3 ms·

Oh yeah, arena allocation is great. I've used it a few times and it's always a huge performance improvement. The requirements are that (i) the objects you're al

Oh yeah, arena allocation is great. I've used it a few times and it's always a huge performance improvement. The requirements are that (i) the objects you're allocating all have the same type, and (ii) they all have the same lifetime (i.e. it's okay not to free any of them while others still live).

knome 2y ago
Why would a rust arena allocator be required to have objects all of the same type in it? No C arena allocator would have such a restriction.
- Rusky 2y ago
  It wouldn't. For some reason in Rust people tend to apply the term "arena" to "vector + indices" as well as the proper definition, which does have that restriction.
o11c 2y ago
"All objects of the same(-sized) type" is typically a constraint of bitmap allocators, not arena allocators.
kragen 2y ago
your (i) is incorrect (arena allocators can allocate objects of different types, and indeed one of their strong points is that they don't suffer from fragmentation) and your (ii) doesn't make sense because not freeing an object is always okay, according to conventional programming language semantics anyway this particular arena allocator does allocate objects of different types, almost arbitrary types in fact: fn alloc_uninitialized<T>(&self) -> NonNull<T> { ... } the safety concern is that nothing outside the arena survive the arena if it contains a pointer into the arena. as the linked micro-book puts it, 'it shouldn't be possible for any arena-allocated [variable] to outlive the memory that it[s data]'s located on.'
- hansvm 2y ago
  > not freeing an object is always okay, according to conventional programming language semantics anyway Conventional semantics have to hit reality eventually. Long-running programs can't just alloc forever without free, hence an arena not being the only allocator in most long-running programs.
  - kragen 2y ago
    yes, this is certainly a weak point in conventional semantics, but not relevant to this issue, which is about ensuring no dangling pointers, not about memory leaks if you're running low on memory, switching to an arena allocator is probably a bad idea, because it will usually increase your memory requirements (by delaying deallocation longer), not decrease them

knome 2y ago

Why would a rust arena allocator be required to have objects all of the same type in it? No C arena allocator would have such a restriction.

Rusky 2y ago
It wouldn't. For some reason in Rust people tend to apply the term "arena" to "vector + indices" as well as the proper definition, which does have that restriction.

o11c 2y ago

"All objects of the same(-sized) type" is typically a constraint of bitmap allocators, not arena allocators.

kragen 2y ago

your (i) is incorrect (arena allocators can allocate objects of different types, and indeed one of their strong points is that they don't suffer from fragmentation) and your (ii) doesn't make sense because not freeing an object is always okay, according to conventional programming language semantics anyway this particular arena allocator does allocate objects of different types, almost arbitrary types in fact: fn alloc_uninitialized<T>(&self) -> NonNull<T> { ... } the safety concern is that nothing outside the arena survive the arena if it contains a pointer into the arena. as the linked micro-book puts it, 'it shouldn't be possible for any arena-allocated [variable] to outlive the memory that it[s data]'s located on.'

hansvm 2y ago
> not freeing an object is always okay, according to conventional programming language semantics anyway Conventional semantics have to hit reality eventually. Long-running programs can't just alloc forever without free, hence an arena not being the only allocator in most long-running programs.
- kragen 2y ago
  yes, this is certainly a weak point in conventional semantics, but not relevant to this issue, which is about ensuring no dangling pointers, not about memory leaks if you're running low on memory, switching to an arena allocator is probably a bad idea, because it will usually increase your memory requirements (by delaying deallocation longer), not decrease them