1Overview: 2 3Zswap is a lightweight compressed cache for swap pages. It takes pages that are 4in the process of being swapped out and attempts to compress them into a 5dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles 6for potentially reduced swap I/O.�� This trade-off can also result in a 7significant performance improvement if reads from the compressed cache are 8faster than reads from a swap device. 9 10NOTE: Zswap is a new feature as of v3.11 and interacts heavily with memory 11reclaim. This interaction has not been fully explored on the large set of 12potential configurations and workloads that exist. For this reason, zswap 13is a work in progress and should be considered experimental. 14 15Some potential benefits: 16* Desktop/laptop users with limited RAM capacities can mitigate the 17������ performance impact of swapping. 18* Overcommitted guests that share a common I/O resource can 19������ dramatically reduce their swap I/O pressure, avoiding heavy handed I/O 20 throttling by the hypervisor.��This allows more work to get done with less 21 impact to the guest workload and guests sharing the I/O subsystem 22* Users with SSDs as swap devices can extend the life of the device by 23������ drastically reducing life-shortening writes. 24 25Zswap evicts pages from compressed cache on an LRU basis to the backing swap 26device when the compressed pool reaches its size limit. This requirement had 27been identified in prior community discussions. 28 29Zswap is disabled by default but can be enabled at boot time by setting 30the "enabled" attribute to 1 at boot time. ie: zswap.enabled=1. Zswap 31can also be enabled and disabled at runtime using the sysfs interface. 32An example command to enable zswap at runtime, assuming sysfs is mounted 33at /sys, is: 34 35echo 1 > /sys/module/zswap/parameters/enabled 36 37When zswap is disabled at runtime it will stop storing pages that are 38being swapped out. However, it will _not_ immediately write out or fault 39back into memory all of the pages stored in the compressed pool. The 40pages stored in zswap will remain in the compressed pool until they are 41either invalidated or faulted back into memory. In order to force all 42pages out of the compressed pool, a swapoff on the swap device(s) will 43fault back into memory all swapped out pages, including those in the 44compressed pool. 45 46Design: 47 48Zswap receives pages for compression through the Frontswap API and is able to 49evict pages from its own compressed pool on an LRU basis and write them back to 50the backing swap device in the case that the compressed pool is full. 51 52Zswap makes use of zpool for the managing the compressed memory pool. Each 53allocation in zpool is not directly accessible by address. Rather, a handle is 54returned by the allocation routine and that handle must be mapped before being 55accessed. The compressed memory pool grows on demand and shrinks as compressed 56pages are freed. The pool is not preallocated. By default, a zpool of type 57zbud is created, but it can be selected at boot time by setting the "zpool" 58attribute, e.g. zswap.zpool=zbud. It can also be changed at runtime using the 59sysfs "zpool" attribute, e.g. 60 61echo zbud > /sys/module/zswap/parameters/zpool 62 63The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which 64means the compression ratio will always be 2:1 or worse (because of half-full 65zbud pages). The zsmalloc type zpool has a more complex compressed page 66storage method, and it can achieve greater storage densities. However, 67zsmalloc does not implement compressed page eviction, so once zswap fills it 68cannot evict the oldest page, it can only reject new pages. 69 70When a swap page is passed from frontswap to zswap, zswap maintains a mapping 71of the swap entry, a combination of the swap type and swap offset, to the zpool 72handle that references that compressed swap page. This mapping is achieved 73with a red-black tree per swap type. The swap offset is the search key for the 74tree nodes. 75 76During a page fault on a PTE that is a swap entry, frontswap calls the zswap 77load function to decompress the page into the page allocated by the page fault 78handler. 79 80Once there are no PTEs referencing a swap page stored in zswap (i.e. the count 81in the swap_map goes to 0) the swap code calls the zswap invalidate function, 82via frontswap, to free the compressed entry. 83 84Zswap seeks to be simple in its policies. Sysfs attributes allow for one user 85controlled policy: 86* max_pool_percent - The maximum percentage of memory that the compressed 87 pool can occupy. 88 89The default compressor is lzo, but it can be selected at boot time by setting 90the ���compressor��� attribute, e.g. zswap.compressor=lzo. It can also be changed 91at runtime using the sysfs "compressor" attribute, e.g. 92 93echo lzo > /sys/module/zswap/parameters/compressor 94 95When the zpool and/or compressor parameter is changed at runtime, any existing 96compressed pages are not modified; they are left in their own zpool. When a 97request is made for a page in an old zpool, it is uncompressed using its 98original compressor. Once all pages are removed from an old zpool, the zpool 99and its compressor are freed. 100 101A debugfs interface is provided for various statistic about pool size, number 102of pages stored, and various counters for the reasons pages are rejected. 103