bumpalo VS wg-allocators

Long story short, heap allocation is painfully slow. Any sort of malloc will always be slower than a custom pool or a bump allocator, because it has a lot more context to deal with.

Rust makes it especially hard to use custom allocators, see bumpalo for example [0]. To be fair, progress is being made in this area [1].

Theoretically one can use a "handle table" as a replacement for pools, you can find relevant discussion at [2].

[0] https://github.com/fitzgen/bumpalo

There are ways to accomplish this as well. Different allocator libraries exist for this kind of scenario, namely bumpallo which allocates a larger block of memory from the kernel, and allocates quickly thereafter. That would amortize the cost of memory allocations in the way I think you're after?

Meaning, storing a lot of things in the same block of allocated memory? Vec is a thing, you know. There's also a bump allocator library.

There are data structures like slotmap, and relatively low-level crates like bumpalo. This is not to say that either fits your use case, just that you definitely have access to the necessary parts to fit what you describe.

Sometimes you can put everything in a bump allocator, then when you're done, free the entire bump allocator in one go. https://docs.rs/bumpalo/

This is useful with crates like bumpalo which give you bump-allocation arenas whose lifetimes are tied to the objects they allocate.

TSC doesn't need to "stick around", right? Just a run-once and the program is over?

In those cases, https://github.com/fitzgen/bumpalo works amazingly as an arena. You can pretty much forget about reference counting and have direct references everywhere in your graph. The disadvantage is that it's hard to modify your tree without leaving memory around.

We use it extensively in http://github.com/dioxusLabs/dioxus and don't need to worry about Rc anywhere in the graph/diffing code.

Probably bumpalo, but then its Box will have a lifetime parameter - bumpalo::boxed::Box<'a, dyn MyTrait>

Strictly speaking, Rust doesn't need this as a built-in language feature, because its design allows it to be implemented as a third-party library: https://docs.rs/bumpalo

The biggest problem is that there's some awkwardness around RAII; I'm not sure whether that could have been avoided with a different approach.

Of course, ideally you'd want it to be compatible with the standard-library APIs that allocate. This is implemented, but is not yet at the point where they're sure they won't want to make backwards-incompatible changes to it, so you can only use it on nightly. https://doc.rust-lang.org/stable/std/alloc/trait.Allocator.h...

Or are you suggesting that the choice of allocator should be dynamically scoped, so that allocations that occur while the bump allocator is alive automatically use it even if they're in code that doesn't know about it? I think it's not possible for that to be memory-safe; all allocations using the bump allocator need to know about its lifetime, so that they can be sure not to outlive it, which would cause use-after-free bugs. I'm assuming that Odin just makes the programmer responsible for this, and if they get it wrong then memory corruption might occur; for a memory-safe language like Rust, that's not acceptable.

Tracking issue for Storages, and a TLDR on what it is

I must have gotten confused, since from your brief discussion with CAD97 it seemed like there was a way for the concepts to live separately and that Storage could complicate things in comparison. But if implementing Allocator in terms of Storage is basically equivalent and Storage is flexible enough that I could write one to pass memory out to unsafe code, that works just as well.

https://rust-lang.github.io/rfcs/1974-global-allocators.html was the original RFC.

My vague understanding is that there's a working group https://github.com/rust-lang/wg-allocators

The further I get from working on Rust day to day, the less I know about these things, so that's all I've got for you.

If you self-reference using pointers and guarantee the struct will never move, you don't even need unsafe. If you self-reference using offsets from the struct's base pointer, you need a splash of unsafe but your struct can be freely moved without invalidating its self-referential "pointers".

Per-struct allocators are a work in progress (see https://github.com/rust-lang/wg-allocators/issues/48).

Not sure what "non thread local addresses" means, but in my experience Rust is pretty good at sending data between threads (without moving it).

Besides that allocation is not really a problem for no_std. It's resolved by using alloc crate directly, so anything usable with custom allocators is supported. Example in dasp sources - https://github.com/RustAudio/dasp/blob/master/dasp_slice/src/boxed.rs#L14-L19 . Also worth looking at this issue to check what is usable already - https://github.com/rust-lang/wg-allocators/issues/7

But that's on track for rust as well: https://github.com/rust-lang/wg-allocators/issues/7

Here: https://github.com/rust-lang/rfcs/pull/1398. there is also a working group for this: https://github.com/rust-lang/wg-allocators.

It's unstable. wg-allocators contains discussions about design and a tracking issue for collections that need an allocator https://github.com/rust-lang/wg-allocators/issues/7