Llogiq on stuff

Rust Performance Pitfalls

Overall, Rust is pretty good for performance. Write the most simple, naive stuff, and it will usually run within a factor of two from optimized C/C++ code, without any further performance work on the code. With some investment into optimizations, matching or exceeding C’s speed should be possible in most cases. However, Rust makes some tradeoffs for different reasons than sheer speed, so here’s a handy list of some things that may bite you and how you can speed them up.

Before we come to the list, please remember that this is only general advice that may or may not apply to your specific situation. As Kirk Pepperdine always exhorts: “Measure, don’t guess!”.

Update: redditor MEaster did some benchmarks and was so nice to share them. Thanks!

Ask for --release

The first stumbling block is that Rust by default compiles (and runs) in debug mode. This is fast to compile, but does next to no optimizations, so your code will likely run slow as molasses.

Every now and then, someone logs on IRC, rust-users or /r/rust asking “why is Rust so slow?” only to be blown away by how fast it is once you ask for it. So use cargo run --release to run your code if you want it fast.

I’d like to add that this is actually a good default, letting you test your code quickly without much hassle, leaving the heavyweight optimizations until you’re sufficiently confident it does the right thing. No need to waste cycle optimizing a program that doesn’t work correctly.

Also if you want to leverage the full potential of your CPU, -C target-cpu=native can give you a bit of extra oomph. Note that for some CPUs the target detection is broken and will result in illegal opcodes in your binaries. I get this with my intel Core m3-6y30, and using -C target-cpu=skylake works like a charm for me.

Bonus: You can also add some .cargo directory within any directory from the project root upwards (many people will have their projects somewhere in their home directory, so if they use rustup, they’ll already have it). If this directory contains a config file, its contents will be parsed for defaults. So insert e.g.:

[target.`cfg(any(windows, unix))`]
rustflags = ["-C target-cpu=native"]

Unbuffered IO

By default, Rust uses unbuffered File IO. So when you write files, wrap them in a BufWriter / BufReader, unless you have good reason to forgo the speedup – constrained memory, using a custom buffering scheme, specific need for high write granularity. Also if you read/write the whole thing at once, buffering won’t help you.

Similarly, 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.


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.

Reading lines

Similarly, the Read::lines() iterator is very easy to use, but it has one downside: It allocates a String for each line. Manually allocating and reusing a String will reduce memory churn and may gain you a bit of performance.


for line in buf.lines() {
    let line = line.unwrap();
    // do something with line

should become this to remove the extra allocation per line:

let mut line = String::new(); // may also use with_capacity if you can guess
while buf.read_line(&mut line).unwrap() > 0 {
    // do something with line
    line.clear(); // clear to reuse the buffer

str vs. [u8]

Many operations can be done regardless the string encoding. Yet Rust insists on using UTF-8 encoding, and will check this on string creation. This is a very good thing, as it allows us to rely on valid UTF-8 data, which allows Rust to implement quite fast, yet correct string handling methods, once we’ve paid the entrance fee by way of UTF-8 checking.

To get rid of the checks, we can either use bytes directly (usually via Vec<u8> / &[u8]) or, if we are absolutely sure the input will be valid UTF-8, use str::from_utf8_unchecked(_) (WARNING: this will require unsafe and break your code in surprising ways should the input not be valid UTF-8).

The regex crate has a regex::bytes submodule containing all functions to work with byte slices where regex does with &strings

Most parsing crates work on byte slices already to avoid UTF-8 checking.

I won’t give an example here, as the operations are too diverse to hope covering even the most popular cases.

Making a Hash of it

A lot has been written about the topic of hashing (including by me)

Like with UTF-8 strings, Rust chooses a sensible, safe default and allows you to override that choice should you be so inclined. For example, the Rust compiler uses the fnv crate for the FNV hash function which speedily gets good results with short-ish strings.

Don’t do anything before you’ve measured and have a good idea of your hash key distribution. Depending on this, you may be able to get some respectable gains, but the price in carefully choosing the hash function and setting up a good benchmark regime is steeper than with the other suggestions.

As a baseline check, you can (if your keys implement Ord) replace your hash map/set with a BTreeMap/BTreeSet and look at the performance. Also in some cases, the ordermap provides maps that improve memory locality (good for fast iteration) at the cost of some memory.

Don’t Index, Iterate (in simple cases)

While codegen for complex iterator chains may be suboptimal as of yet, in most simple cases using an iterator will be faster than an indexed loop. So this:

for i in 0..(xs.len()) {
    let x = xs[i];
    // do something with x

should really be this:

for x in &xs {
    // do something with x

However, there are some caveats:

  • if you use the index i elsewhere, use for (i, x) in xs.iter().enumerate()
  • the iterator loop borrows xs whereas the indexed loops does so only for the indexing operation. So you may need to index to pass the borrow checker. Still, some access patterns may be supported by iterator methods (e.g. xs.iter().windows(2) if you want to access each two neighboring elements).
  • note this also means you cannot modify xs while iterating over it. However, there are specialized iterators to e.g. remove elements in-place (Drain).

Needless collect()

Also regarding iterators, if you use FromIterator::collect(), be aware that you incur allocation for a new collection and force evaluation for each iteration step. So before writing out that collect, ask yourself if you really need it. This:

let nopes : Vec<_> = bleeps.iter().map(boop).collect();
let frungies : Vec<_> = nopes.iter().filter(|x| x > MIN_THRESHOLD).collect();

Could most probably be this:

let frungies : Vec<_> = bleeps.iter()
                              .filter(|x| x > MIN_THRESHOLD)

Depending on what we do with our frungies, we may even get rid of the second collect(). Watch out for side effects, which might be relevant and change evaluation order if a collect() is removed.

Avoid needless allocation

Rust gives us a good number of tools to avoid needless allocations. It however also gives us tools to do them if we’re so inclined. Especially when starting out and getting into the first disagreements with the borrow checker, often a simple fix is to call .to_owned() or .clone(). However, while this keeps the code simple, performance may suffer.

As there are multiple techniques to avoiding allocation, I’ll only list those that I deem both useful and sufficiently easy to implement:

  • Use &str instead of &String (or even String unless you explicitly want to consume it)
  • Similarly, use &[T] (slices) instead of &Vec<T> (or even Vec<T>). Also use &mut [T] instead of &mut Vec<T> if you have no resizing operation.
  • For static values, you can often get away with arrays instead of Vecs. Just reference them to get a slice.
  • If you cannot replace your owned value with a borrow, consider using a Cow. For example, it is often possible to replace String with Cow<'static, str> and Vec<T> with Cow<'a, [T]> if you can borrow in some but not all cases.
  • Sometimes, when changing an enum, we want to keep parts of the old value. Use mem::replace to avoid needless clones.

Avoid other needless work

Sometimes, adding a bit more lazyness can make a positive difference. For example, in the name of readability, people may convert this:

match my_option {
    Some(foo) => frobnicate(foo),
    None => calculate_default_frob(),

to my_option.map_or(calculate_default_frob(), frobnicate), which calculates the default frob even when there is a foo. Using my_option.map_or_else(calculate_default_frob, frobnicate) solves this particular case. Note that while I wrote the fn names directly, sometime you need a closure to capture some arguments (e.g. on Result::ok_or_else(..)) or to auto-dereference them (e.g. &Boxes will be dereferenced to refs to their contents). The easiest way then is to always use a closure and let clippy tell you when to remove it.

Also Rust uses LLVM, which performs dead-store analysis, which may optimize away the needless default calculation. However, this analysis doesn’t always work, so as always measure the effect.


That’s all I have for now. Do you have navigated other pitfalls? Discuss on /r/rust or rust-users!