1.. _zsmalloc:
2
3========
4zsmalloc
5========
6
7This allocator is designed for use with zram. Thus, the allocator is
8supposed to work well under low memory conditions. In particular, it
9never attempts higher order page allocation which is very likely to
10fail under memory pressure. On the other hand, if we just use single
11(0-order) pages, it would suffer from very high fragmentation --
12any object of size PAGE_SIZE/2 or larger would occupy an entire page.
13This was one of the major issues with its predecessor (xvmalloc).
14
15To overcome these issues, zsmalloc allocates a bunch of 0-order pages
16and links them together using various 'struct page' fields. These linked
17pages act as a single higher-order page i.e. an object can span 0-order
18page boundaries. The code refers to these linked pages as a single entity
19called zspage.
20
21For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE
22since this satisfies the requirements of all its current users (in the
23worst case, page is incompressible and is thus stored "as-is" i.e. in
24uncompressed form). For allocation requests larger than this size, failure
25is returned (see zs_malloc).
26
27Additionally, zs_malloc() does not return a dereferenceable pointer.
28Instead, it returns an opaque handle (unsigned long) which encodes actual
29location of the allocated object. The reason for this indirection is that
30zsmalloc does not keep zspages permanently mapped since that would cause
31issues on 32-bit systems where the VA region for kernel space mappings
32is very small. So, before using the allocating memory, the object has to
33be mapped using zs_map_object() to get a usable pointer and subsequently
34unmapped using zs_unmap_object().
35
36stat
37====
38
39With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via
40``/sys/kernel/debug/zsmalloc/<user name>``. Here is a sample of stat output::
41
42 # cat /sys/kernel/debug/zsmalloc/zram0/classes
43
44 class  size almost_full almost_empty obj_allocated   obj_used pages_used pages_per_zspage
45    ...
46    ...
47     9   176           0            1           186        129          8                4
48    10   192           1            0          2880       2872        135                3
49    11   208           0            1           819        795         42                2
50    12   224           0            1           219        159         12                4
51    ...
52    ...
53
54
55class
56	index
57size
58	object size zspage stores
59almost_empty
60	the number of ZS_ALMOST_EMPTY zspages(see below)
61almost_full
62	the number of ZS_ALMOST_FULL zspages(see below)
63obj_allocated
64	the number of objects allocated
65obj_used
66	the number of objects allocated to the user
67pages_used
68	the number of pages allocated for the class
69pages_per_zspage
70	the number of 0-order pages to make a zspage
71
72We assign a zspage to ZS_ALMOST_EMPTY fullness group when n <= N / f, where
73
74* n = number of allocated objects
75* N = total number of objects zspage can store
76* f = fullness_threshold_frac(ie, 4 at the moment)
77
78Similarly, we assign zspage to:
79
80* ZS_ALMOST_FULL  when n > N / f
81* ZS_EMPTY        when n == 0
82* ZS_FULL         when n == N
83