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A lot of the memory management in Plash is done using regions. Regions work like this:
You create a region. This allocates a medium-sized block (eg. 1k) initially.
You allocate blocks from the region.
You free the whole region. This frees all the blocks that were allocated from it.
Regions don't provide a way to free individual blocks in the region. This means that allocation is fast, because there's no fragmentation within a region. Deallocation is fast too, because all the blocks are freed in one step.
The main reason for using regions in Plash is convenience. Deallocation becomes much less of a burden, and is easier to get right, so the chances of memory leaks are reduced. If a complex structure is region-allocated, you don't need to traverse the structure to free each node individually.
Using regions works well when allocation and deallocation follow the structure of the function call tree: when a structure is not used outside of the function that allocates it or the parent of the function that allocates it. When this is not the case, and the amounts of storage allocated are large, regions are not so good.
Plash uses reference counting for storage management when regions are not appropriate.
It is possible to attach explicit finalisers to a region, so that when the region is free, other resources are freed, eg. file descriptors can be closed.