock();
    writeln!(lock, "{}", header);
    for line in lines {
        writeln!(lock, "{}", line);
        writeln!(buf, "{}", line);
    }
    writeln!(lock, "{}", footer);
}   // end scope to unlock stdout and flush/close buf> 
This locks only once and writes only once the buffer is filled (or buf is closed), so it should be much faster.

Similarly, for network IO, you may want to use buffered IO.

How can I improve the performance of my Rust code compared to Python code?

1 Answers1