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0 pointsbrundolf5y ago0 comments

This is one of the psychological traps I've experienced writing Rust: in Python or Java you can't hope to make things more efficient at a certain point. You couldn't get clever with string re-use if you tried, etc. So you're content to just move forward and be practical.

Whereas in Rust you know that it's probably possible to use a &str there instead of a String. If you just bang on it a little longer, you can make it just a bit more optimal. The clone()s and the Vec::new()s are all explicit, making them feel heavier than those other languages, when in reality they're still quite a bit lighter.

You usually don't have to optimize them out! Your code will probably be faster than Java even with a bunch of clones, etc! But there's this temptation that's really hard for programmers to resist, to sink hours and hours into making things just slightly more optimal given the opportunity. You have to consciously talk yourself out of that if you want to be productive.

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masklinn5y ago

> The clone()s and the Vec::new()s are all explicit, making them feel heavier than those other languages, when in reality they're still quite a bit lighter.

That's… not exactly true. `Vec::new` is completely free so that's a different debate, but much as in C++ or C allocations in Rust are much more expensive than in managed languages:

1. system allocators genuinely suck, all of them, though some more than others (iirc macos' is especially bad)

2. managed languages can much more easily specialised allocation strategies (freelists, bump allocators, type-custom allocation strategies), this is either difficult / impossible (no custom allocators) or way more painful (manually pass custom allocators in) in Rust

As a result, allocations in Rust are really tremendously slow by default, it's not too rare to see questions about Rust programs which are slower than managed equivalents, in release mode. Because the rust program is allocation heavy (relatively), it might have 10% the allocations of the non-rust program but the allocations are 100 times more expensive.

brundolfOP5y ago

Interesting, I hadn't heard this before. Do you have any resources I could read?

Edit: I do know that languages where strings are immutable use that fact to do lots of optimization (automatically sharing "copied" strings and just cloning reference-counters, for example), but you can accomplish some of this in Rust too with Rc<> or even persistent data structures if you really want to. And of course there are cases where it's more efficient to actually mutate a string, which these languages can't do. But it sounds like you're talking about something else?

estebank5y ago

There are three strategies that other languages use automatically that you can also apply in Rust by hand: string interning, small string optimization and "Copy-on-write". For the first and second, you can use some existing crate like https://docs.rs/string-interner/0.12.2/string_interner/ and https://github.com/rust-analyzer/smol_str. For the later, you can use Cow<'_, str> https://doc.rust-lang.org/std/borrow/enum.Cow.html. I'm having trouble thinking of a case where Rc<String> would be appropriate.

pdimitar5y ago

> You have to consciously talk yourself out of that if you want to be productive.

That's 100% correct but there's one problem: how do you write down your future improvement opportunities?

Skip 2 optimizations here, 3 optimizations there and after 30 PRs you can easily have notes spanning 100 bullet points on what to optimize one day in the future Soon™.

I am still not at what I'd call an expert level in Rust -- and the skill ceiling is quite high -- so maybe with time I'd start to automatically spot these optimization opportunities?

But before I am at that level I'd prefer to either keep hugely long notes or, ideally, have a tool recommend optimizations to me.

SAI_Peregrinus5y ago

The simplest tool is to just grep for various constructs that copy or allocate. Clippy, the Rust linter, will find some of these (as well as other issues, some of which tend to cause performance problems).

The better tool is to profile the code, then optimize hotspots. There's a Rust Performance Book with a list of profilers known to work with Rust programs[1]. Rustc does support Profile Guided Optimization[2] which is often pretty good at speeding up the output even without any code changes.

[1] https://nnethercote.github.io/perf-book/profiling.html [2] https://doc.rust-lang.org/rustc/profile-guided-optimization....

pdimitar5y ago

Helpful links, thank you!

> The simplest tool is to just grep for various constructs that copy or allocate.

Do you happen to have a curated list somewhere?

SAI_Peregrinus5y ago

For allocations (more expensive in many cases) the Rust container cheat sheet[1] is helpful. Box, Vec, Rc, Arc, and Mutex are the standard types which allocate.

Not sure of a list for things that copy their data. The Clone trait is the obvious one, and it requires calling clone() to make the copy. EG `val.clone()`. So searching for `.clone()` will get you those. But other things like to_string are expensive, and From/Into are sometimes expensive.

[1] https://docs.google.com/presentation/d/1q-c7UAyrUlM-eZyTo1pd...

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masklinn5y ago

> The clone()s and the Vec::new()s are all explicit, making them feel heavier than those other languages, when in reality they're still quite a bit lighter.

That's… not exactly true. `Vec::new` is completely free so that's a different debate, but much as in C++ or C allocations in Rust are much more expensive than in managed languages:

1. system allocators genuinely suck, all of them, though some more than others (iirc macos' is especially bad)

brundolfOP5y ago

Interesting, I hadn't heard this before. Do you have any resources I could read?

estebank5y ago

pdimitar5y ago

> You have to consciously talk yourself out of that if you want to be productive.

That's 100% correct but there's one problem: how do you write down your future improvement opportunities?

Skip 2 optimizations here, 3 optimizations there and after 30 PRs you can easily have notes spanning 100 bullet points on what to optimize one day in the future Soon™.

I am still not at what I'd call an expert level in Rust -- and the skill ceiling is quite high -- so maybe with time I'd start to automatically spot these optimization opportunities?

But before I am at that level I'd prefer to either keep hugely long notes or, ideally, have a tool recommend optimizations to me.

SAI_Peregrinus5y ago

[1] https://nnethercote.github.io/perf-book/profiling.html [2] https://doc.rust-lang.org/rustc/profile-guided-optimization....

pdimitar5y ago

Helpful links, thank you!

> The simplest tool is to just grep for various constructs that copy or allocate.

Do you happen to have a curated list somewhere?

SAI_Peregrinus5y ago

For allocations (more expensive in many cases) the Rust container cheat sheet[1] is helpful. Box, Vec, Rc, Arc, and Mutex are the standard types which allocate.

[1] https://docs.google.com/presentation/d/1q-c7UAyrUlM-eZyTo1pd...

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