| .. _z3fold: |
| |
| ====== |
| z3fold |
| ====== |
| |
| z3fold is a special purpose allocator for storing compressed pages. |
| It is designed to store up to three compressed pages per physical page. |
| It is a zbud derivative which allows for higher compression |
| ratio keeping the simplicity and determinism of its predecessor. |
| |
| The main differences between z3fold and zbud are: |
| |
| * unlike zbud, z3fold allows for up to PAGE_SIZE allocations |
| * z3fold can hold up to 3 compressed pages in its page |
| * z3fold doesn't export any API itself and is thus intended to be used |
| via the zpool API. |
| |
| To keep the determinism and simplicity, z3fold, just like zbud, always |
| stores an integral number of compressed pages per page, but it can store |
| up to 3 pages unlike zbud which can store at most 2. Therefore the |
| compression ratio goes to around 2.7x while zbud's one is around 1.7x. |
| |
| Unlike zbud (but like zsmalloc for that matter) z3fold_alloc() does not |
| return a dereferenceable pointer. Instead, it returns an unsigned long |
| handle which encodes actual location of the allocated object. |
| |
| Keeping effective compression ratio close to zsmalloc's, z3fold doesn't |
| depend on MMU enabled and provides more predictable reclaim behavior |
| which makes it a better fit for small and response-critical systems. |
