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I initially read your username as boehm, and I was like wow, ok, this is a guy who knows his memory. :)

What situations would an arena allocator prove problematic or non-optimal, aside from the many allocations/deallocations scenario?

This is an area I'm very interested in, so any info would be appreciated.

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In general, everything allocated within an arena has its lifetime tied to that arena. In lots of situations this is a fine or even desirable property (e.g., a single request context in a server application), but can be a tough restriction to work with in situations where you need fine-grained deallocations and possibly want to reuse freed space. The lifetime property can also be a pain to work with in multithreaded scenarios, where you might have multiple threads needing to access data stored in a single arena. Another situation that comes to mind is large long-lived allocations where you might want to have some manual defragmentation in place for performance reasons.

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