diff options
Diffstat (limited to 'Documentation/vm/zswap.txt')
| -rw-r--r-- | Documentation/vm/zswap.txt | 36 |
1 files changed, 28 insertions, 8 deletions
diff --git a/Documentation/vm/zswap.txt b/Documentation/vm/zswap.txt index 8458c0861e4e..89fff7d611cc 100644 --- a/Documentation/vm/zswap.txt +++ b/Documentation/vm/zswap.txt @@ -32,7 +32,7 @@ can also be enabled and disabled at runtime using the sysfs interface. An example command to enable zswap at runtime, assuming sysfs is mounted at /sys, is: -echo 1 > /sys/modules/zswap/parameters/enabled +echo 1 > /sys/module/zswap/parameters/enabled When zswap is disabled at runtime it will stop storing pages that are being swapped out. However, it will _not_ immediately write out or fault @@ -49,14 +49,26 @@ Zswap receives pages for compression through the Frontswap API and is able to evict pages from its own compressed pool on an LRU basis and write them back to the backing swap device in the case that the compressed pool is full. -Zswap makes use of zbud for the managing the compressed memory pool. Each -allocation in zbud is not directly accessible by address. Rather, a handle is +Zswap makes use of zpool for the managing the compressed memory pool. Each +allocation in zpool is not directly accessible by address. Rather, a handle is returned by the allocation routine and that handle must be mapped before being accessed. The compressed memory pool grows on demand and shrinks as compressed -pages are freed. The pool is not preallocated. +pages are freed. The pool is not preallocated. By default, a zpool of type +zbud is created, but it can be selected at boot time by setting the "zpool" +attribute, e.g. zswap.zpool=zbud. It can also be changed at runtime using the +sysfs "zpool" attribute, e.g. + +echo zbud > /sys/module/zswap/parameters/zpool + +The zbud type zpool allocates exactly 1 page to store 2 compressed pages, which +means the compression ratio will always be 2:1 or worse (because of half-full +zbud pages). The zsmalloc type zpool has a more complex compressed page +storage method, and it can achieve greater storage densities. However, +zsmalloc does not implement compressed page eviction, so once zswap fills it +cannot evict the oldest page, it can only reject new pages. When a swap page is passed from frontswap to zswap, zswap maintains a mapping -of the swap entry, a combination of the swap type and swap offset, to the zbud +of the swap entry, a combination of the swap type and swap offset, to the zpool handle that references that compressed swap page. This mapping is achieved with a red-black tree per swap type. The swap offset is the search key for the tree nodes. @@ -74,9 +86,17 @@ controlled policy: * max_pool_percent - The maximum percentage of memory that the compressed pool can occupy. -Zswap allows the compressor to be selected at kernel boot time by setting the -“compressor” attribute. The default compressor is lzo. e.g. -zswap.compressor=deflate +The default compressor is lzo, but it can be selected at boot time by setting +the “compressor” attribute, e.g. zswap.compressor=lzo. It can also be changed +at runtime using the sysfs "compressor" attribute, e.g. + +echo lzo > /sys/module/zswap/parameters/compressor + +When the zpool and/or compressor parameter is changed at runtime, any existing +compressed pages are not modified; they are left in their own zpool. When a +request is made for a page in an old zpool, it is uncompressed using its +original compressor. Once all pages are removed from an old zpool, the zpool +and its compressor are freed. A debugfs interface is provided for various statistic about pool size, number of pages stored, and various counters for the reasons pages are rejected. |