FWIW, if you aren’t interleaving other allocations (which includes on other threads), the ArenaAllocator in Zig doesn’t have this problem. If you attempt to resize the most recent allocation, it will do so in place if possible.
All of my "arenas" have an additional fixed-length list of function pointers that they call in sequence before resetting/de-allocating the memory. That way they can manage any form of memory (or non-memory resource) you want:
char *dat = malloc(42);
arena_push_dtor(ar, dat, free);
// use dat
Neatly solves the problem of stuff that's too awkward to put in linear memory while still letting you be lazy about cleanup.
Also: if you don't need the contiguity you can simply break up your dynamic array into linked buckets the same way the arena internally does with its own memory. Iteration and random access will still be fast.
This article (and the previous one) is a little weird, because nowhere in either of these is a discussion about why you would use an arena, and I'm not certain the author understands that very fundamental concept.
You use an arena when you have computation that might consume a bunch of scratch memory, and then you want to free all of that memory at once when the computation is done. It's basically dynamically-scoped memory, very similar to stack-allocating a big structure and letting the stack pointer free it, but without the requirement that you know the size of all the memory you're allocating at compile time, or that allocations follow a strict LIFO ordering. They can be very fast for the same reason that stack allocation and copying GC is very fast: it's just bumping an allocation pointer. But unlike copying GC, deallocation is fast too, because you just free the whole arena at once.
free() being a no-op with arenas follows naturally from that, and is basically the whole point.
But ArrayLists also not playing nicely also follows naturally from it. An ArrayList (or Vector) has its own dynamic memory management; it transparently reallocates when you run out of space. This is useful for convenience, but if you're aiming for performance - which is basically the main reason to use an Arena - you want to be a little bit more careful about copies and allocations.
Typically, the common pattern for arena-based allocation is that you have one pass to plan out & compute where everything goes (where you're allocating just metadata, and metadata is storing pointers, sizes, and lengths of the actual data), and then you do one pass to allocate and write out the output data structure. That way everything is copied no more than once: you copy and compute your results and write them directly to the output data structure, then free the arena with all the scratch work all at once. Object serialization is the canonical example, and indeed common serialization libraries like Protobufs or Apache Arrow are big users of arenas. But if you're just accumulating things in an ArrayList and letting it automatically resize, you're doing it wrong.
> performance - which is basically the main reason to use an Arena
I'd argue the main reason Zig/C programmers use arenas is for correctness, not performance. You might think of arenas as a performance thing if you consider the alternative to be a GC, but the alternative in Zig/C is usually to do things manually.
> I know this is obvious, but I never actually thought about it. I'm probably not the only one.
It's pretty obvious, and I have thought about it, but I bet this is the sort of thing that I would reach for, forget about and lose some amount of time chasing down, so it's nice to have periodic reminders.
As an aside: sometimes, a linked list is the right datastructure.
https://ziglang.org/documentation/0.16.0/std/#std.heap.Arena...
Also: if you don't need the contiguity you can simply break up your dynamic array into linked buckets the same way the arena internally does with its own memory. Iteration and random access will still be fast.
You use an arena when you have computation that might consume a bunch of scratch memory, and then you want to free all of that memory at once when the computation is done. It's basically dynamically-scoped memory, very similar to stack-allocating a big structure and letting the stack pointer free it, but without the requirement that you know the size of all the memory you're allocating at compile time, or that allocations follow a strict LIFO ordering. They can be very fast for the same reason that stack allocation and copying GC is very fast: it's just bumping an allocation pointer. But unlike copying GC, deallocation is fast too, because you just free the whole arena at once.
free() being a no-op with arenas follows naturally from that, and is basically the whole point.
But ArrayLists also not playing nicely also follows naturally from it. An ArrayList (or Vector) has its own dynamic memory management; it transparently reallocates when you run out of space. This is useful for convenience, but if you're aiming for performance - which is basically the main reason to use an Arena - you want to be a little bit more careful about copies and allocations.
Typically, the common pattern for arena-based allocation is that you have one pass to plan out & compute where everything goes (where you're allocating just metadata, and metadata is storing pointers, sizes, and lengths of the actual data), and then you do one pass to allocate and write out the output data structure. That way everything is copied no more than once: you copy and compute your results and write them directly to the output data structure, then free the arena with all the scratch work all at once. Object serialization is the canonical example, and indeed common serialization libraries like Protobufs or Apache Arrow are big users of arenas. But if you're just accumulating things in an ArrayList and letting it automatically resize, you're doing it wrong.
I'd argue the main reason Zig/C programmers use arenas is for correctness, not performance. You might think of arenas as a performance thing if you consider the alternative to be a GC, but the alternative in Zig/C is usually to do things manually.
It's pretty obvious, and I have thought about it, but I bet this is the sort of thing that I would reach for, forget about and lose some amount of time chasing down, so it's nice to have periodic reminders.
As an aside: sometimes, a linked list is the right datastructure.