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Comment by masklinn | Hacker News Reader

> Where we are now is better

Not really. Lots of rust users come from high level languages so when people come in and complain about rust being unexpectedly slow allocation issues are in the top 5 at least. It’s incredible how shit the standard allocators are in almost all systems.

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They’re not. They are supposed to be general-purpose allocators with requirements that may not match your workload.

> They’re not.

Are too

> They are supposed to be general-purpose allocators with requirements that may not match your workload.

I don’t think they match any workload which exists anywhere anymore. Possibly their workload existed back in the 90s.

Jemalloc is also a general-purpose allocator, it‘s the one freebsd uses.

Isn’t this inevitable if the user allocates each time a struct/array is created, rather than allocating upfront?

Meh. Users not used to tracking allocations keep to the same regime (lots of allocations), but because the platform allocators are utterly terrible the allocation overhead is orders of magnitude higher than in even a relatively basic runtime, thus the program is dog slow.

This is not inevitable, platforms could provide allocators which are less awful. Obviously they can’t be as fast as specialised runtime facilities but when you see the gains many applications get by just swapping in jemalloc or some such…

I would simplify it and say it's an issue with developers who don't understand manual memory management (allocating memory pools up front is just 1 such strategy for manual memory management).

That’s not a simplification, that’s a misattribution.

It’s not the beginner’s fault that most system allocators suck so hard you must do your utmost to limit their use.

Otherwise known as Victim Blaming. C developers are that old guy who white knuckles everything and calls people weak for wanting help.

Many of the features in modern programs didn’t exist in the 90’s not because they hadn’t been thought of but because people spent all their energy getting the first fifty features to work reliably. Every app had a couple cool features. Most of them are de rigeur now because we can.

This is akin to the idea that a compiler will eventually solve performance problems.

We're still waiting on that to happen.

There's a difference between a developer who understands manual memory management choosing to use a tech that does it for you and a developer choosing to use a tech that does it for you so they don't have to learn manual memory management.

manual memory management is table stakes for any halfway competent developer.

Rust used to use jemalloc, which was arguably the better default. For some reason they switched it to the system allocator when they added support for specifying the global allocator

jemalloc was not the default on every platform, but was on some. It adds a non-trivial amount of binary size to every program, even ones that don't need the performance boost, and that is an area Rust is often criticized vs C.

That said, there are a few reasons beyond just that. For the primary sources: https://github.com/rust-lang/rust/issues/36963

> the platform allocators are utterly terrible the allocation overhead is orders of magnitude higher than in even a relatively basic runtime, thus the program is dog slow

Huh. I’ve seen lots of people write slow Rust code because they didn’t realize there were allocations. But the complaint is usually “my rust program is only 5x faster than my equivalent python program instead of 100x how come”?

Can you point out some good blog posts about Rust or C++ being slower than a “basic runtime” due to inferior allocator?

If rust isn’t doing escape analysis for stack vs heap allocation then what is even the fucking point of this language? I would have thought that was the first thing implemented.

Rust doesn’t allocate anything on the heap unless you tell it to. When you tell it too, it puts it on the heap. The target use case is as a systems language.

Escape analysis as you’re alluding to isn’t needed in this model because the amount of times this helps you (ie you put it on the heap but the compiler can figure out it can live on the stack) is about 0. You need escape analysis in managed memory languages where everything is nominally a heap allocation and the compiler is responsible for clawing back performance through escape analysis.

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