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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

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.

Locke1689 10y ago
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.