summaryrefslogtreecommitdiffstats
path: root/Documentation/mm/z3fold.rst
diff options
context:
space:
mode:
Diffstat (limited to 'Documentation/mm/z3fold.rst')
-rw-r--r--Documentation/mm/z3fold.rst28
1 files changed, 28 insertions, 0 deletions
diff --git a/Documentation/mm/z3fold.rst b/Documentation/mm/z3fold.rst
new file mode 100644
index 0000000000..25b5935d06
--- /dev/null
+++ b/Documentation/mm/z3fold.rst
@@ -0,0 +1,28 @@
+======
+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.