admin-guide/mm/zswap.rst

8 Zswap is a lightweight compressed cache for swap pages. It takes pages that are
26 Zswap evicts pages from compressed cache on an LRU basis to the backing swap
40 When zswap is disabled at runtime it will stop storing pages that are
42 back into memory all of the pages stored in the compressed pool.  The
43 pages stored in zswap will remain in the compressed pool until they are
45 pages out of the compressed pool, a swapoff on the swap device(s) will
46 fault back into memory all swapped out pages, including those in the
52 Zswap receives pages for compression from the swap subsystem and is able to
53 evict pages from its own compressed pool on an LRU basis and write them back to
60 pages are freed.  The pool is not preallocated.  By default, a zpool
68 The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which
70 zbud pages).  The zsmalloc type zpool has a more complex compressed page
102 compressed pages are not modified; they are left in their own zpool.  When a
104 original compressor.  Once all pages are removed from an old zpool, the zpool
107 Some of the pages in zswap are same-value filled pages (i.e. contents of the
108 page have same value or repetitive pattern). These pages include zero-filled
109 pages and they are handled differently. During store operation, a page is
115 pressure on swap (this will result in flipping pages in and out zswap pool
118 refuse taking pages into zswap pool until it has sufficient space if the limit
119 has been hit. To set the threshold at which zswap would start accepting pages
133 Note that if the store failures are recurring (for e.g if the pages are
135 writeback (because the same pages might be rejected again and again).
138 can be advantageous to proactively write these cold pages to swap and reclaim
148 of pages stored, same-value filled pages and various counters for the reasons
149 pages are rejected.