Correct way to use arena allocators?

[lethal whale]

Hi, 0.5x dev here. Just wanted to hear some human feedback on the "zig" way of handling arenas and allocators. So far I have just been defining a bunch of arenas, each one corresponding to lifetimes of data so that I can reset them granularly. Also, if I pass an arena allocator into an ArrayList, do I ever need to call ArrayList.deinit()? Is this correct? ```zig pub var respArena: std.heap.ArenaAllocator = std.heap.ArenaAllocator.init(std.heap.page_allocator);
pub const respAllocator: std.mem.Allocator = respArena.allocator();

pub var persistentArena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
pub const persistentAllocator = persistentArena.allocator();

pub var browserArena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
pub const browserAllocator = browserArena.allocator();

[pastel basin]

Yes you are thinking correctly. Arenas are really good at pinning a lifetime of complex nested datastructures for example.

I wouldnt back the arenas with page allocator though unless you preheat them

[lethal whale]

-So the answer is no, you don't need to deinit() the data structure if you reset the arena elsewhere?
-I thought backing up with the page allocator just means it mmaps (capacity % page) pages

[pastel basin]

No indeed, if your arrays contain things that hold OS resources however then those ofc need to be deinit

but if it's only memory, once the arena is gone all the memory it held is gone

[pastel basin]

yes page allocator does that, but since it always gets pages of memory, your allocations will be fragmented and far away, unless you preheat your arenas first

arenas will eventually defragment themselves if you use reset, but in general it's better back them up with general purpose allocator instead of page