1===== 2zswap 3===== 4 5Overview 6======== 7 8Zswap is a lightweight compressed cache for swap pages. It takes pages that are 9in the process of being swapped out and attempts to compress them into a 10dynamically allocated RAM-based memory pool. zswap basically trades CPU cycles 11for potentially reduced swap I/O. This trade-off can also result in a 12significant performance improvement if reads from the compressed cache are 13faster than reads from a swap device. 14 15Some potential benefits: 16 17* Desktop/laptop users with limited RAM capacities can mitigate the 18 performance impact of swapping. 19* Overcommitted guests that share a common I/O resource can 20 dramatically reduce their swap I/O pressure, avoiding heavy handed I/O 21 throttling by the hypervisor. This allows more work to get done with less 22 impact to the guest workload and guests sharing the I/O subsystem 23* Users with SSDs as swap devices can extend the life of the device by 24 drastically reducing life-shortening writes. 25 26Zswap evicts pages from compressed cache on an LRU basis to the backing swap 27device when the compressed pool reaches its size limit. This requirement had 28been identified in prior community discussions. 29 30Whether Zswap is enabled at the boot time depends on whether 31the ``CONFIG_ZSWAP_DEFAULT_ON`` Kconfig option is enabled or not. 32This setting can then be overridden by providing the kernel command line 33``zswap.enabled=`` option, for example ``zswap.enabled=0``. 34Zswap can also be enabled and disabled at runtime using the sysfs interface. 35An example command to enable zswap at runtime, assuming sysfs is mounted 36at ``/sys``, is:: 37 38 echo 1 > /sys/module/zswap/parameters/enabled 39 40When zswap is disabled at runtime it will stop storing pages that are 41being swapped out. However, it will _not_ immediately write out or fault 42back into memory all of the pages stored in the compressed pool. The 43pages stored in zswap will remain in the compressed pool until they are 44either invalidated or faulted back into memory. In order to force all 45pages out of the compressed pool, a swapoff on the swap device(s) will 46fault back into memory all swapped out pages, including those in the 47compressed pool. 48 49Design 50====== 51 52Zswap receives pages for compression from the swap subsystem and is able to 53evict pages from its own compressed pool on an LRU basis and write them back to 54the backing swap device in the case that the compressed pool is full. 55 56Zswap makes use of zpool for the managing the compressed memory pool. Each 57allocation in zpool is not directly accessible by address. Rather, a handle is 58returned by the allocation routine and that handle must be mapped before being 59accessed. The compressed memory pool grows on demand and shrinks as compressed 60pages are freed. The pool is not preallocated. By default, a zpool 61of type selected in ``CONFIG_ZSWAP_ZPOOL_DEFAULT`` Kconfig option is created, 62but it can be overridden at boot time by setting the ``zpool`` attribute, 63e.g. ``zswap.zpool=zbud``. It can also be changed at runtime using the sysfs 64``zpool`` attribute, e.g.:: 65 66 echo zbud > /sys/module/zswap/parameters/zpool 67 68The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which 69means the compression ratio will always be 2:1 or worse (because of half-full 70zbud pages). The zsmalloc type zpool has a more complex compressed page 71storage method, and it can achieve greater storage densities. 72 73When a swap page is passed from swapout to zswap, zswap maintains a mapping 74of the swap entry, a combination of the swap type and swap offset, to the zpool 75handle that references that compressed swap page. This mapping is achieved 76with a red-black tree per swap type. The swap offset is the search key for the 77tree nodes. 78 79During a page fault on a PTE that is a swap entry, the swapin code calls the 80zswap load function to decompress the page into the page allocated by the page 81fault handler. 82 83Once there are no PTEs referencing a swap page stored in zswap (i.e. the count 84in the swap_map goes to 0) the swap code calls the zswap invalidate function 85to free the compressed entry. 86 87Zswap seeks to be simple in its policies. Sysfs attributes allow for one user 88controlled policy: 89 90* max_pool_percent - The maximum percentage of memory that the compressed 91 pool can occupy. 92 93The default compressor is selected in ``CONFIG_ZSWAP_COMPRESSOR_DEFAULT`` 94Kconfig option, but it can be overridden at boot time by setting the 95``compressor`` attribute, e.g. ``zswap.compressor=lzo``. 96It can also be changed at runtime using the sysfs "compressor" 97attribute, e.g.:: 98 99 echo lzo > /sys/module/zswap/parameters/compressor 100 101When the zpool and/or compressor parameter is changed at runtime, any existing 102compressed pages are not modified; they are left in their own zpool. When a 103request is made for a page in an old zpool, it is uncompressed using its 104original compressor. Once all pages are removed from an old zpool, the zpool 105and its compressor are freed. 106 107Some of the pages in zswap are same-value filled pages (i.e. contents of the 108page have same value or repetitive pattern). These pages include zero-filled 109pages and they are handled differently. During store operation, a page is 110checked if it is a same-value filled page before compressing it. If true, the 111compressed length of the page is set to zero and the pattern or same-filled 112value is stored. 113 114To prevent zswap from shrinking pool when zswap is full and there's a high 115pressure on swap (this will result in flipping pages in and out zswap pool 116without any real benefit but with a performance drop for the system), a 117special parameter has been introduced to implement a sort of hysteresis to 118refuse taking pages into zswap pool until it has sufficient space if the limit 119has been hit. To set the threshold at which zswap would start accepting pages 120again after it became full, use the sysfs ``accept_threshold_percent`` 121attribute, e. g.:: 122 123 echo 80 > /sys/module/zswap/parameters/accept_threshold_percent 124 125Setting this parameter to 100 will disable the hysteresis. 126 127Some users cannot tolerate the swapping that comes with zswap store failures 128and zswap writebacks. Swapping can be disabled entirely (without disabling 129zswap itself) on a cgroup-basis as follows:: 130 131 echo 0 > /sys/fs/cgroup/<cgroup-name>/memory.zswap.writeback 132 133Note that if the store failures are recurring (for e.g if the pages are 134incompressible), users can observe reclaim inefficiency after disabling 135writeback (because the same pages might be rejected again and again). 136 137When there is a sizable amount of cold memory residing in the zswap pool, it 138can be advantageous to proactively write these cold pages to swap and reclaim 139the memory for other use cases. By default, the zswap shrinker is disabled. 140User can enable it as follows:: 141 142 echo Y > /sys/module/zswap/parameters/shrinker_enabled 143 144This can be enabled at the boot time if ``CONFIG_ZSWAP_SHRINKER_DEFAULT_ON`` is 145selected. 146 147A debugfs interface is provided for various statistic about pool size, number 148of pages stored, same-value filled pages and various counters for the reasons 149pages are rejected. 150