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authorMike Rapoport <rppt@linux.vnet.ibm.com>2018-03-21 22:22:47 +0300
committerJonathan Corbet <corbet@lwn.net>2018-04-16 23:18:15 +0300
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tree3994f40f1f93aec279d0b5c9117c0085a9f9ab03 /Documentation/vm/z3fold.rst
parent3406bb5c64a091ad887c3fb339ad88e9e88ef938 (diff)
downloadlinux-ad56b738c5dd223a2f66685830f82194025a6138.tar.xz
docs/vm: rename documentation files to .rst
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com> Signed-off-by: Jonathan Corbet <corbet@lwn.net>
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+.. _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.