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Fri, 20 Apr 2007

Power-of-two allocators.


Eric Dumazet has rised an interesting question about existing power-of-two allocator related to no-mmu implementation of the - is it possible to allocate higher-order page and then return part of it as unused (for example if someone has allocated 10-order page and then return 8-order part).
As far as I can tell (I'm not absolutely sure though) it is impossible with SLAB one - each page can only be 'split' into the same-sized parts, so either 10-order, or two 9-order, or 4 8-order, but not one 8-order and one 10-order minus 8-order.
That was one of the reasons I created network allocator, which I proved does fix such power-of-two overhead in the single page, i.e. blocks are combined when freed to form bigger one, and it is possible to allocate exactly requested block not aligned to power-of-two boundary.
But my allocator did not get enough attention (did I say that already for something unrelated? :), so it was a bit postponed.
Let's see, if there will be some interesting suggestions in the thread.

Update: David Howells of RedHat seems to be sure, that it is possible to allocate an order-10 page, then release part of it (say an order-8 subpage). But from the whole thread it seems that he says about no-mmu case, which can work on top of SLOB allocator in some embedded system.

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