Comment by hgs3

There is also the double-sided arena allocator which uses one contiguous buffer but grows in both directions (front-to-back and back-to-front). When allocating memory from it you need to indicate whether you want memory from the front or back. The allocator is out-of-memory when both font and back meet.

The double-sided approach is useful for various purposes. For instance you can allocate short-lived data from the front and long-lived data from the back. It also makes better use of free space: with two separate arena allocators one could be out-of-memory but the other might have free space. With the double-sided approach all memory is fair game.