How does a fixed-cell object pool reclaim memory differently from an arena reset at a phase boundary?

## Two different reclamation contracts Both designs exist to stop paying for individual releases, but they stop paying in opposite ways. - An **arena** removes the per-object release entirely and substitutes one bulk release at a phase boundary. Reclamation is **temporal**: it happens when the phase ends, for everything at once. - A **pool** keeps the per-object return but makes it trivial: the returned thing is not released to anything, it goes back on a free set of identical cells and is lent out again. Reclamation is **circulatory**: memory is not given up, it is reused. That is why a pool needs its cells to be interchangeable. If every cell is the same size and shape, any free cell satisfies any request with no fitting decision — take the head of the free set and return. Mixed sizes reintroduce the search the design was meant to avoid. ## What a borrow and a return actually do 1. `acquire` removes a cell from the free set and hands it to the caller. If the free set is empty, the pool either creates a cell (a pool that may grow), makes the caller wait, or fails the request outright — and which of those it does is a design decision, not a detail. 2. The caller uses the cell for as long as it needs, holding the only handle to it by convention. 3. `release` puts the cell back on the free set, at which point the caller must stop using it. A pool that is allowed to grow allocates only when every cell is out on loan simultaneously, so in steady state the allocation path costs a list operation and nothing else. ## Side by side | | Fixed-cell pool | Phase-reset arena | |---|---|---| | Unit of reuse | One cell, returned individually | The whole region, at the boundary | | Object shapes | One shape, interchangeable | Anything, any size | | Reclamation event | Every `release` | Only the reset | | Needs an explicit return | Yes — a missed one is a leak | No | | State of what you get | Whatever the last user left | Fresh, never handed out before | | Footprint driver | Peak simultaneous borrowers | Total allocated in one phase | | Natural fit | Long-running steady state | Work with a real boundary | ## The obligations each creates The pool's obligation is the **return**. Every acquire must be matched, on every path including the failure paths, or the cell is gone for good: it is reachable from the borrower, it is not on the free set, and there is no boundary event that sweeps it up. A pool therefore reintroduces exactly the bug the arena abolished, in a smaller and more visible form — the pool drains, and the symptom is that borrowers start waiting or failing rather than that memory climbs. The pool's second obligation is **state**. A cell handed to a new borrower carries whatever the previous one left in it, so correctness now depends on somebody clearing it. That hazard is large enough to be worth treating on its own. The arena's obligation is the opposite shape: nothing to return, but nothing may **outlive** the reset, and the whole phase's allocation is held until then. ## Choosing between them Ask two questions: - **Is there a real boundary?** If the work has a point at which everything scratch can be dropped — a response written, a frame presented, a batch committed — the arena's one-reset release is available and costs almost nothing. - **Is there a bounded number of simultaneous users of one expensive shape?** If the costly thing is creating the object rather than the memory itself, and only so many are ever in flight, the pool's circulation is the better answer, and the pool's size becomes a capacity decision. The two are not exclusive. A service can pool the small number of large, expensive buffers it hands to its I/O layer, and give each request an arena for the many small scratch objects it builds and drops. What they must not do is share a discipline: an object taken from a pool follows the return rule, and an object taken from an arena follows the boundary rule, and mixing the two is how pointers into a reset region end up on a pool's free set.