This article is about 10 times longer than it needs to be. Also no need for the dripping condescension. I’m even among the most receptive to what he’s trying to say, but got exhausted after about 20% of the article and had to give up.
Also the author's condescension about automatic memory management is... telling. Remembering to call "free" is only one small part of why tools like RAII are good, and arena allocators do not help with the other parts.
The overall goal is to be able to write correct, reliable, performant software. Arena allocators help prevent missed or double frees, but they don't help with the other memory safety issues. For example, sometimes objects in different arenas need to reference each other and C does not help prevent you from accessing those references after one of the arenas has been freed.
On top of that there are general issues with arena allocators:
1. They can be inefficient for resizable collections where you don't know the total length in advance. The multiple re-allocations results in a lot of wasted space in the arena.
2. In C, it's implicit which objects should belong to which arena.
3. It's not composable. A library doesn't necessarily know which objects should use which arena, or may not even support arena allocation at all.
4. There are other resources than just memory. There's a reason it's called RAII and not MAII - because it allows you to clean up all kinds of resources (eg. file handles) when an arena allocator doesn't typically support any kind of destructor.
5. It requires more thought to structure your program in this way. For some programs that may be effort well spent, but a lot of programs are not bound by allocation performance, and for those programs not thinking about allocation at all leaves more time for thinking about correctness in other aspects.
6. Languages which do automatic memory management can still support arenas, and may offer additional benefits when they do (eg. Rust's explicit lifetimes can tie an object to the arena it came from).
7. The stack and heap are global resources that most code can simply assume exist, and use with no extra ceremony. When you have arenas in play, these need to be passed as additional arguments. Adding a new allocation to a function can require sweeping changes to add a new arena parameter to every function above it in the call stack.
4. Nothing prevents arena allocators to track other resources than memory.
7. This is not a big problem in practice from my experience. You can just do like Zig and always pass an allocator or you can use global/thread local variables like PostgreSQL.
But I also would like to add my biggest issue with arena allocators: they do nothing against use after free.
> 6. Languages which do automatic memory management can still support arenas, and may offer additional benefits when they do (eg. Rust's explicit lifetimes can tie an object to the arena it came from).
Languages with automatic memory management don't expose the arenas as an implementation detail, and so they are free to work out how to compose them, and when to change their minds.
Java has had copying collectors practically from the beginning, and that ability to rewrite pointers after the fact affords you the ability to work with heuristics that have known failure modes, where any mistake becomes a use-after-free bug. Moving an object out of a volatile arena is just an inconvenience. So it evolved from generational collectors, to thread local arenas, to thread local arenas where some objects that are likely to survive are never allocated into the arena but 'pre-tenured'. This process really hit its stride with the introduction of escape analysis, which is a logical ancestor of today's borrow checker. With escape analysis you know for certain some objects never escape, in which case you can even inline simple objects (eg, register allocation). And if an arena has an escape probability of 0, there's less guard logic to deal with, and there is some short circuiting one can do when considering the root set for Mark and Sweep.
You might prefer a public, conversational setting at the conference instead of a personal blog post. That said the article made useful technical points. And he wasn't hurling expletives or denigrating any individual.
I hope you can appreciate that there’s room between “technically correct” and “hurling expletives” for some someone to simply dislike an article. I regret my response struck a nerve—it certainly was not meant as an attack on its author.
again: nothing like this was ever presented to us, except as some sage wisdom from upperclassmen who had figured it out on their own. you learn malloc/free when you learn C, then you learn new/delete/RAII when you learn C++, and you get a brief overview of how garbage collection broadly works in higher-level languages, and that's it, that's the education about memory management. I had never explicitly heard of the concept of a "lifetime" until Rust started gaining traction. it wasn't until working on my sophomore year game engine that an upperclassman explained the idea of building a memory-managing array-like data structure that, instead of reallocating when the capacity is reached, instead leaves the existing items where they were and allocates a new "page" of memory, so as to ensure pointers to existing items remain functional.
hell, I'd never even been exposed to the idea of a per-frame "scratch" pool allocator until I saw Jon Blow talk about having such a thing as a built-in feature of his new programming language. once I saw how it worked, I was floored—it's such an easy-to-explain concept, infinitely useful, basically gives you "garbage collection for free" for any number of one-off things you need allocations for in a given frame of a video game (or other interactive application)—without the need for Actual Garbage Collection!
I agree that this article is a bit lengthy, but in my opinion it does a great job of setting up the knowledge necessary to understand what it's talking about, so I'm not really sure how I'd do a better job. what in particular did you find condescending about it, especially within the first fifth of the article?
---
[0] from what I've heard, this from-scratch game-engine-creation is now gone from the curriculum entirely, and now students just make shit in unreal or unity, which is tragic, imo.
Here's that Jonathan Blow stream if anyone else is curious: https://www.youtube.com/watch?v=ciGQCP6HgqI&t=130s
It's impossible for me not to read this without imagining a sneering tone.
And a few paragraphs later:
"As you may be able to tell from my tone, I think this way of thinking is nonsensical."
As for your post, it's hard for me to get worked up about whatever the game programming curriculum is these days. There are more important things in the world. Good to keep things in perspective: it's school... for game programming.
Okay. Yeah, that doesn't seem great.
> There are more important things in the world. Good to keep things in perspective: it's school... for game programming.
Wait a second. Isn't your own statement kind of hard to read without a sneering tone?
In defense of videogames. It's only a massive industry that drives technological developments massively useful in other industries; while at the same time, providing entertainment, recreation, and escape for billions of people.
In defense of complaining about sneering tones while having one yourself: <space intentionally left blank>
> Education around memory management—as it was presented to me—was purely a historical, academic endeavor. How did the Linux kernel originally do memory management? Let’s do an assignment on that subject so you can see how gross it is! Eww, look at that malloc! Weird! Oh, don’t forget to free it! But don’t worry, kiddo; in next class, you can return to your “safe” and “managed” padded-room languages where bugs and instabilities are “impossible” (or so they claim).
was your CS education appreciably different to what the author wrote above? is this not the increasingly-prevailing attitude among contemporary programmers?
> As for your post, it's hard for me to get worked up about whatever the game programming curriculum is these days. There are more important things in the world. Good to keep things in perspective: it's school... for game programming.
ok, should I refrain from posting unless I have something to say about world hunger or climate change or something?? hopefully you saw the point I was making which is that memory management techniques like those described in the article are pretty crucial for making high-performance real-time interactive applications, and hopefully you understand that not being taught about such things at a school whose whole point is to teach such things is pretty crazy—especially since, from the looks of things, these things aren't really being taught in school anywhere else, either!
do you disagree with the author about the importance of manual memory management techniques and theory? if not, then what other point were you trying to make with that last paragraph? your reply is very confusing to me.
---
EDIT:
> If you slow down
> Maybe because it's a relatively inconsequential topic (in the grand scheme of things) we can approach it more calmly and soberly instead of with a hyperventilating tone.
> I would also say that while it can be difficult for some people to approach tendentious issues without being combative, it is indeed possible. Some might even say it's a useful skill to spend some time picking up.
these are more condescending than anything in the article—projecting irrationality onto others is unbecoming.
> That said, whether or not I agree with the author isn't salient
it should be! discussions about programming techniques and methodologies are far more interesting and relevant than sentence after sentence explaining why you think the tone of the introduction of a lengthy article is Bad And Wrong, and, despite refusing to read the rest of said article, claiming that said article's length is also problematic.
Manual memory management is definitely an interesting topic of discussion, with important ramifications in the field of software engineering. Like you said, world hunger and climate change it is not. Maybe because it's a relatively inconsequential topic (in the grand scheme of things) we can approach it more calmly and soberly instead of with a hyperventilating tone.
I would also say that while it can be difficult for some people to approach tendentious issues without being combative, it is indeed possible. Some might even say it's a useful skill to spend some time picking up.