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I really don't understand this article, and the claims really rub me the wrong way. The main point it makes is, again "He perfectly demonstrates one of the poi

I really don't understand this article, and the claims really rub me the wrong way. The main point it makes is, again "He perfectly demonstrates one of the points my “Oxidizing” article was making: with Rust and WebAssembly we have reliable performance without the wizard-level shenanigans that are required to get the same performance in JavaScript." This doesn't make a lot of sense as a claim. Why? Because underneath all that rust .... is an optimizing compiler, and it happens the author has decided to stay on the happy path of that. There is also an unhappy path there. Is that happy path wider? Maybe. It's a significantly longer and more complex optimization pipeline just to wasm output, let alone the interpretation of that output. I have doubts it's as "reliable" as the author claims (among other things, WebAssembly is still an experimental target for LLVM). Adding the adjective "reliable" repeatedly does not make it so. Let's ignore this though, because there are easier claims to pick a bone with. It also tries to differentiate optimizations between the two in ways that don't make sense to me: "In some cases, JITs can optimize away such allocations, but (once again) that depends on unreliable heuristics, and JIT engines vary in their effectiveness at removing the allocations." I don't see a guarantee in the rust language spec that these allocations will be optimized away. Maybe i missed it. Pointers welcome. Instead, i have watched plenty of patches to LLVM go by to try to improve it's heuristics (oh god, there's that evil word they used above!) for removing allocations for rust. They are all heuristic based, they deliberately do not guarantee attempting to remove every allocation (for a variety of reasons). In general, it can be proven this is a statically undecidable problem for a language like rust (and most languages), so i doubt rustc has it down either (though i'm sure it does a great job in general!) The author also writes the following: "WebAssembly is designed to perform well without relying on heuristic-based optimizations, avoiding the performance cliffs that come if code doesn’t meet those heuristics. It is expected that the compiler emitting the WebAssembly (in this case rustc and LLVM) already has sophisticated optimization infrastructure," These two sentences literally do not make sense together. The "sophisticated optimization infrastructure" is also using heuristics to avoid expensive compilation times, pretty much all over the place. LLVM included. Even in basic analysis, where it still depends on quadratic algorithms in basic things. If you have a block with 99 stores, and ask LLVM's memory dependence analysis about the dependency between the first and the last, you will get a real answer. If you have 100 stores, it will tell you it has no idea. What happened to reliable? Why does this matter? For example: Every time rust emits a memcpy (which is not infrequent), if there are more than 99 instructions in between them in the same block, it will not eliminate it, even if it could. Whoops. Thats' a random example. These things are endless. Because compilers make tradeoffs (and because LLVM has some infrastructure that badly needs rewriting/reworking). These "sophisticated optimization infrastructures" are not different than JITs in their use of heuristics. They often use the same algorithms. The only difference is the time budget allocated to them and how expensive the heuristics let things get. There may be good reasons to want to write code in rust and good reasons to believe it will perform better, but they certainly are not the things mentioned above. Maybe what the author really wants to say is "we expect the ahead of time compiler we use is better and more mature than most JITs and can spend more time optimizing". But they don't. Maybe it would also surprise the author to learn that their are JITs that beat the pants off LLVM AOT for dynamic languages like javascript (they just don't happen to be integrated into web browsers). But instead, they make ridiculous claims about heuristics and JITs. Pretending the compiler they use doesn't also depend, all over the place, on heuristics and other things is just flat out wrong. At least to me (and i don't really give a crap about what programming language people use), it makes it come off as rampant fanboism. (Which is sad, because i suspect, had it been written less so, it might be actually convincing)

tmandry 9y ago
To address your question about allocations, in Rust, you always know when you are allocating on the heap vs on the stack. The way you write your code guarantees it. Stack allocations are “basically free” compared to the heap because the memory management overhead is negligible or nonexistent. There are also guarantees about when objects you allocate on the heap are freed; there is no garbage collection. This is in contrast to JavaScript where objects are always allocated on the heap, and are garbage collected. So Rust gives you a lot more control and flexibility about how memory is managed. This might or might not matter for your use case, of course. There are compiler optimizations and heuristics -on top of that-, to be sure, but you end up with a lot more guarantees about how your code executes, because that’s what the language is designed for. EDIT: If you want to learn more about memory management in Rust, see https://doc.rust-lang.org/book/first-edition/the-stack-and-the-heap.html https://doc.rust-lang.org/book/first-edition/the-stack-and-t...
- DannyBee 9y ago
  "here are also guarantees about when objects you allocate on the heap are freed; there is no garbage collection. This is in contrast to JavaScript where objects are always allocated on the heap, and are garbage collected." Surely you realize that this, and what the author wrote, are basically the same ever rehashed GC vs non-GC language discussion. Performance characteristics of each is not anywhere near as simple as any of these things claim, so i'm going to leave this alone. "EDIT: If you want to learn more about memory management in Rust, see https://doc.rust-lang.org/book/first-edition/the-stack-and-t..." https://doc.rust-lang.org/book/first-edition/the-stack-and-t... Again, i see no guarantees here. I see it saying things like "This program has one variable binding, x. This memory needs to be allocated from somewhere. Rust ‘stack allocates’ by default, which means that basic values ‘go on the stack’." Can someone point me to an actual language level guarantee in a spec? I looked, and i don't see it. I can't see anything that makes it non-conformant to build a rust compiler that dynamically allocates and places all of these on the heap, and is very non-constant time for local variable allocation. But again, i didn't spend more than a few minutes browsing, so i may have missed it (For example, https://doc.rust-lang.org/reference/memory-allocation-and-lifetime.html says nothing here, the requirements i can find in this entire chapter can easily be met by using dynamic allocation. There are not guarantees on space or time usage that would require a real stack :P). In fact, the vast majority of requirements here look like they could be met by a garbage collected heap/stack. It would be horribly inefficient, but ... I'm totally willing to believe nobody has written down a good enough spec yet, just saying i don't see it ATM :) I want to strongly differentiate between what "one implementation of rust does" and what "the language guarantees". Because if you are going to claim it's rust that makes the guarantee, as the author did, you should be able to back the claim up.

tmandry 9y ago

To address your question about allocations, in Rust, you always know when you are allocating on the heap vs on the stack. The way you write your code guarantees it. Stack allocations are “basically free” compared to the heap because the memory management overhead is negligible or nonexistent. There are also guarantees about when objects you allocate on the heap are freed; there is no garbage collection. This is in contrast to JavaScript where objects are always allocated on the heap, and are garbage collected. So Rust gives you a lot more control and flexibility about how memory is managed. This might or might not matter for your use case, of course. There are compiler optimizations and heuristics -on top of that-, to be sure, but you end up with a lot more guarantees about how your code executes, because that’s what the language is designed for. EDIT: If you want to learn more about memory management in Rust, see https://doc.rust-lang.org/book/first-edition/the-stack-and-the-heap.html https://doc.rust-lang.org/book/first-edition/the-stack-and-t...

DannyBee 9y ago
"here are also guarantees about when objects you allocate on the heap are freed; there is no garbage collection. This is in contrast to JavaScript where objects are always allocated on the heap, and are garbage collected." Surely you realize that this, and what the author wrote, are basically the same ever rehashed GC vs non-GC language discussion. Performance characteristics of each is not anywhere near as simple as any of these things claim, so i'm going to leave this alone. "EDIT: If you want to learn more about memory management in Rust, see https://doc.rust-lang.org/book/first-edition/the-stack-and-t..." https://doc.rust-lang.org/book/first-edition/the-stack-and-t... Again, i see no guarantees here. I see it saying things like "This program has one variable binding, x. This memory needs to be allocated from somewhere. Rust ‘stack allocates’ by default, which means that basic values ‘go on the stack’." Can someone point me to an actual language level guarantee in a spec? I looked, and i don't see it. I can't see anything that makes it non-conformant to build a rust compiler that dynamically allocates and places all of these on the heap, and is very non-constant time for local variable allocation. But again, i didn't spend more than a few minutes browsing, so i may have missed it (For example, https://doc.rust-lang.org/reference/memory-allocation-and-lifetime.html says nothing here, the requirements i can find in this entire chapter can easily be met by using dynamic allocation. There are not guarantees on space or time usage that would require a real stack :P). In fact, the vast majority of requirements here look like they could be met by a garbage collected heap/stack. It would be horribly inefficient, but ... I'm totally willing to believe nobody has written down a good enough spec yet, just saying i don't see it ATM :) I want to strongly differentiate between what "one implementation of rust does" and what "the language guarantees". Because if you are going to claim it's rust that makes the guarantee, as the author did, you should be able to back the claim up.