Comment by mike_hearn | Hacker News Reader

Comment by mike_hearn | Hacker News Reader

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Arena allocation in C/C++ is doing something similar to generational GC. I was about to type "simulating it" but that's not quite right ... they exploit similar observations to similar effects in different ways.

In C/C++ you have to manually decide how short lived allocations are ahead of time and either stack or arena allocate them. Freeing all the allocations on the stack or the arena is then very fast.

In a generational GC the programmer doesn't annotate allocations like that ahead of time. Instead it is assumed that all allocations might be short lived. How long they actually live is then tracked and eventually they're moved out of the "arena" (young gen space) by copying them. But again, freeing the contents of the young gen is very fast because you don't actually touch dead data at all, you only copy the live objects.

I've heard of GCs that can actually do "pre-tenuring": when the VM knows an allocation will definitely live for a long time based on profiling data it can be allocated directly into the old gen space. That reduces your minor GC pause times further.

All this adds overhead of course, but makes automatic something C/C++ devs would do themselves. Given the potentially catastrophic security consequences of screwing up in the C/C++ model I've become a lot less sympathetic to that approach over the years. How much extra performance is a security advisory and possible resulting botnet worth to you?

Arenas have fast allocation (because you often have no freelist and just increment the end). They also allow you to quickly reclaim old data, which is important for a database because a query might run long enough for the allocated memory to be considered old (for one example). And they don't require compaction or copying, because objects of the same lifetime are physically near each other in memory.

It's fair to draw an analogy to generational GC, but they aren't the same.

Rust is an example of a language that can do arena allocation without the dangers of C++.

EDIT: Based on my experience with postgresql, and what I've learned about rust so far, I am starting to believe that object lifetime is a fundamental concept in CS. It's possible that sophisticated GCs have, to some degree, hidden the importance.

Arenas have fast allocation (because you often have no freelist and just increment the end). They also allow you to quickly reclaim old data, which is important for a database because a query might run long enough for the allocated memory to be considered old (for one example). And they don't require compaction or copying, because objects of the same lifetime are physically near each other in memory.

This is exactly how the Gen0 nursery works. The surviving objects get copied to Gen1 before the Gen0 bump pointer is reset. Of course, in C++ arenas there cannot be surviving objects or that is implemented manually.

The main difference here is that what is in the nursery is decided by the GC, whereas a manual arena requires you to pre-decide what is in the arena or not.

Arenas also have the benefit of incredibly quick allocation as well. Malloc/new can easily get you into performance trouble just as well as free.

WRT security Rust is my go-to these days(they also support both typed and un-typed arenas). Much better memory safety guarantees and C ABI makes embedding in a larger application straightforward.

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