3 ms·

I'm a little bit confused. Every object that gets allocated has to be freed at some point. That cost is the same no matter how the memory is managed. Are you t

by wycats 12y ago

I'm a little bit confused. Every object that gets allocated has to be freed at some point. That cost is the same no matter how the memory is managed. Are you talking about an additional cost beyond "freeing memory that was allocated"?

krig 12y ago
It sounds like they are talking about a semi-space collector, which I can imagine might in theory be faster than scoped allocation since allocation is a simple pointer bump (just like stack allocation) but when garbage collecting you only have to traverse the live set. However, semi-space collectors can't collect non-memory resources without additional metadata, and they have a memory overhead of at least 2x the maximum live set. I have a hard time imagining even a semispace collector outperforming stack allocation. Of course if your program is short-lived you can just allocate a sufficiently large heap, pointer bump the whole way and free all non-memory resources on program termination, which ought to be cheaper (but probably not that much in practice due to cache effects).
- wycats 12y ago
  In Rust, you often use stack allocation, which, as you pointed out, is much more efficient. There's some heap allocation in Rust, but in my experience it's actually a minority of all allocations. Rust also has an Arena library (http://doc.rust-lang.org/0.11.0/arena/index.html http://doc.rust-lang.org/0.11.0/arena/index.html), which can be used for cases where you know that you have enough memory for a given operation and are willing to throw out the entire thing when the operation (or program ;) ) is done.
  - spullara 12y ago
    Arena allocation has similar efficiencies as GC.
    - pcwalton 12y ago
      Except for not having the mark phase (also not having to copy). The mark phase can take a lot of time.
  - eddyb 12y ago
    That link is outdated, see http://doc.rust-lang.org/arena/ http://doc.rust-lang.org/arena/.
viraptor 12y ago
Not necessarily the same. For example for arena allocation, you can drop lots of related objects at the same time. If you don't have to run destructors on them, the cost will be much lower than any other way of managing them.

by wycats 12y ago

krig 12y ago

It sounds like they are talking about a semi-space collector, which I can imagine might in theory be faster than scoped allocation since allocation is a simple pointer bump (just like stack allocation) but when garbage collecting you only have to traverse the live set. However, semi-space collectors can't collect non-memory resources without additional metadata, and they have a memory overhead of at least 2x the maximum live set. I have a hard time imagining even a semispace collector outperforming stack allocation. Of course if your program is short-lived you can just allocate a sufficiently large heap, pointer bump the whole way and free all non-memory resources on program termination, which ought to be cheaper (but probably not that much in practice due to cache effects).

wycats 12y ago
In Rust, you often use stack allocation, which, as you pointed out, is much more efficient. There's some heap allocation in Rust, but in my experience it's actually a minority of all allocations. Rust also has an Arena library (http://doc.rust-lang.org/0.11.0/arena/index.html http://doc.rust-lang.org/0.11.0/arena/index.html), which can be used for cases where you know that you have enough memory for a given operation and are willing to throw out the entire thing when the operation (or program ;) ) is done.
- spullara 12y ago
  Arena allocation has similar efficiencies as GC.
  - pcwalton 12y ago
    Except for not having the mark phase (also not having to copy). The mark phase can take a lot of time.
- eddyb 12y ago
  That link is outdated, see http://doc.rust-lang.org/arena/ http://doc.rust-lang.org/arena/.

viraptor 12y ago

Not necessarily the same. For example for arena allocation, you can drop lots of related objects at the same time. If you don't have to run destructors on them, the cost will be much lower than any other way of managing them.