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zswap: update docs for runtime-changeable attributes 

Change the Documentation/vm/zswap.txt doc to indicate that the "zpool" and
"compressor" params are now changeable at runtime.

Signed-off-by: Dan Streetman <ddstreet@ieee.org>
Cc: Seth Jennings <sjennings@variantweb.net>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Dan Streetman 2015-09-09 15:35:25 -07:00 committed by Linus Torvalds
parent 90b0fc26d5
commit 9c4c5ef376
1 changed files with 28 additions and 8 deletions
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Documentation/vm/zswap.txt
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@ -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 An example command to enable zswap at runtime, assuming sysfs is mounted
at /sys, is: 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 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 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 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. 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 Zswap makes use of zpool for the managing the compressed memory pool. Each
allocation in zbud is not directly accessible by address. Rather, a handle is 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 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 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 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 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 with a red-black tree per swap type. The swap offset is the search key for the
tree nodes. tree nodes.
@ -74,9 +86,17 @@ controlled policy:
* max_pool_percent - The maximum percentage of memory that the compressed * max_pool_percent - The maximum percentage of memory that the compressed
pool can occupy. pool can occupy.
Zswap allows the compressor to be selected at kernel boot time by setting the The default compressor is lzo, but it can be selected at boot time by setting
“compressor” attribute. The default compressor is lzo. e.g. the “compressor” attribute, e.g. zswap.compressor=lzo. It can also be changed
zswap.compressor=deflate 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 A debugfs interface is provided for various statistic about pool size, number
of pages stored, and various counters for the reasons pages are rejected. of pages stored, and various counters for the reasons pages are rejected.