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Diffstat (limited to 'Documentation/vm/zsmalloc.txt')
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diff --git a/Documentation/vm/zsmalloc.txt b/Documentation/vm/zsmalloc.txt deleted file mode 100644 index 64ed63c4f69d6..0000000000000 --- a/Documentation/vm/zsmalloc.txt +++ /dev/null @@ -1,70 +0,0 @@ -zsmalloc --------- - -This allocator is designed for use with zram. Thus, the allocator is -supposed to work well under low memory conditions. In particular, it -never attempts higher order page allocation which is very likely to -fail under memory pressure. On the other hand, if we just use single -(0-order) pages, it would suffer from very high fragmentation -- -any object of size PAGE_SIZE/2 or larger would occupy an entire page. -This was one of the major issues with its predecessor (xvmalloc). - -To overcome these issues, zsmalloc allocates a bunch of 0-order pages -and links them together using various 'struct page' fields. These linked -pages act as a single higher-order page i.e. an object can span 0-order -page boundaries. The code refers to these linked pages as a single entity -called zspage. - -For simplicity, zsmalloc can only allocate objects of size up to PAGE_SIZE -since this satisfies the requirements of all its current users (in the -worst case, page is incompressible and is thus stored "as-is" i.e. in -uncompressed form). For allocation requests larger than this size, failure -is returned (see zs_malloc). - -Additionally, zs_malloc() does not return a dereferenceable pointer. -Instead, it returns an opaque handle (unsigned long) which encodes actual -location of the allocated object. The reason for this indirection is that -zsmalloc does not keep zspages permanently mapped since that would cause -issues on 32-bit systems where the VA region for kernel space mappings -is very small. So, before using the allocating memory, the object has to -be mapped using zs_map_object() to get a usable pointer and subsequently -unmapped using zs_unmap_object(). - -stat ----- - -With CONFIG_ZSMALLOC_STAT, we could see zsmalloc internal information via -/sys/kernel/debug/zsmalloc/<user name>. Here is a sample of stat output: - -# cat /sys/kernel/debug/zsmalloc/zram0/classes - - class size almost_full almost_empty obj_allocated obj_used pages_used pages_per_zspage - .. - .. - 9 176 0 1 186 129 8 4 - 10 192 1 0 2880 2872 135 3 - 11 208 0 1 819 795 42 2 - 12 224 0 1 219 159 12 4 - .. - .. - - -class: index -size: object size zspage stores -almost_empty: the number of ZS_ALMOST_EMPTY zspages(see below) -almost_full: the number of ZS_ALMOST_FULL zspages(see below) -obj_allocated: the number of objects allocated -obj_used: the number of objects allocated to the user -pages_used: the number of pages allocated for the class -pages_per_zspage: the number of 0-order pages to make a zspage - -We assign a zspage to ZS_ALMOST_EMPTY fullness group when: - n <= N / f, where -n = number of allocated objects -N = total number of objects zspage can store -f = fullness_threshold_frac(ie, 4 at the moment) - -Similarly, we assign zspage to: - ZS_ALMOST_FULL when n > N / f - ZS_EMPTY when n == 0 - ZS_FULL when n == N |