1 ====== 1 ======
2 z3fold 2 z3fold
3 ====== 3 ======
4 4
5 z3fold is a special purpose allocator for stor 5 z3fold is a special purpose allocator for storing compressed pages.
6 It is designed to store up to three compressed 6 It is designed to store up to three compressed pages per physical page.
7 It is a zbud derivative which allows for highe 7 It is a zbud derivative which allows for higher compression
8 ratio keeping the simplicity and determinism o 8 ratio keeping the simplicity and determinism of its predecessor.
9 9
10 The main differences between z3fold and zbud a 10 The main differences between z3fold and zbud are:
11 11
12 * unlike zbud, z3fold allows for up to PAGE_SI 12 * unlike zbud, z3fold allows for up to PAGE_SIZE allocations
13 * z3fold can hold up to 3 compressed pages in 13 * z3fold can hold up to 3 compressed pages in its page
14 * z3fold doesn't export any API itself and is 14 * z3fold doesn't export any API itself and is thus intended to be used
15 via the zpool API. 15 via the zpool API.
16 16
17 To keep the determinism and simplicity, z3fold 17 To keep the determinism and simplicity, z3fold, just like zbud, always
18 stores an integral number of compressed pages 18 stores an integral number of compressed pages per page, but it can store
19 up to 3 pages unlike zbud which can store at m 19 up to 3 pages unlike zbud which can store at most 2. Therefore the
20 compression ratio goes to around 2.7x while zb 20 compression ratio goes to around 2.7x while zbud's one is around 1.7x.
21 21
22 Unlike zbud (but like zsmalloc for that matter 22 Unlike zbud (but like zsmalloc for that matter) z3fold_alloc() does not
23 return a dereferenceable pointer. Instead, it 23 return a dereferenceable pointer. Instead, it returns an unsigned long
24 handle which encodes actual location of the al 24 handle which encodes actual location of the allocated object.
25 25
26 Keeping effective compression ratio close to z 26 Keeping effective compression ratio close to zsmalloc's, z3fold doesn't
27 depend on MMU enabled and provides more predic 27 depend on MMU enabled and provides more predictable reclaim behavior
28 which makes it a better fit for small and resp 28 which makes it a better fit for small and response-critical systems.
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