Monday, December 19, 2011

Using partitions as a free list

Yesterday I discussed my search for some help with implementing a free list in RTEMS. Today I'm glad to say I was able to use partitions to achieve my objective of a growable free list—something akin to a slab allocator. A fundamental limit, the maximum number of partitions, still exists but partitions can have an arbitrary size; these limits might be circumvented safely by making each partition much larger than the previous or by combining old partitions into new ones. Further improvements can be made, but for now I'm satisfied.

Iterate over available partitions until one returns a free object. If none succeeds, create a new partition (malloc a new set of objects) and get a free object from the new partition.

I have not handled what to do if partition creation fails. Performance could be improved by tracking the non-full partitions.

Return the object to its partition. I made this easy by storing a handle to the owning partition as part of each object. Non-invasive techniques, such as tracking address ranges for each partition, could be used for a generic solution.

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